The effect of corrosion control treatments and biofilm disinfection on unlined ferrous pipes by Calvin Glenn Abernathy A thesis submitted in partial fulfillment Of the requirements for the degree of Doctor of Philosophy in Civil Engineering Montana State University © Copyright by Calvin Glenn Abernathy (1998) Abstract: The occurrence of microbially related water quality problems has long been a concern to consumers and to water suppliers. Microbially related water quality problems are of particular interest because ferrous materials have been found to support larger populations of attached microorganisms (biofilms) even in the presence of disinfectants. These systems are now being required to implement corrosion control programs to reduce the quantities of lead and copper that leach from plumbing materials into the finished water. Many utilities have reservations about the implementation of a corrosion control program because most corrosion inhibitors contain phosphorus, which is an essential nutrient for microbial growth. It is therefore the purpose of this research to investigate the numerous interactions between disinfectants (free chlorine and monochloramine) and several corrosion control methods and determine how they influence microbial growth in distribution systems. To investigate these numerous interactions, studies were conducted using bench-scale and pilot-scale facilities, chemostats, and various other laboratory systems. Bench-scale facilities consisted of continuous flow annular reactors using unlined ductile iron or unlined mild steel materials. Pilot-scale facilities consisted of a 5-loop system of 4-inch mild steel pipe located at the Bozeman Water Treatment Plant. A series of experiments were conducted, using each type of facility, to evaluate the effect that various combinations of disinfectant and corrosion control treatments would have on distribution biofilms. Results from these experiments have demonstrated that use of an effective corrosion control treatment will typically reduce microbial populations within the distribution system. Our studies have shown that corrosion products have a significant influence on the amount of microorganisms that a pipe material can support. Goethite (α-FeOOH), the most common corrosion product found in distribution systems, is of particular importance because it is able to adsorb and transform humic substances to more bioavailable forms, resulting in increased substrate for attached microorganisms. This study has demonstrated that an effective corrosion control program will reduce the amount of goethite formed on a ferrous pipe, and will consequently reduce the amount of bioavailable carbon on the surface and reduce habitat for problematic microorganisms.  THE EFFECT OF CORROSION CONTROL TREATMENTS AND BIOFILM DISINFECTION ON UNLINED FERROUS PIPES By Calvin Glenn Abernathy A thesis submitted in partial fulfillment Of the requirements for the degree of Doctor of Philosophy in Civil Engineering MONTANA STATE UNIVERSITY Bozeman, Montana November, 1998 APPROVAL of a thesis submitted by Calvin Glenn Abernathy This thesis has been read by each member of the thesis committee and has been found to be satisfactory regarding content, English usage, format, citations, bibliographic style, and consistency, and is ready for submission to the College of Graduate Studies. Dr. Anne Camper Approved for the Department of Civil Engineering Dr. Don Rabern (Signature) Date Approved for the College of Graduate Studies (SJdnatyre;) Date Dr. Joseph J. Fedock Ill STATEMENT OF PERMISSION TO USE In presenting this thesis in partial fulfillment of the requirements for a doctoral degree at Montana State University-Bozeman, I agree that the Library, shall make it available to borrowers under rules of the Library. I further agree that copying of this thesis is allowable only for scholarly purposes, consistent with “fair use” as prescribed in the U S. Copyright Law. Request for extensive copying or reproduction of this thesis should be referred to University Microfilms International, 300 North Zeeb Road, Ann Arbor, Michigan 48106, to whom I have granted “the exclusive right to reproduce and distribute my dissertation in and from microform along with the non-exclusive right to reproduce and distribute my abstract in any format in whole or in part.” Signature Date ACKNOWLEDGEMENTS The completion of this thesis represents the end of one dream and hopefully the beginning of a new career. The dream began in 1987 and has taken several stops, right turns, and wrong turns. The stops and right turns have proven to be inspirational moments that have identified who I am, while the wrong turns have proven to be the turns that directed back to my dream. In reality, the wrong turns were actually the right turns that helped me identify who I am and what I wanted out of life. I would like to thank several people who were once my idols that I did not know, then my mentors, and my closest friends. These include but are not limited to the late Jack Thompson, Paul Prendeville, Carlton Wilder, Bill McKenzie, Adnan Shindala, Dennis Truax, and Mark Brock. On the personal side, I wish to thank my parents and brother for their moral support over the past few years, and cheering me up when the chips were down. A special thanks also goes to the many special people at Montana State University. These include Anne Camper and Warren Jones for giving me the opportunity to continue my education and teaching me more than a thing or two. I would also like to thank Lu Goodrum for all of her help over the years. A special thanks also goes to Stacia Kimbell, an REU student for the summer of 1998, for all of her help and smiles, and to John Neuman for making the lab a safe place to work and for being such a dear friend. VTable off Comteimfis 1 INTRODUCTION 1 1.1 Goals and Objectives 2 1.2 Experimental Approach 3 2 LITERATURE REVIEW 4 2.1 Regulatory Impacts 5 2.1.1 Surface Water Treatment Rule 5 2.1.2 Lead and Copper Rule 7 2.1.3 Disinfectant and Disinfection Byproduct Rule 8 2.2 Physical, Chemical, and Biological Properties of Distribution 10 Biofilms 2.2.1 The Structure of Distribution Biofilms 10 2.2.2 Types of Microorganisms Found in Distribution 11 Biofilms 2.3 Interactions Between Pipe Materials and Distribution Biofilms 12 2.3.1 Pipe Material Properties 14 2.3.2 Formation of Corrosion Products 15 2.3.3 Physical/Chemical Properties of Corrosion Products 15 2.4 Interactions Between Organics and Distribution Biofilms 17 2.4.1 Types of Organics in Finished Water 18 2.4.2 Transformation of TOC Bioavailability by 22 Disinfectants 2.4.3 Transformation of TOC Bioavailability by 23 Corrosion Product Adsorption 2.5 Interactions Between Disinfectants and Distribution Biofilms 27 2.5.1 Microbial Inactivation in the Bulk Fluid by Disinfectants 28 2.5.2 Disinfection of Distribution Biofilms 29 2.6 Interactions Between Corrosion Inhibitors and Distribution 32 Biofilms 2.6.1 Adverse Water Quality Concerns Caused by Corrosion 32 Control Methods 2.6.2 Mechanisms of Corrosion Control Methods 34 2.6.3 Use of Corrosion Indices 37 2.7 Biofilm Modeling Systems 40 2.7.1 Requirements of a Biofilm Model , 41 2.7.2 Deficiencies of Existing Biofilm Models 42 2.8 Key Findings 43 3 MATERIALS AND METHODS 48 3.1 Microbial and Chemical Measurement Techniques 48 3.1.1 Bulk Fluid Measurements 48 3.1.2 Biofilm Density Measurements 49 3.1.3 Chemical Measurements 50 3.1.4 Corrosion Product Mass Measurements 50 3.1.5 Statistical Methods 52 3.2 Chemostat Operation 53 3.2.1 Coliform Culture Preparation and Conditioning 53 3.2.2 Chemostat Startup Procedures 54 3.3 Bench-scale Studies 56 3.3.1 Bench-scale Startup Procedures 60 3.4 Pilot-scale Studies ' 61 3.4.1 Pretreatment Facilities 61 3.4.2 Pipe Loop System 62 3.4.3 Pilot-plant Startup Procedures 65 3.5 Miscellaneous Laboratory Studies 66 3.5.1 Batch-scale Adsorption Studies of GAC Corrosion 67 Products 3.5.2 Disinfectant Demand Studies 68 4 BENCH-SCALE STUDIES 70 4.1 Bench-scale Studies with Free Chlorine without Corrosion 70 Control vi Vll 4.1.1 Experimental Design 71 4.1.2 Results 73 4.2 Bench-scale Studies with Free Chlorine and Monochloramine 78 without Corrosion Control 4.2.1 Experimental Design 82 4.2.2 Results 82 4.3 Bench-scale Studies with Free Chlorine and Corrosion Inhibitors 86 4.3.1 Experimental Design 87 4.3.2 Results 87 4.4 Bench-scale Studies with Monochloramine and Corrosion 93 Inhibitors 4.4.1 Experimental Design 93 4.4.2 Results 93 4.5 Bench-scale Studies with Free Chlorine and Polyphosphate 99 4.5.1 Experimental Design 99 4.5.2 Results 99 4.6 Key Findings 104 5 PILOT-SCALE STUDIES 108 5.1 Pilot-scale Studies with Free Chlorine and Corrosion Control 109 5.1.1 Experimental Design 109 5.1.2 Results and Discussion 110 5.2 Pilot-scale Studies with Monochloramine and Corrosion Control 116 5.2.1 Experimental Design 118 5.2.2 Results and Discussion 118 5.3 Pilot-scale Studies with Free Chlorine, Monochloramine and Zinc 125 Orthophosphate 5.3.1 Experimental Design 126 5.3.2 Results and Discussion 126 5.4 Key Findings 130 6 MISCELLANEOUS LABORATORY STUDIES 133 6.1 Adsorption and Bioavailability Studies of Humics and Corrosion 133 Products 6.1.1 Experimental Design 134 6.1.2 Results and Discussion * 136 6.2 Batch-scale Adsorption Properties of Corrosion Products 137 6.2.1 Experimental Design 137 6.2.2 Results and Discussion 139 6.3 Disinfectant Demand Studies of Corrosion Products 142 6.3.1 Experimental Design 143 6.3.2 Results and Discussion 143 6.4 pH Inhibition o i Klebsiella pneumoniae 146 6.4.1 Experimental Design 147 6.4.2 Results and Discussion 148 6.5 Key Findings 150 7 DISCUSSION, APPLICATIONS AND CONCLUSIONS 151 7.1 Biofilm Support Mechanisms Provided by Corrosion Products 151 7.1.1 Chemical Properties of Corrosion Products 152 7.1.2 Electrostatic Charges of Corrosion Products 158 7.2 Distribution System Biofilm Control Strategies 160 7.2.1 Changing Water Supplies 162 7.2.2 Enhanced Coagulation 162 7.2.3 Treatment Plant Optimization 163 7.2.4 Advanced Oxidation and Biological Treatment 165 7.2.5 Membrane Treatment 166 7.2.6 Changing Secondary Disinfectants 167 7.2.7 Implementation of a Corrosion Control Program 168 7.2.8 Distribution Pipe Replacement and Rehabilitation 169 7.2.9 Distribution Flushing Programs 170 LITERATURE CITED 171 viii IX Lisfi of Tables 2.1 Problematic Microorganisms in Water Distribution Systems 12 3.1 Chemical Analysis Methods and Equipment 51 4.1 Operational Conditions of Bench-scale Study (Step 2) Using Variable Free Chlorine Residuals without Corrosion Control 72 4.2 Operational Conditions of Bench-scale Study (StepS) Using Variable Free Chlorine Residuals without Corrosion Control 72 4.3 Seasonal Finished Water Quality Changes in Bozeman, MT 73 4.4 Biofilm Density Reaction Rates at Various Chlorine Residual Levels without the Use of Corrosion Inhibitors 82 4.5 Operational Conditions of Bench-scale Study Comparing Free Chlorine and Monochloramine without Corrosion Control 83 4.6 . Operational Conditions of Bench-scale Study Comparing Free Chlorine with Zinc Orthophosphate and Polyphosphate 88 4.7 Statistical Analysis of Biofilm Densities Using a 95% Confidence Level for Bench-scale Experiment Using Free Chlorine and Corrosion Inhibitors 91 4.8 Operational Conditions of Bench-scale Study Comparing Monochloramine with Zinc Orthophosphate and Polyphosphate 94 4.9 Statistical Analysis of Biofilm Densities Using a 95% Confidence Level for Bench-scale Experiment Using Monochloramine and Corrosion Inhibitors 97 4.10 Operational Conditions of Bench-scale Study Using Constant Free Chlorine Dosage and Variable Polyphosphate Levels 101 4.11 Results from Bench-scale Study Using Constant Free Chlorine Dosage and Variable Polyphosphate Levels 103 5.1 Operational Conditions of Pilot-scale Experiments Using Free Chlorine and Corrosion Control Treatments 110 5.2 Statistical Analysis of Biofilm Densities Using a 95% Confidence Level for Pilot-scale Experiment Using Free Chlorine and Corrosion Control Treatments 114 5.3 Operational Conditions of Pilot-scale Experiment Using Monochloramine and Corrosion Control Treatments 119 X5.4 Statistical Analysis of Biofilm Using a 95% Confidence Level for Pilot-scale Experiments Using Monochloramine and Corrosion Control Treatments 5.5 Operational Conditions of Pilot-scale Experiments Using Free Chlorine or Monochloramine with and without Zinc Orthophosphate 7.1 Partial List of Possible Reactions That Produce Corrosion Products 123 127 153 LSsft ©f Figures 2.1 Microbially Influenced Corrosion 16 2.2 Major Groups of TOC Available for Microbial Growth in 21 Drinking Water 2.3 Humic Substance Interaction with Iron Oxides 24 2.4 Biotransformation Pathways of Total Organic Carbon 26 3.1 Typical Chemostat Schematic 55 3.2 Field Type and Lab Type Annular Reactors 57 3.3 Typical Annular Reactor System 59 3.4 Pretreatment Facilities for the Pilot-plant System 63 3.5 Schematic of Pilot-scale Distribution System 64 4.1 Bench-scale Studies with Free Chlorine without Corrosion 74 Control - Process Flow Schematic 4.2 Biofilm Density as a Function o f Chlorine Residual 75 4.3 Corrosion Product Mass and Biofilm Populations at Various 77 Chlorine Residuals 4.4 Biofilm Density as a Function of Corrosion Products and Cb 79 Residual 4.5 Biofilm Density Comparisons Between Free Chlorine and 84 Monochloramine 4.6 Photographs of Reactor Walls after Experiment Using Free 85 Chlorine and Monochloramine 4.7 Biofilm Density Comparisons Using Free Chlorine and 89 Corrosion Inhibitors 4.8 Summary Data from Bench-scale Experiment Using Free 90 Chlorine and Corrosion Inhibitors 4.9 Photographs of Reactor Walls after Experiment Using Free 92 Chlorine and Corrosion Inhibitors Biofilm Density Comparisons Using Monochloramine and Corrosion Inhibitors xi 4.10 95 , 4.11 Summary Data from Bench-scale Experiment Using 96 Monochloramine and Corrosion Inhibitors 4.12 Photographs of Reactor Walls after Experiment Using 98 Monochloramine and Corrosion Inhibitors 4.13 Bench-scale Studies with Free Chlorine with Corrosion 100 Control - Process Flow Schematic 4.14 Biofilm Density, Chlorine Residual, Phosphate Residual, and 102 Corrosion .Product Mass Comparisons 4.15 Photographs of Coupons Prior to Sample Collection 105 4.16 Photographs of Coupons after Sample Collection 106 5.1 Biofilm Densities for Pilot-scale Experiments Using Free 112 Chlorine and Corrosion Control Treatments 5.2 Biofilm Density, Cl2 and PO4 Levels for Pilot-scale . 113 Experiments Using Free Chlorine and Corrosion Control T reatments 5.3 Photographs of Coupons after Experiments Using Free 117 Chlorine and Corrosion Control Treatments 5.4 Biofilm Densities for Pilot-scale Experiments Using 121 Monochloramine and Corrosion Control Treatments 5.5 Biofilm Density, NH2CI and PO4 Levels for Pilot-scale 122 Experiments Using Monochloramine and Corrosion Control Treatments 5.6 Photographs of Coupons after Experiments Using 124 Monochloramine and Corrosion Control Treatments 5.7 Biofilm Densities for Pilot-scale Experiments Using Free 128 Chlorine and Monochloramine as Secondary Disinfectants with and without Zinc Orthophosphate 5.8 Biofilm Density, Cl2 or NH2CI1 and PO4 Levels for 129 Experiments Using Free Chlorine or Monochloramine as Secondary Disinfection with and without Zinc Orthophosphate 5.9 Photographs of Coupons after Experiments Using Free 131 Chlorine or Monochloramine as Secondary Disinfection with and without Zinc Orthophosphate xii Xlll 6.1 Experimental Setup for Corrosion Products Study 135 6.2 Bioavailability Study of Adsorbed Humics on Corrosion 138 Products 6.3 Adsorption Isotherm for GAC and Humic Substances 140 6.4 Adsorption Isotherm for Magnetite (Fe3O4) and Humic 141 Substances 6.5 Disinfectant Demand Study Using Free Chlorine and 144 Corrosion Products 6.6 Disinfectant Demand Study Using Monochloramine and 145 Corrosion Products 6.7 pH Inhibition eft Klebsiella pneumoniae 149 7.1 Corrosion Product Pathways 155 7.2 Typical Structure of a Corrosion Tubercle 156 7.3 pH of Zero Charge (PZC) for Selected Materials 159 XlV ABSTRACT The occurrence of microbially related water quality problems has long been a concern to consumers and to water suppliers. Microbially related water quality problems are of particular interest because ferrous materials have been found to support larger populations of attached microorganisms (biofilms) even in the presence of disinfectants. These systems are now being required to implement corrosion control programs to reduce the quantities of lead and copper that leach from plumbing materials into the finished water. Many utilities have reservations about the implementation of a corrosion control program because most corrosion inhibitors contain phosphorus, which is an essential nutrient for microbial growth. It is therefore the purpose of this research to investigate the numerous interactions between disinfectants (free chlorine and monochloramine) and several corrosion control methods and determine how they influence microbial growth in distribution systems. To investigate these numerous interactions, studies were conducted using bench-scale and pilot-scale facilities, chemostats, and various other laboratory systems. Bench-scale facilities consisted of continuous flow annular reactors using unlined ductile iron or unlined mild steel materials. Pilot-scale facilities consisted of a 5-loop system of 4-inch mild steel pipe located at the Bozeman Water Treatment Plant. A series of experiments were conducted, using each type of facility, to evaluate the effect that various combinations of disinfectant and corrosion control treatments would have on distribution biofilms. Results from these experiments have demonstrated that use of an effective corrosion control treatment will typically reduce microbial populations within the distribution system. Our studies have shown that corrosion products have a significant influence on the amount of microorganisms that a pipe material can support. Goethite (a-FeOOH), the most common corrosion product found in distribution systems, is of particular importance because it is able to adsorb and transform humic substances to more bioavailable forms, resulting in increased substrate for attached microorganisms. This study has demonstrated that an effective corrosion control program will reduce the amount of goethite formed on a ferrous pipe, and will consequently reduce the amount of bioavailable carbon on the surface and reduce habitat for problematic microorganisms. 1Chapter 1 Hotrod MCtDOim For the past 20 years, the United States government has been implementing drinking water regulations with the intent of reducing the health risk associated with drinking water. For the most part these regulations have focused on increased microbial disinfection, reducing the amount of disinfection by­ products and reducing the leaching of lead and copper from distribution piping. In the years to come, new regulations will be promulgated that will not only lower the maximum contaminant levels (MCLs) of various disinfection by-products, but for the first time place a maximum limit on chlorine residuals found in distribution systems. In view of these new regulations, many utilities are concerned about maintaining chlorine residuals high enough to control biofilms and the possible impacts of increased microbial growth resulting from phosphorus based corrosion inhibitors. Although the federal regulations have changed significantly in the past 20 years and will continue to change well into the next millennium, microbial biofilms will still be present in water distribution systems. Biofilms have been shown to be responsible for positive conforms in water samples along with causing taste and odor complaints from consumers(LeChevallier, et al 1996; van der Wende and Characklis 1990; AWWA 1996). In the years to come, the water industry will be implementing numerous treatment changes to stay one step ahead of the 2regulations. In many cases, utilities will be using a trial-and-error approach to stay in regulatory compliance without knowing the impact a treatment change may have on the biostability of the finished water. Some questions that are frequently pondered by water suppliers are: © Will the addition of phosphorus based corrosion inhibitors increase microbial growth of distribution biofilms? o Will lower chlorine residuals be able to effectively control microbial regrowth in the distribution system? o Will chloramines be strong enough to control microbial regrowth events even though it is a weaker disinfectant? In view of the above listed concerns, a better understanding is needed about the interactions between pipe materials, disinfectants, and corrosion inhibitors, on distribution biofilms. It is therefore the purpose of this thesis to investigate and discuss many of these interactions, and hopefully provide better guidance on how treatment changes may affect distribution system biofilms. 1.1 Goals and Objectives The main goal of this work is to gain a better working knowledge of the interactions between pipe materials, organics, corrosion inhibitors and how each of these influence the disinfection of distribution biofilms. Since the complete inactivation of distribution biofilms is not possible, this thesis will focus on the effect that corrosion control treatments have on the control of biofilms on unlined ferrous pipes. 31.2 Experimental Approach To investigate the numerous interactions between corrosion control treatments and the disinfection of distribution biofilms, a four-step approach was implemented. This approach consisted of (a) a comprehensive literature review, (b) bench-scale studies utilizing annular reactors, (c) pilot-scale studies, and (d) ancillary laboratory experiments. Results from these four steps brought to light several new relationships between corrosion products, bioavailable carbon, and distribution biofilms. Regulation or control of the formation rates of corrosion products may prove to be a key factor in reducing water quality problems associated with distribution biofilms. 4Chapter 2 Literature Review The removal or inactivation of microorganisms in potable water is perhaps the leading health concern of consumers. Although the inactivation and removal of microorganisms from the finished water leaving a water treatment plant may appear to be simple, maintaining low levels of microorganisms in the distribution system is seldom, if at all, accomplished even in the presence of a disinfectant residual. The presence of microorganisms in water distribution systems is widespread because the microorganisms are able to accumulate and colonize on the interior surfaces of drinking water pipes. Once attached, they develop a physical and chemical structure that enables them to modify the microenvironment of the pipe surface in a manner that allows them to optimize their metabolism and become highly resistant to disinfectants (Characklis and Marshall 1990; LeChevaIIier, etal 1988). In consideration of the numerous water quality problems associated with microorganisms in water supply, a better understanding of how implementation of processes to meet the federal regulations influences distribution biofilms so that water suppliers can develop more efficient ways to minimize water quality problems. It is therefore the purpose of this chapter to: (a) discuss the federal regulations and (b) discuss the numerous interactions between pipe materials, 5organics, disinfectants, corrosion control methods and how each of these components affect the formation and inactivation of distribution biofilms. 2.1 Regulatory Impacts The 1986 and 1996 Safe Drinking Water Act (SDWA) Amendments will require many water utilities to modify current treatment and distribution practices. The goal of these regulations is to reduce the microbial and chemical health risk of the finished water by increasing disinfection efficacy along with reducing the concentrations of lead and copper, disinfection byproducts, and disinfectant residuals within the distribution system. Although these regulations will result in lower health risk, many utilities will use a trial-and-error approach to compliance, and may actually increase water quality problems associated with the presence of microorganisms in the distribution system if improperly addressed. In view of the above, it is therefore the purpose of this section to discuss the existing and proposed SDWA regulations, and define how irrational implementation may result in increased microbial related water quality problems. Regulations that will be addressed will include the Surface Water Treatment Rule, Lead and Copper Rule, and the Disinfectant and Disinfection Byproduct Rule. 2.1.1 Surface Water Treatment Rule The assumption in developing the Surface Water Treatment Rule (SWTR) was that surface waters and groundwater under the direct influence of surface 6waters are at risk of contamination by Giardia lamblia and other protozoa, viruses, and pathogenic microorganisms (Bryant, et al 1992). The goal of this rule was to provide a minimum level of protection from illnesses caused by these organisms by specifying a minimum 3-log (99.9 percent) inactivation and/or removal of Giardia cysts and a 4-log (99.99 percent) inactivation and/or removal of enteric viruses prior to final distribution. The SWTR also established a minimum disinfectant residual (either free or total chlorine) of 0.2 mg/L entering the distribution system and requires each utility to maintain a detectable residual throughout the distribution system. As a result of the SWTR, the microbial water quality supplied by water suppliers has improved, primarily from increased disinfection at the water treatment plant. Increased disinfection efficacy will minimize the possible “breakthrough” of microorganisms from the treatment system, and will likely reduce the quantity of microorganisms that enter the distribution system. One possible adverse impact of the SWTR occurs when utilities increase disinfectant dosages or change primary disinfectants to comply with the “CT” requirements of the SWTR. In many cases it has been found that an increase in disinfectant dosage - particularly with the strong oxidants such as ozone, chlorine dioxide, and free chlorine - results in increased bioavailable organics in the finished water (Bryant, et al 1992). The increase of bioavailability of organics may result in increased microbial growth within the distribution system, even 7though disinfection efficacy was increased at the water treatment plant. Additional information on this subject can be found in Section 2.4 of this chapter. 2.1.2 Lead and Copper Rule The Lead and Copper Rule (LCR) was promulgated in June 1991 and requires utilities to maintain lead and copper concentrations, at the consumer tap below an action level of 15 pg/L and 1.3 mg/L respectively (AWWA 1992). In the event that the lead and/or copper levels exceed the specified action limit, the water supplier must implement a response plan that will minimize the leaching of lead and copper from pipes and plumbing fixtures. The response plan is typically to implement a corrosion control program that interferes with the corrosion process, resulting in reduced leaching of lead and copper from piping and plumbing materials. A corrosion control plan typically includes the use of pH adjustment of the finished water with sufficient alkalinity to promote the coating of the interior surface of pipes and plumbing materials with calcium carbonate (CaCOs), or by applying a corrosion inhibitor (AWWA 1992). The selection and implementation of corrosion control programs should be done with caution, because they can have an impact on both the formation of disinfection byproducts and possibly increase microbial growth in the distribution system (AWWA 1992). Of particular concern of these corrosion control techniques are: (a) the increase in tota l. trihalomethane (TTHM) formation resulting from higher pH values (Symons, et al 1982) and (b) the fact that most corrosion inhibitors contain phosphorus which is 8an essential nutrient for microbial growth (Brock, et al 1994). It has also been demonstrated numerous times that the implementation of a corrosion control program reduces biofilm densities (Schreppel and Geiss 1996; Lowther and Moser 1984). Discussions about how these corrosion control techniques may increase microbial related water quality problems are presented in Section 2.6 of this chapter. 2.1.3 Disinfectant and Disinfection Byproduct Rule The proposed Disinfectant/Disinfection Byproduct Rule (D/DBPR) is expected to lower the current THM levels and establish many new limits on various other disinfection byproducts and disinfectants in the finished water. This rule will likely have the most dramatic impact on water quality because it will apply to almost all water suppliers, regardless of size, and for the first time establish a maximum disinfectant residual in the distribution system. In summary, the goal of the D/DBPR is to optimize the removal of organic carbon, minimize the formation of disinfection byproducts, and to minimize disinfectant dosages. The implementation of the D/DBPR will drastically reduce the chemical health risk associated with disinfection, but may also reduce both primary and secondary disinfection efficacies. The implementation of the D/DBPR may cause widespread microbial water quality problems if not addressed properly by water suppliers. Perhaps the highest at risk are the smaller utilities that currently serve less than 10,000 people. These utilities do not have to comply with current TTHM limits, but will 9soon be required to meet TTHM levels as low as 80 jug/L and maintain disinfectant residuals less than 4 mg/L in the distribution system. Many of these utilities will struggle with coliform violations during the warmer months when microbial activity is at its highest level, and may not be able to control water quality problems with disinfectant residuals less than 4 mg/L and still comply with TTHM limits. A second group of utilities that will possibly have adverse microbial water quality problems are those utilities that currently have TTHM levels between 80 and 100 jug/L. Many of these utilities will attempt to lower disinfectant residuals to reduce disinfection byproduct levels, only to result in reduced control of existing distribution biofilms. A third group of utilities that may have problems with the delicate balance between disinfection byproducts and microbial inactivation will be the utilities that modify current primary disinfection practices by using more powerful disinfectants such as ozone or chlorine dioxide. The popularity of these powerful disinfectants is rapidly gaining because they are known to produce fewer regulated disinfection byproducts and are more effective at inactivating Cryptosporidium.(DeMers L.D. and Renner 1992; AVWVA 1998). Although the use of these primary disinfectants will likely result in lower disinfection byproducts in the finished water, these disinfectants are known to increase the bioavailability of organics for downstream treatment processes or in the distribution system 10 (Pontius 1990; Langlais1 et al 1991). Additional information on this subject can be found in Section 2.4 of this chapter. 2.2 Physical, Chemical and Biological Properties of Distribution Biofilms Disinfectants such as free chlorine or monochloramine have been used for nearly 100 years to reduce microbial populations in potable water supplies. Both free chlorine and monochloramine can be highly efficient at inactivating microorganisms in the bulk fluid, but have been found to be up to 2400 times less efficient at inactivating distribution biofilms (LeChevallier, et al 1988; McMath1 et al 1997; Geldreich 1996). Low inactivation efficacy of biofilms can be attributed to the physical and chemical properties of a biofilm which enable the microorganisms to accumulate nutrients and to develop resistance mechanisms to various disinfectants (Chen, et al 1993; van der Wende and Characklis 1990; LeChevallier, et al 1996; LeChevallier 1990; Geldreich 1996). 2.2.1 The Structure of Distribution Biofiims In general, biofilms consists of four components. The first three components include a consortium of microorganisms, extracellular polymeric substances (EPS) excreted by the microorganisms, and water (Characklis and Marshall 1990). The fourth component can be classified as organic and inorganic particles that adhere to the EPS and may originate from the bulk fluid, the pipe surface, or from inactivated cells from within the biofilm (LeChevallier, et al 1996; LeChevallier 1990; van der Wende and Characklis 1990; LeChevallier, et al 1993). EPS production will vary in composition and quantity depending on 11 the bacteria present and may be influenced by environmental conditions (Koudjonou, et al 1997). The development and accumulation of these four components creates a variety of niches that favor the metabolisms of aerobic, facultative, and anoxic microorganisms (van der Wende and Characklis 1990; Geldreich 1996), EPS, and the accumulation of corrosion products and particulates exert a significant disinfectant demand and provides a shield that protects microorganisms from lethal levels of disinfectants (LeChevallier, et al 1996; van der Wende and Characklis 1990; Geldreich 1996; LeChevallier, et al 1993). 2.2.2 Types of Microorganisms Found in Distribution Biofilms As result of the numerous niches formed within a biofilm, a distribution system can support a variety of microorganisms as described above. Microorganisms found in distribution biofilms may include conforms, actinomyces, molds, fungi, nitrifying bacteria, iron oxidizing bacteria, sulfate reducing bacteria (SRBs), and possibly even Giardia cysts or Cryptosporidium oocysts (LeChevallier 1990; AWWA 1996; Geldreich 1996; Keevel 1997; Camper 1994). Possible water quality problems associated with these microorganisms are identified in Table 2.1. 12 Table 2.1 Problematic Microorganisms in Water Distribution Systems Type o f Microorganism Infrastructure or Water Quality Problem' .... • . ■ Conforms Positive samples may be a violation of the Total Coliform Rule for large utilities and a violation of the Total Conform Rule for small utilities. Actinomycetes, Molds, and Fungi Produce earthy-musty-moldy taste and odor compounds. Commonly found in surface waters. Iron Bacteria Oxidize soluble iron to precipitate forms increasing the mass of corrosion products on pipe walls and pump casings. Excessive iron deposits causes increased pipe friction and lower pump efficacy. Sulfate Reducing Bacteria (SRBs) Reduces sulfate to hydrogen sulfide creating rotten egg taste and odor. Increases corrosion rates. Nitrifying Bacteria Oxidizes ammonia to nitrate. Consumes alkalinity, which may result in pH reduction. Protozoans Will not reproduce in biofilm, but may reside in biofilm. 2.3 Interactions Between Pipe Materials and Distribution Biofilms Most water distribution lines constructed today utilize polyvinyl chloride (PVC), cement lined ductile iron, or prestressed concrete cylinder pipes (PCCP). However, prior to the 1940’s when these modern materials were not universally available, most water distribution systems were constructed with unlined mild steel and unlined cast-iron pipes. Although these materials are seldom used today, they currently comprise about 22 percent of all distribution pipes in the 13 United States(LeChevallier 1997). For many of the larger water distribution systems (>50,000 people), the average age of the oldest section of the distribution system is typically greater than 50 years (Haas 1998). In view of this, it can be concluded that these distribution systems will have substantial quantities of unlined cast iron pipes. In many older cities, unlined cast-iron and unlined ductile-iron pipes may consist of more than 80 percent of the entire distribution system. The materials used in water distribution systems appear to be one of the most important factors that influence the proliferation of distribution biofilms (Camper, et al 1996). In a recent survey (LeChevallier, et al 1996), it was found that water distribution systems that contain large quantities of unlined cast-iron and unlined ductile-iron pipes frequently experience problems with coliform violations and taste and odor complaints(LeChevallier, et al 1996; van der Wende and Characklis 1990; van der Kooij and Oorhuizen 1997). Researchers have also found that pipe materials support different quantities of microorganisms even when the influent water quality is the same for each material (Chen, et al 1993; Camper, et al 1996; Olios, et al 1997; Delanoue, et al 1997). These researchers have documented that unlined mild steel, followed by unlined cast-iron, and unlined ductile-iron surfaces will support significantly higher biofilm densities than non-ferrous materials (LeChevallier 1997; Delanoue, et al 1997). 14 2.3.1 Pipe Material Properties In consideration of the facts presented above, one must ponder why unlined mild steel, unlined cast-iron and unlined ductile-iron pipes are capable of supporting higher biofilm densities than non-ferrous materials. The answer to this question is likely to be related to the amount iron present in each material. A typical mild steel will contain approximately 99.12 percent iron by weight, while cast-iron and ductile-iron contain 93.18 and 92.66 percent respectively (Singley and Ahmadi 1985). Although the differences between these percentages may appear to be insignificant, the amount of exposed iron surface area is substantial when the specific gravity of the non-ferrous materials are taken into consideration. The exposed non-ferrous materials amount to 1.1 percent of the total area for mild steel and 17 and 18.5 percent for cast-iron and ductile-iron pipes, respectively (Singley and Ahmadi 1985). Exposed iron is critical because the formation of iron based corrosion products is directly related to release of dissolved Fe2+ from the pipe surface. Corrosion products exert a chlorine demand and can accumulate nutrients for the growth of microorganisms (van der Wende and Characklis 1990; LeChevaIIier, et al 1996; LeChevaIIier, et al 1993). Corrosion products are of interest because systems with significant quantities of corrosion product mass have been found to have substantially higher microbial densities (Rice, et al 1991; Herson, et al 1991; van der Kooij and Oorhuizen 1997; McMath, et al 1997; Crayton, et al 1997; Martin, et al 1982; LeChevaIIier1 et al 1991; LeChevaIIier1 et al 1993; Camper 1994). 15 2.3.2 Formation of Corrosion Products The formation of corrosion products in water distribution systems results from the release of Fe2"1" ions from the pipe surface that react with various electron acceptors such as carbonate, oxygen, and free chlorine in the bulk fluid (Singley and Ahmadi 1985). Dissolved iron (Fe2+) can be released from the pipe by either microbiologically influenced corrosion (MIC) or by chemical induced corrosion. MIC is caused by the chemical gradient that develops between the pipe surface beneath a microbial colony and the bulk fluid. The oxygen gradient creates an electrochemical cell that causes the pipe to release a Fe2+ ion and two electrons as illustrated in Figure 2.1 (Videla 1996). Chemically induced corrosion is the result of an electrochemical potential differences between the ions present in the bulk fluid and the pipe surface, which promotes the release of the more thermodynamically stable form of iron (Fe2+) and electrons from the pipe surface. Once Fe2+ is released, it reacts with various electron acceptors to form precipitated corrosion products, or chelates with organic compounds in the bulk fluid or the biofilm matrix (Gu, et al 1994; Parfitt, et al 1977; Benjamin, et al 1990; Weber 1988). 2.3.3 Physical/Chemical Properties of Corrosion Products Goethite (a-FeOOH) and magnetite (FeaO4) are the most common types of corrosion products found on iron pipe surfaces in water distribution systems (Singley and Ahmadi 1985; Smith, et al 1996). The presence of goethite, the O-SO2 Microbial Colony Cathodic Area + H9O + 2e —► Cathodic Area Anodic Area Fe —^ F e 2+ + 2e* Videla, 1996 Figure 2.1 Microbially Influenced Corrosion 17 most abundant corrosion product, is of interest because it is capable of adsorbing organics from the bulk fluid (Gu, et al 1994; Chang, et al 1997; Benjamin, et al 1993; Benjamin and Li 1997; Benjamin, et al 1990; Parfitt, et al 1977; Tipping and Cooke 1982; Characklis 1989). There have even been a few studies where iron oxides have been used as a filter medium to remove natural organic matter (NOM) from water supplies (Benjamin, et al 1993; Benjamin and Li 1997). Considering that corrosion products are iron oxides, the corrosion products on a pipe surface are capable of adsorbing organics from the bulk fluid, hence, I providing a higher concentration of carbon on the pipe surface than in the bulk fluid (van der Wende and Characklis 1990). It is hypothesized that these adsorbed organics exert a substantial chlorine demand along with increasing the bioavailability of carbon substrates for microorganisms. The interactions between organics and corrosion products are discussed in more detail in Section 2.4. 2.4 Interactions Between Organics and Distribution Biofilms The role and significance of organic compounds in drinking water is of concern because many are precursors for various disinfection byproducts such as THM’s, and that many of the microorganisms present in distribution biofilms require the use of organic carbon for metabolism and/or cell synthesis (Camper 1994). This section will discuss the role of dissolved organic compounds typically found in finished water and discuss how they influence the formation of distribution biofilms. 18 2.4.1 Types of Organics in Finished Water - The amount of total organic carbon (TOC) in finished water is often used as a measure for appraising a water’s potential to form disinfection byproducts and to support microbial growth in the distribution system. In many situations this appraisal works reasonably well for assessing the disinfectant byproduct potential of a water, but can be inaccurate for predicting a water’s ability to promote excessive growth of microorganisms in the distribution system. The shortfall of using TOC as an indicator of a water’s ability to support biofilms is that only a small fraction of TOC can be used as a carbon source for microbial growth and energy (Geldreich 1996). This small amount of TOC can be classified as the bioavailable portion of TOC, and will change after rain events and seasonally (Trussell 1998). In view of this, it is possible to have a decrease in TOC and have an increase in bioavailable carbon in the distribution system. In an attempt to quantify the bioavailable portion of TOC, several researchers have developed methods that measure the bioavailability of carbon in water. Results from these studies can be used to assess a water’s potential to support microorganisms in the distribution system. These methods utilize bioassays and are defined as the biodegradable organic carbon (BDOC) and assimilable organic carbon (AOC). An overview of these bioassays are described by Huck and Camper (Huck 1990; Camper 1994). The BDOC is that portion of the TOC in water that can be mineralized by heterotrophic microorganisms (Camper 1994). BDOC can be determined by 19 measuring the difference in TOC between the influent and effluent of a packed bed bioreactor (Lucena, et al 1990). The change in TOC is defined as the BDOC and is typically less than 0.6 mg/L and may be less than 0.1 mg/L for a high quality water. Due to the low levels of BDOC1 the accuracy of TOC equipment must be very high and all glassware must be thoroughly cleaned to obtain statistically reliable results. The AOC has been defined as the portion of BDOC that can be converted to cell mass and is expressed as pg/L acetate equivalents (van der Kooij and Hijnen 1984). The procedure to measure biomass production uses prepared cultures of Pseudomonas fluorescens (P17) and Spirillum (NOX) (van der Kooij and Hijnen 1984). The procedure is time consuming and can take up to 20 days to obtain results. This procedure is considered by many to be excessively labor intensive and may not be entirely representative of the bioavailable carbon, because it only uses two specific microorganisms which may not have similar metabolisms as the microorganisms in the distribution system. Many researchers and water suppliers have used these to determine relationships between BDOC and AOC concentrations and the occurrence of conforms in distribution samples. These studies have found that AOC levels greater than 10-15 pg/L typically supported heterotrophic microbial growth in distribution systems (van der Kooij 1992), while AOC levels greater than 50 pg/L always supported heterotrophic growth (LeChevaIIier 1990; Laurent, et al 1997). 20 It has also been shown that some conforms are unable to grow in waters containing AOC concentrations less than 50 jug/L (LeChevaIIier 1990). In view of the above, it is clear that the amount of bioavailable carbon present in the bulk fluid may have a profound affect on a water’s biostability. However, as addressed in the Section 2.3 and in the concluding sections, the biostability of a water should be based on numerous factors that include material composition of the substratum, organics, disinfectant type and residual, and ions present in the finished water (Laurent, et al 1997; Trussell 1998). To obtain a better working knowledge of BDOC and AOC, the types of organics that are utilized by microorganisms in water distribution systems must be considered. The types of organics commonly found in finished water include humic and non-humic substances. Typically, up to 90 percent humic substances are fulvic acids, with the remaining components consisting of humic acids and humin (Benjamin, et al 1993; Beckett 1990; Chang 1992). The non-humic substances may include carbohydrates, proteins, and lipids (Olios, et al 1997). As illustrated in Figure 2.2, humic substances and humic-bound organics comprise the majority of organics found in a finished water (Kaplan, et al 1994). Humic substances are naturally occurring organic materials that result from the decomposition of vegetative material and residues (Benjamin, et al 1993; Lovley, et al 1996; Stumm and Morgan 1996; Owen, et al 1993). They are long-chain molecules having molecular weights ranging from 500 to 5,000 g/mole (Stumm and Morgan 1996). In the bulk fluid, humic substances are found in tightly coiled Other humic substances (69%) Non-humic bound amino acids (1%) Non-humic bound carbohydrates (8%) Others (13%) Humic bound amino acids (1%) Humic bound carbohydrates (8%) Kaplan 1994 Figure 2.2 Major Groups of TOC Available for Microbial Growth in Drinking Water 22 structures, which decreases their bioavailability to suspended microorganisms (Chang 1992). However, there are several physical/chemical pathways that are capable of transforming the structure of humic substances to forms that increase the bioavailability of these compounds, resulting in an increase in BDOC and AOC levels without an increase in TOC. 2.4.2 Transformation of TOC Bioavailability by Disinfectants The use of disinfectants to inactivate pathogens and control microbial water quality is widespread in most of the world. Disinfectants such as ozone, chlorine dioxide, and free chlorine are used in water treatment processes for a variety of reasons. Although these disinfectants can be highly efficient at inactivating suspended microorganisms, they also react with various organic and inorganic compounds present in the water. The reactions of these disinfectants with humics substances are of concern because; (a) they may lead to the formation of trihalomethanes, and (b) they may transform the structure of humics to smaller more bioavailable molecules (Bryant, et al 1992). As a result, the BDOC and AOC levels increase, perhaps leading to elevated microbial growth in the distribution system (Volk, et al 1997; van der Kooij, et al 1998). The transformation of TOC to a more bioavailable form by disinfectants provides one explanation of why the addition of a disinfectant sometimes increases microbial populations in downstream processes or in the distribution system. 23 2.4.3 Transformation of TOC Bioavailability by Corrosion Product Adsorption Adsorbed humic substances is perhaps the most overlooked component of corrosion products. Humic substances are some of the most powerful metal­ binding agents found in natural organic matter (NOM) (Glaus, et al 1995). The mechanisms by which humic substances adsorb to corrosion products have been proposed to involve: (a) anion exchange, (b) ligand exchange-surface complexion, (c) hydrophobic interaction, (d) hydrogen bonding, (e) cation bridging, and (f) electrostatic interactions (Gu, et al 1994; Glaus, et al 1995; Varadachari, et al 1997; Chang 1992; Stumm and Morgan 1996; Kummert and Stumm 1980; Davis 1982; Weber, et al 1983; Tipping 1981; Tipping and Cooke 1982; Parfitt, et al 1977; Tipping, et al 1981). The most common adsorption mechanism is thought to be ligand exchange between the hydroxyl (OH ) molecule of goethite (a-FeOOH) and the carboxylate groups (COO) of humic substances as illustrated in Figure 2.3 (Parfitt, et al 1977; Gu, et al 1994; Varadachari, et al 1997; Chang, et al 1997). Once adsorbed, humic substances collapse on the surface, allowing for maximum points of interaction and ligand exchange with goethite (Gu, et al 1994; Chang 1992; Stumm and Morgan 1996). As a result of this collapse, the humic molecules becomes uncoiled, increasing the bioavailability of sugars and peptides previously unavailable to microorganisms (Gu, et al 1994). The tails of the adsorbed humic substances may also bond with other humic molecules in the bulk fluid, increasing the mass of humics at the corrosion product-bulk fluid Humic Substance in Bulk Fluid e sugar • peptide E carboxylate Humic Substance Reacting with Goethite Figure 2.3 Humic Substance Interaction with Iron Oxides 25 interface (Gu, et al 1994), and once again increasing the bioavailability of organics for microorganisms. Adsorbed carbon on corrosion products is likely to produce the highest concentration of bioavailable carbon in the water distribution system. As with other systems, a large portion of the adsorbed organics are irreversibly attached while others are loosely attached (Gu, et al 1994). The detachment of these loosely bound humics can be enhanced by surfactants that are excreted by many microorganisms commonly found in distribution biofilms (Georgiou, et al 1992). The increased solubility of these organics by microbial surfactants represents another pathway a biofilm system uses to increase the bioavailability of organic carbon. As illustrated in the preceding paragraphs and summarized in Figure 2.4, the transformation of bound organic carbon to bioavailable forms has numerous pathways. In view of the pathways that occur within the biofilm/corrosion product matrix, it can be concluded that analytical techniques such as BDOC and AOC will underestimate the amount of bioavailable carbon that can be used as a carbon substrate by biofilms. Total Organic Carbon * Disinfection Byproducts Disinfectant Disinfectant Adsorption of TOC on Corrosion Products Biosurfactant Bioavailable Portion of Total Organic Carbon (Available for Microbial Growth) Figure 2.4 Biotransformation Pathways of Total Organic Carbon 27 Secondary disinfectants have been used in the United States to provide protection against cross-connections and to control the growth of microorganisms in the distribution system. Although most water systems maintain detectable disinfectant residuals throughout the distribution system, biofilms continue to grow and create water quality problems. The presence of biofilms promotes positive coliform occurrences in water samples and cause taste and odor complaints from consumers. Free chlorine and monochloramine are the most commonly used secondary disinfectants in the United States (Bryant, et al 1992). In most cases, these disinfectants are capable of minimizing the occurrence of conforms, but neither has been able to completely eliminate the presence of biofilms in the distribution system. The disinfection efficacy of these two disinfectants is different for cell inactivation in the bulk fluid versus the attached microorganisms within the biofilm/EPS/corrosion product matrix. Free chlorine is considered to be the superior disinfectant for inactivating microorganisms in the bulk fluid, but monochloramine, a weaker disinfectant, is typically the superior disinfectant for inactivating biofilms (Olson 1996; Olios, et al 1997; Griebe, et al 1994; Srinivasan, et al 1995). To explain why a weaker disinfectant is superior at inactivating biofilms, the reactivity of disinfectants in the bulk fluid and biofilms 2.5 Interactions Between Disinfectants and Distribution Biofilms must be taken into consideration. 28 2.5.1 Microbial Inactivation in the Bulk Fluid by Disinfectants The mechanisms of disinfectants on bulk fluid microorganisms have been known for many years. Microoganisms that are suspended in the bulk fluid are exposed to disinfectants from every possible angle (LeChevallier, et al 1988). As a result of this exposure, the disinfectant with the highest oxidizing strength will be quickest and most efficient disinfectant. In view of this, free chlorine has been found to be the most efficient bulk fluid secondary disinfectant, because it is a stronger oxidant than monochloramine, although the disinfection kinetics will vary depending on: (a) type of microorganism, (b) age of microorganism, and (c) the nutrient concentrations of the bulk fluid (LeChevallier, et al 1988; Carson, et al 1972). The differences in efficacy between these two disinfectants can be attributed to the reactivity of the disinfectant with the specific components of the cell wall of a microorganism. Free chorine will oxidize virtually every component of a bacterial cell, which results in a quick destruction of the cell wall. Monochloramine reacts specifically with nucleic acids, tryptophan, and sulfur- containing amino acids and is not known to react with EPS or sugars which results in a much slower death (van der Wende and Characklis 1990; LeChevallier 1997). The slow reactivity of monochloramine is the reason why it is seldom recognized as a primary disinfectant (Olson 1996), 29 The objective of an effective biofilm disinfection scheme is to provide a lethal concentration of disinfectant to inactivate attached microorganisms. This is seldom accomplished in a distribution biofilm because of the mass transfer limitations created by the EPS and corrosion products that embed the microbial cells of a biofilm (LeChevaIIier 1990; Chen and Stewart 1996; LeChevaIIier1 et al 1991). This section will present information on the causes of mass transfer limitations and discuss how biofilms resist disinfectants. Overcoming mass transfer limitations is the key to minimizing the presence and problems caused by distribution biofilms. These mass transfer limitations are the result of fast consumption of the disinfectant near the surface of the biofilm/bulk fluid interface along with diffusion restrictions caused by corrosion products and EPS (van der Wende and Characklis 1990; Koudjonou, et al 1997; Martin, et al 1982; LeChevallier, et al 1993; Srinivasan, et al 1995; DE Beer, et al 1994). The reaction limitation is caused by the consumption of chlorine with EPS, corrosion products, and the numerous organics adsorbed to the corrosion products (Gate!, et al 1998; Srinivasan, et al 1995; Koudjonou, et al 1997). The consumption of chlorine by these components and the diffusion limitation caused by the porosity of these components severely restricts the penetration of a lethal dose of chlorine to the inner core of a corrosion tubercle. The inner core region of a corrosion tubercle is thought to be void of oxygen, 2.5.2 Disinfection of Distribution Biofilms 30 creating a niche for microorganisms such as conforms and sulfate reducing bacteria (SRBs) (Lovley, et al 1996; AVWVA 1996; LeChevaIIier, et al 1993). To overcome diffusion/reaction mass transfer limitations, many utilities have attempted to increase the disinfectant residual in the bulk fluid, which enables the disinfectant to penetrate deeper into the biofilm/corrosion product matrix. This approach has worked for a number of systems, but may not be an acceptable solution if TTHM concentrations exceed allowable levels. LeChevaIIier (LeChevaIIier 1990) summarized numerous cases where utilities increased chlorine residuals and were not successful at reducing coliform levels in water samples. To complicate matters, the proposed Disinfectant/Disinfection Byproduct Rule (D/DBPR) will for the first time establish a maximum disinfectant residual in the distribution system of 4 mg/L and will lower allowable TTHM levels from 100 pg/L to 80 pg/L. Another approach to overcoming these diffusion/reaction limitations is to use a disinfectant that is not as reactive with EPS, organics, and corrosion products. Many researchers and utilities have found that monochloramine closely fits these criteria and has been successfully implemented in numerous cases (LeChevallier, et al 1996; LeChevaIIier 1997; Camper, et al 1997; Griebe, et al 1994). Monochloramine has about the same diffusivity as free chlorine and is not as reactive with organics, EPS, or corrosion products (LeChevallier 1990; van der Wende and Characklis 1990). The low reactivity of monochloramine enables it to penetrate deeper in the biofilm/corrosion product matrix and provide 31 a higher degree of biofilm inactivation (LeChevaIIier 1990). The lower reactivity of monochloramine also allows utilities to more easily maintain higher disinfectant residuals in the outer extremes of the distribution system without producing significant levels of TTHMs (LeChevaIIier 1997). As previously discussed, the use of disinfectants can actually increase the bioavailability of organics in the bulk fluid and in the biofilm/corrosion product matrix. At low disinfectant residuals the diffusion/reaction limitations restrict the penetration of the disinfectant to the extreme depths of the biofilm/corrosion product matrix. The reactivity of the disinfectants with corrosion products and other matter creates increased chloride levels, which have been shown to accelerate corrosion (Videla 1996; Singley and Ahmadi 1985). As a result, the combination of these mechanisms creates a system that becomes self-sustaining for microorganisms within the biofilm/corrosion product matrix. Although the use of disinfectants can cause an increase in corrosion product formation, there will be a disinfectant residual level in which the diffusion/reaction limitations are exceeded, enabling the disinfectant to penetrate deeper in the biofilm/corrosion product matrix. The deeper penetration increases the inactivation of microorganisms resulting in a decrease in biofilm density. This disinfectant level can be defined as the threshold residual and will change dependent on the pipe material and water quality. 32 Corrosion control has been used in industry for a number of years to minimize the impact of corrosion related water quality problems, extend the useful life of pipe materials, and to reduce energy requirements in pump/pipeline systems. In the drinking water industry, corrosion control is used to minimize the leaching potential of lead and copper into the finished water as required by the Lead and Copper Rule (AWWA 1992). In most cases, water utilities use either corrosion inhibitors or pH adjustment as a method of corrosion control. This research will focus on the use of two corrosion inhibitors (zinc orthophosphate and polyphosphate) and pH adjustment and evaluate how each may affect the formation of distribution biofilms. 2.6.1 Adverse Water Quality Concerns Caused by Corrosion Control Methods There are several water quality changes associated with the implementation of a corrosion control program. These water quality changes may lead to conditions that may result in increased levels of microorganisms in the distribution system. These problems are associated with the reduced disinfectant efficacy of free chlorine at high pH values, and the possible increase microbial growth by the addition of phosphorus based corrosion inhibitors. Utilities that use pH adjustment as a corrosion control method face two potential water quality problems. These problems include: (a) increased TTHM levels and, (b) reduction in disinfection efficacy of free chlorine. Increases in 2.6 Interactions Between Corrosion Inhibitors and Distribution Biofilms 33 TTHM levels may occur after pH adjustment because the formation rate of TTHM typically increases with increasing pH (Symons, et al 1982). Although TTHM increases may not be substantial, even a small increase may cause some utilities to be in violation of the current or future TTHM limits. Another problem with pH adjustment may be the reduced disinfection efficacy of free chlorine at higher pH values (Snoeyink and Jenkins 1980; AVWVA 1998; Pontius 1990; Montgomery 1985). This is the result of a shift in equilibrium between hypochlorous acid (HOCI) and hypochlorite ion (OCI ) (Stumm and Morgan 1996; Pontius 1990; Snoeyink and Jenkins 1980; AWWA 1998). At pH values below 7.50, free chlorine is a stronger disinfectant because most of the free chlorine is in the form of HOCI. At pH values above 7.50, OCI" a much weaker disinfectant is the dominant species, resulting in lower disinfectant demands but a less efficient disinfectant. The use of corrosion inhibitors is also of concern to water suppliers because most are phosphorus based. Phosphorus is an essential nutrient for microbial growth. The fear of phosphorus originates from the wastewater industry where phosphorus is the limiting nutrient for phototrophic (use light as an energy source) microorganisms such as algae. Since distribution biofilms are carbon limited (LeChevaIIier 1990; Camper, et al 1996), the addition of phosphorus will seldom result in increased microbial levels, unless the finished water is phosphorus limited (Haas, et al 1988) or exposed to sunlight in uncovered finished water reservoirs (LeChevallier, et al 1996). It has also been found that 34 the total organic carbon increases downstream of uncovered finished water storage reservoirs (LeChevallier, et al 1996). The increase in TOC is likely the result of endogenous respiration and decay of phototrophic microorganisms once they enter the light-free distribution system. Another possible concern about the use of zinc orthophosphate is the presence of zinc in wastewater effluents (Geldreich 1996). Zinc' levels in wastewater effluents, in many cases, limits the dose of zinc orthophosphate in the drinking water. If the zinc orthophosphate dose is not adequate, the user is required to change to a different corrosion control program to maintain compliance with the Lead and Copper Rule. As a result, of these problems, several corrosion inhibitor manufacturers have changed the formulation of zinc orthophosphate, so that it now contains less zinc. 2.6.2 Mechanisms of Corrosion Control Methods The process of corrosion is a natural chemical reaction where elements in the pipe material are transformed to a more thermodynamically stable state. Corrosion can be minimized by reducing the rate at which ions such as Fe+2, lead, and copper are released from the pipe surface. Corrosion control can be accomplished by making it more difficult for these molecules to either approach or vacate the vicinity of a cathode and anode, or by altering the electrical potential of the surface (Benjamin, et al 1990). 35 The two most common mechanisms that can be implemented to interfere with the corrosion process are (a) coating the surface with a compound that limits the transport of molecules from the pipe surface, or (b) altering the surface chemistry of the pipe. The first mechanism is defined as precipitation, while the latter is defined as passivation. Both mechanisms are discussed in the following paragraphs. The most commonly practiced precipitation method is accomplished by controlling the pH of the finished water. In the pH adjustment method, the pH of the bulk fluid is raised to a level that saturates the water with calcium carbonate (CaCO3) provided that adequate alkalinity and calcium is present. Once saturation is reached, the pipe surface is coated with a layer of CaCO3, which forms a diffusion-limited barrier between the pipe surface and the bulk fluid. The diffusion-limited barrier minimizes the release rate of Fe+2 molecules from the pipe surface, hence, reducing the corrosion rate. This process is controlled by maintaining a near zero Langelier Saturation Index (LSI), which is a measure of the number of pH units away from the saturation pH of CaCO3 (Pontius 1990; Singley 1981; Singley and Ahmadi 1985). The LSI can be determined by a variety of spreadsheet programs. (Trussell 1998; Holm and Schock 1998) pH adjustment, with sufficient alkalinity, can also stabilize monochloramine residuals in the distribution system. Another possible benefit of pH adjustment is that some microorganisms are inhibited by pH. Provided that the pH required to achieve CaCO3 36 precipitation is high enough to inhibit microorganisms, a reduction in microbial populations may result. This may be particularly attractive if commonly found conforms, such as Klebsiella pneumonia, are inhibited by high pH levels. Klebsiella pneumonia is commonly found in soils and is known to remove iron from solution by attacking the organic portion of iron-organic molecules (Alexander 1977). It has also been found that Klebsiella pneumonia, the most common coliform found in water distribution systems (Geldreich 1996), is inhibited by pH values above 9.0 (Martin, et al 1982). In view of this inhibition, many utilities that have old pipes with poor joints, high water tables, and water hammer problems (LeChevaIIier 1998) may find pH adjustment as an attractive management approach to minimize the growth and survival of Klebsiella pneumonia in the distribution system. It is therefore possible that pH adjustment will not only minimize leaching of iron, lead and copper, but also selectively inhibit the growth of commonly found conforms in distribution systems. Most of the corrosion inhibitors used today utilize passivation to minimize the corrosion rate. This process forms a chemical scale on the pipe surface mainly from the reaction of phosphate (PO4) with Fe+2 and Fe+3 molecules to form a Fe-PO4. The FePO4 film is a more thermodynamically stable form of corrosion product, and is not easily converted to other forms of corrosion products that have carbon adsorption capabilities. The presence of this film on the pipe surface develops a protective stable layer that minimizes the diffusion rate of Fe+2 from the pipe surface, hence, reducing the corrosion rate. 37 The use of corrosion inhibitors may also cause short-term coliform problems in the distribution system. These problems are the result of the action of phosphates on existing corrosion tubercles and the formation of FePO4 at the pipe surface. As a result of the PO4 interactions, the cohesive strength of corrosion tubercles are much lower, resulting in rapid sloughing (Geldreich 1996). Since conforms are frequently found in these corrosion tubercles, coliform excursions may be experienced. Frequent flushing and controlled increases of disinfectant residuals may minimize the impact of these sloughing events soon after the addition of the corrosion inhibitor (Geldreich 1996). As discussed above, the use of corrosion control practices such as pH adjustment and phosphorus based corrosion inhibitors change the type of corrosion products normally found on pipe materials. The use of these corrosion I control methods will reduce the amount of goethite (a-FeOOH) formed along with changing the electrostatic surface characteristics of these materials. The electrostatic surface charge is of significance because the charge regulates the rate of adsorption of humic substances to the pipe surface (Stumm and Morgan 1996; Chang 1992; Beckett 1990) along with the electrostatic adhesion of microorganisms to the surface (Stumm and Morgan 1996). 2.6.3 Use of Corrosion Indices For many years, researchers have attempted to develop an index that will quantify the interactions between a pipe material and finished water that result in corrosion. Since it has been found that systems with high corrosion rates and 38 unlined pipe typically have biofilm related water quality problems, the use of a corrosion index could be a valuable tool to evaluate the efficacy of a corrosion control program. Indexes commonly used include the Langelier Saturation Index (LSI)(Singley 1981; Pontius 1990; Singley and Ahmadi 1985; LeChevaIIier1 et al 1993), Riddick Index (RI)(Singley 1981; Pontius 1990), and the Larson Index (LI) (Singley 1981; Pontius 1990; Singley and Ahmadi 1985). These indexes may be used to estimate a water’s corrosion potential prior to the implementation of a corrosion control program. However, they should not be used to evaluate the effectiveness of a corrosion inhibitor. An evaluation of each index will be discussed in the following paragraphs. The Langelier Saturation Index (LSI) is probably the most used and improperly used index in the water industry. The LSI is an indicator for the saturation of CaCO3 in a water. The LSI should be used to determine if CaCO3 is precipitating on the pipe surface and not be used if a corrosion inhibitor is present. LSI should also be used with caution if high levels of sulfate (SOD are present, because the formation of CaSC^ will lead to inaccurate results (Singley 1981) and an ineffective corrosion control program. Use of LSI should also be avoided in very soft-low alkalinity water, because it is well known that these waters cannot be supersaturated with CaCO3, regardless of how high the pH is raised (Singley 1981). The Riddick Index (RI) is an empirical equation that considers many factors that contribute to corrosion. This index provides values that can be used 39 to quantify the corrosion potential of a water. The empirical equation is as follows (Pontius 1990): m = ^ - IC O 2 + {(h a rd n e s s -A ik )+ C t + 2 M ? ( ^ ) ( ^ ) ] (1) where hardness and alkalinity (ALK) are in mg/L as CaCO3, NO3 is in mg/L as nitrogen, and the remaining parameters in mg/L (Singley 1981; Pontius 1990). Values of Rl less than 25 indicate noncorrosive water, 26 to 50 indicate a moderately corrosive water, 51 to 75 a corrosive water, and values greater than 75 very corrosive water (Singley 1981; Pontius 1990). The use of the Rl should be limited to soft water (Singley 1981), and should not be used to evaluate the efficacy of a corrosion inhibitor, because it does not take into consideration the effects of the passivating properties of phosphate. The Larson Index (LI) has been used by a number of utilities as a measure of corrosion potential (LeChevallier, et al 1991; LeChevaIIier, et al 1993). This index evaluates the effects of chlorides (Cl"), sulfates (SCV) and bicarbonate (HCO3 ) on corrosion (Pontius 1990; Singley 1981). Chlorides have been found to break down passive films (Videla 1996), while sulfate levels create a high potential for microbially influenced corrosion by sulfate reducing bacteria. The Larson Index (LI) can be determined by evaluating the following equation where all concentrations are expressed in moles/L. [cn+2[so;] (2 ) [HC(J3) 40 German regulations (DIN 50930, 1980) recommend that the LI be less than 1.0 (Singley and Ahmadi 1985). Researchers from the American Water Works Service Company, Inc. have also found that monochloramine was more effective with a LI less than 1.0, while free chlorine was much less efficient at inactivating biofilms at LI greater than 0.5 (LeChevallier, et al 1991; LeChevaIIier1 et al 1993). The use of the Larson Index can provide a indication of long-term effects of corrosion and as an indicator for determining disinfection efficacy. However, as with the previous indexes, it does not provide any information about the formation of passivating films of corrosion inhibitors, and it also does not address the precipitation formation of CaCOs. 2.7 Biofilm Modeling Systems The use of computer models to mathematically simulate dynamic systems has been widespread for the past 15 years. The use of these models has provided substantial advances in decision-making, design, and rapid evaluation of numerous chemical and/or biological systems. These models are capable of simulating many complex conditions using a variety of analytical and numerical techniques. The accuracy of these models is limited to systems with respect to the microbial community, reactor configuration, kinetic expressions, and substrate concentration ranges for which they were designed.(Wanner and Gujer 1986) 41 2.7.1 Requirements of a Biofilm Model As stated above, it is essential that a model be capable of accurately portraying the numerous physical, chemical, and biological reactions that occur within the system to be modeled. This is particularly true for the complex interactions that occur within bulk fluid and biofilm systems. Systems that need to be accounted for in an oligotrophic biofilm model include: (a) the physical, chemical and biological activity in the bulk fluid, (b) external mass transfer resistance between the bulk fluid and the biofilm, (c) detachment of microorganisms from the biofilm to the bulk fluid, (d) diffusion of disinfectants, and nutrients into the biofilm, (e) chemical and biological reactions within the biofilm, (f) physical and chemical reactions with inorganic compounds within the biofilm/corrosion product matrix, and (g) the physical and chemical reactions that occur between the biofilm and the substratum (Characklis and Marshall 1990; Weberand DiGiano 1996). Incorporation of these criteria will likely result in a model for many systems, provided that: (a) the biological and chemical kinetics, (b) absence of unaccounted for inorganic materials such as corrosion products, and (c) water quality conditions (both biological and chemical), are nearly constant. The accuracy of a model is determined by comparing the results of the model with data from a prototype or full-scale system. 42 2.7.2 Deficiencies of Existing Biofilm Models As previously discussed, many biofilm models can be used to accurately simulate the net affect of numerous environmental factors. These models are effective because they are used within the substrate ranges and kinetic ranges of the system. Models for high substrate systems can be simplified by neglecting many of the insignificant kinetic and surface interactions that have very little effect on the overall accuracy of the model. However, when low substrate systems such as those found in drinking water systems are modeled, these small kinetic and surface interactions can not be eliminated because they will likely significant and would have notable effects on the accuracy of the model. Items that are typically negIectied in high substrate systems but can not be omitted in a low substrate system include: o the effect of substratum on the formation of distribution biofilms as discussed in Section 2.3 of this chapter. o the consumption of organic substrates by corrosion products as discussed in Section 2.3 of this chapter. o the changes in the bioavailability of humic substances adsorbed to corrosion products as discussed in Section 2.4 of this chapter. o the effects of biosurfactants on the availability of humic substances adsorbed to corrosion products as discussed in Section 2.4 of this chapter. o the changes in the microbial niches caused by corrosion tubercles as discussed in Section 2.2 of this chapter. 43 o the changes in microbial growth rates (kinetics) associated with the numerous types of microorganisms present in the numerous niches found in a corrosion product/biofilm. ° the effect of EPS (chlorine demand, diffusion resistance, and nutrient accumulation) on biofilm disinfection (LeChevaIIier 1990; van der Wende and Characklis 1990; Koudjonou, et al 1997). o the biotransformation of carbon by disinfectants as discussed in Section 2.4 of this chapter. As discussed above, the simplification of a biofilm model - by eliminating many reactions that are insignificant in high substrate models - will not be able to accurately predict/simulate the microbial and nutrient conditions present in a distribution biofilm. In view of this, it is clear that an accurate model for distribution biofilms will need to include these components. This will result in a highly complex model that will require substantial field measurements and verification. 2.8 Key Findings As presented above, a distribution biofilm is a unique system that has many physical, chemical, and biological properties that aid in the protection and survival of microorganisms. In retrospect, these properties enable the distribution system to serve as a plug flow reactor that is highly reactive with chlorine, adsorbs and accumulates carbon and other nutrients, and supports copious quantities of attached microorganisms. Several of the key features presented in this section are listed below: 44 o Low disinfectant inactivation efficacy of distribution biofilms can be attributed to the physical and chemical properties of a biofilm which enable the microorganisms to accumulate nutrients and to develop diffusion/reaction resistance mechanisms to disinfectants. o Biofilms consist of a consortium of microorganisms, EPS produced by microorganisms, water, and organic and inorganic substances adsorbed from the bulk fluid or substratum. The biofilm/corrosion product matrix develops a variety of niches that favor the metabolisms of aerobic, facultative, and anoxic microorganisms. o Biofilms can be home to various microorganisms that include heterotrophic bacteria, conforms, actinomyces, molds, fungi, nitrifying bacteria, iron oxidizing bacteria, sulfate reducing bacteria, and possibly act as a reservoir for Giardia cysts and/or Cryptosporidium occysts. o The presence of microorganisms may lead to taste and odor complaints by consumers and violations of the Safe Drinking Water Act (conforms). o Distribution systems that contain large quantities of unlined cast-iron pipe typically have the most problems with biofilm related water quality problems. o Non-ferrous pipe materials typically support fewer microorganisms than ferrous materials. 45 © The presence of corrosion products on a pipe surface creates mass transfer limitations of oxygen and disinfectants. These conditions may lead to the formation of facultative and/or anoxic zones, which are known to support conforms and sulfate reducing bacteria. o Total organic carbon measurements do not accurately predict a water’s potential to support microbial growth in the distribution system. o The amount of bioavailable total organic carbon is only a small fraction of the total organic carbon. o The majority of bioavailable carbon is comprised of humic substances, carbohydrates, and amino acids. o Goethite (a-FeOOH), the most common corrosion product found on distribution pipes, is known to adsorb humic substances from the bulk fluid. o Organic carbon can be in either the solute or adsorbed forms. © Non-bioavailable total organic carbon can be transformed into bioavailable carbon by reacting with disinfectants, adsorbing to corrosion products, and from biosurfactant reactions. o BDOC and AOC measurements will not totally quantify the amount of carbon that is bioavailable within a biofilm. 46 o Free chlorine is the superior disinfectant for inactivating microorganisms in the bulk fluid. © Monochloramine is a superior disinfectant for inactivating distribution biofilms on ferrous materials, because it is less reactive with corrosion products, organics, and EPS. o Using pH control as a corrosion control method may cause elevated levels of TTHMs in the distribution system. The efficacy of free chlorine is reduced at higher pHs due to a shift in equilibrium between HOCI and ocr. o pH levels above 9.0 may inhibit the growth of Klebsiella pneumoniae, the most common coliform found in water distribution systems. o The use of the Langelier Saturation Index (LSI) is most appropriate when pH adjustment is used as a corrosion control method, and should not be used in conjunction with corrosion inhibitors. o With few exceptions, heterotrophic microorganisms found in the water distribution system are carbon limited, not phosphorus limited.The addition of phosphorus based corrosion inhibitors will not typically result in increased microbial levels unless the finished water is exposed to sunlight in an uncovered reservoir. 47 o Widespread sloughing events may occur in the distribution system shortly after the addition of corrosion inhibitors due to the "softening" of existing corrosion tubercles. o The use of corrosion indices such as the Riddick Index (RI) and the Larson Index (LI) are not appropriate in assessing the efficacy of a corrosion control program, because they do not account for the passivating properties of phosphate. 48 Chapter 3 !Materials and Methods The purpose of this chapter is to: (a) describe the materials and methods used to collect, quantify, and statistically evaluate data, and (b) describe the facilities used to conduct bench-scale, pilot-scale, and laboratory studies. Details on the specific experimental designs are described in the relative chapters associated with bench-scale (Chapter 4), pilot-scale (Chapter 5), and laboratory studies (Chapter 6). 3.1 Microbial and Chemical Measurement Techniques Microbial bulk fluid and biofilm measurements were taken using sterile techniques and analyzed using the spread plate method as described in Section 9215 of Standard Methods. 3.1.1 Bulk Fluid Measurements Bulk fluid samples were collected in empty sterile test tubes. For bulk fluid samples collected from disinfected systems, a small amount of sodium thiosulfate was added to the test tube prior to collection to neutralize any disinfectant present. Dilutions (10'1, 10"2) were then prepared, using a 1-ml pipetteman (P-1000 as distributed by Fisher Scientific) into sterile test tubes, each containing 9-ml of sterile water. Quantification of samples was then determined by plating 100pl or 1 ml of the appropriate dilution on triplicate plates 49 of R2A or mT7. Heterotrophic plate counts (HPCs) were quantified using R2A media and incubated for 7 days at room temperature, while conforms were quantified on mT7 media and incubated at 35°C in a Lab-line Imperial III incubator for 24 hours. After incubation, colonies were counted and the number of colony forming units (CPU) per ml determined by the following equation: ColonyCount CFU dilution (3) Ctrr1 m s^ 3.1.2 Biofilm Density Measurements Biofilm samples were collected from annular reactor coupons or slides and from coupons from the pilot-scale facilities by scraping all corrosion products and biofilm into a1 OO-ml beaker containing 10-ml of sterile water. For biofilm samples collected from disinfected systems, a small amount of sodium thiosulfate was added to the 10-ml of sterile water prior to collection to neutralize any disinfectant present. The biofilm/corrosion product slurry was then poured into a sterile test tube and transported to the laboratory for processing. Biofilm samples were homogenized, using a Janke & Kunkel Model T25S1 Homogenizer, for 2 minutes at 20,000 rpm to disperse bacteria and corrosion products prior to preparing dilutions (10‘1, 10'2, 10"3, 10"4, and 10'5). Heterotrophic and coliform populations for the biofilm samples were quantified by the spread plate method as described in Section 9215 of Standard Methods, using triplicate plates of R2A and mT7 agar plates respectively. R2A plates were incubated for 7 days at room temperature and mT7 plates incubated at 35°C in a Lab-line Imperial III 50 incubator. After incubation, the plates were counted and the biofilm density determined by evaluating the following equation: ColonyCount CFU __ dilution (4 ) cm mis (cm ) 3.1.3 Chemical Measurements In addition to microbial samples, numerous other physical and chemical parameters were routinely analyzed using the methods described in Table 3.1. Flow rates and temperature were routinely monitored and adjusted while numerous chemical parameters were also analyzed. 3.1.4 Corrosion Product Mass Measurements Corrosion product masses were determined by filtering the corrosion product/biofilm slurry, as described in Section 3.1.2, on Watman 3 filter paper and measured on a Mettler AE 200 balance. Filters were initially rinsed and oven dried at 105°C for at least 2 hours and the initial weight recorded. After filtering the corrosion product/biofilm slurry, the filters were again dried 105°C for at least 2 hours and the final weight measured. The difference between the two weight measurements represents 90 percent of the total corrosion product mass on the slide. The remaining 10 percent of corrosion product mass were used in preparing dilutions as described in Section 3.1.2. 51 Table 3.1 Chemical Analysis Methods and Equipment Chemical : Method Equipment Parameter, units pH Section 4500 of Standard Accumet Model 50, Fisher Methods Scientific Total Chlorine, mg/L DPD Method. Hach Method Hach DR 2000 8021 Spectrophotometer Free Chlorine, mg/L DPD Method. Hach Method Hach DR 2000 8167 Spectrophotometer Magnesium and Calmagite Colorimetric Hach DR 2000 Calcium Hardness, mg/L as CaCO3 Method. Hach Method 8030 Spectrophotometer Alkalinity, mg/L as Section 2320 of Standard 50-ml burette and 0.036 N CaCO3 Methods Sulfuric Acid Nitrate Nitrogen, Cadmium Reduction Hach DR 2000 mg/L Method. Hach Method 8039 Spectrophotometer Phosphorus, mg/L Orthophosphate Method. Hach DR 2000 Hach Method 8048 Spectrophotometer Sulfate, mg/L Sulfa Ver 4 Method. Hach Hach DR 2000 Method 8051 Spectrophotometer Dissolved and Total Ferrozine Method. Hach Hach DR 2000 Iron, mg/L Method 8147 Spectrophotometer (Dissolved iron filtered with a 0.2 pm filter) Chloride, mg/L Mercuric Thiocyanate Hach DR 2000 Method. Hach Method 8113 Spectrophotometer Silica, mg/L Silicomolybdate Method. Hach DR 2000 Hach Method 8185 Spectrophotometer Total Organic Infrared Combustion Shimadzu Model TOC-5000A Carbon, mg/L Method. Standard Method 5310 Total Organic Carbon Analyzer 52 3.1.5 Statistical Methods The objective of the statistical analysis was to determine if a specific treatment significantly increases or decreases biofilm densities when compared against another treatment system. For these experiments, two controls were used. The first control was a non-chlorinated (or non-chloraminated) system while the second control was a chlorinated (or chloraminated) system, without the use of any type of corrosion control method. In all experiments, organic and nutrient feeds were constant for each system. Some experiments incorporated variable disinfectant dosages, while other experiments utilized constant disinfectant feeds but with variable dosages of corrosion inhibitors. Since one of the goals of this project was to determine the numerous interactions between pipe materials, disinfectants, and corrosion inhibitors on distribution biofilms, a multiple comparison analysis of variance was utilized. Prior to conducting the statistical analysis, all microbial data were logic transformed to stabilize the variance. Data were then analyzed using Fisher’s Multiple Comparison Tests with MINITAB™ software using all microbial data collected during the last 4 weeks of the experiment. The last 4 weeks represents steady state conditions. Experiments that utilized the laboratory style annular reactors (see Section 3.3), multiple samples were collected at the end of the experiment and statistically analyzed as described above. 53 3.2 Chemostat Operation Prepared cultures of conforms were introduced to all field-type annular reactors (Chapter 4), two of the pilot-scale experiments (Chapter 5), and the pH inhibition experiment (Chapter 6). Conforms used were originally isolated from conform outbreaks in distribution systems. Conforms included Klebsiella pneumoniae, Klebsiella oxytoca, Enterobacter aerogenes, E. cloacae, and Escherichia coli. These cultures were maintained as frozen stocks at -YO0C. 3.2.1 Coliform Culture Preparation and Conditioning Isolated colonies were obtained for each conform by streaking frozen stocks of each conform on R2A agar. Batch cultures of each conform were prepared by placing an isolated colony of the respective conforms in a sterile 250-ml flask containing 100-ml of sterile water containing 2500 pg C/L substrate (equal carbon amounts from sodium acetate, sodium benzoate, propionaldehyde, parahydroxybenzoic acid, and ethanol), 500 jug/L of nitrate (potassium nitrate), and 500 pg/L of phosphate (equal molar amounts of potassium phosphate and potassium phosphate monobasic). Each batch culture was then placed in a shaking incubator for 20 hours at room temperature. Five mis from each batch culture were then placed in another sterile 250-ml flask, using the same nutrients described above, and incubated again for 20 hours. Klebsiella pneumonia cultures used in pH inhibition studies (Chapter 6) were prepared by placing an isolated colony in a sterile test tube containing 10 54 ml of sterile 1% R2A broth for 20 hours. One ml of the culture was then transferred to a sterile 250-ml flask containing 100 ml of sterile 0.05% R2A broth and incubated in a shaking incubator for 20 hours at room temperature. 3.2.2 Chemostat Startup Prior to chemostat startup, each chemostat, associated solution bottles and tubing were autoclaved. Chemostats were inoculated with 5 ml of each culture when a 4.4 L chemostat was used (bench-scale and pilot-scale studies) and 1ml of Klebsiella pneumoniae for the 500 ml chemostats used in the pH inhibition studies. Nutrient feeds (carbon substrate, nitrogen, phosphorus, or R2A broth) were introduced at 10% of the concentration used in the last batch culture, at a flow rate that would produce a growth rate of 0.05 hr"1 (20-hour residence time). A 20 hour residence time was used because it has been found that conforms grown at low growth rates and low nutrient conditions are more resistant to the environmental conditions found in a water distribution system (Camper, et al 1991; LeChavallier, et al 1988; Camper 1995). Each chemostat was mixed and aerated using filtered (0.2 pm filter) air supplied by an aquarium pump. Chemostats were operated for approximately 4 days to establish steady- state conditions prior to inoculation of the bench-scale and pilot-scale facilities or for pH adjustment for the pH inhibition studies. A typical chemostat setup is presented in Figure 3.1. Pump Air Pump 0.2(1 AirVent Chemostat Discharge Nutrient Solution Figure 3.1 Typical Chemostat Schematic 56 3.3 Bench-scale Studies Bench-scale experiments reported here employed two different types of annular reactors. These reactors were either the field type (Model 920WD) or the laboratory type (Model 920LJ) as manufactured by BioSurface Technologies Corporation (Bozeman, Montana) and illustrated in Figure 3.2. Each reactor consists of a stationary outer cylinder and a rotating inner cylinder. The outer cylinder for the field type units is constructed of unlined ductile-iron pipe with 30 removable coupons each with a surface area of 1.27 cm2. The outer cylinders of the laboratory units were made of glass with 20 removable mild-steel slides, each with a surface area of 19.05 cm2, were located on the rotating inner cylinder of the laboratory units. In both reactor types, the inner cylinder rotates to simulate shear stresses that would be present on a pipe surface at low velocities. Low shear stresses were used to simulate low flow conditions in distribution systems that have been found to be problematic locations for the formation of biofilms. The total liquid volume of each reactor is 1,100 ml (0.29 gal.) which provides a high surface area to volume ratio. Due to the high surface area to volume ratio, the annular reactors are highly biased towards surface interactions associated with microbial attachment/detachment (biofilms) and chemical reactions. As a result, annular reactors will have higher disinfectant demands and higher microbial counts in the bulk fluid than an actual distribution system. Bench-scale studies using the field type reactors used a hydraulic residence time of 120 minutes,while the lab style reactors utilized a hydraulic residence time of a) Field Type Annular Reactor ©Rob Wilkeb) Lab Type Annular Reactor Figure 3.2 Field Type and Lab Type Annular Reactors 58 90 minutes to maintain control of disinfectant residuals. As result of the short residence time, any increase in microbial populations in the effluent will be from the detachment of microorganisms from the biofilm (van der Wende and Characklis 1990; Camper 1995). The annular reactors were operated in parallel, to allow multiple conditions to be evaluated simultaneously. Experiments used three, four or six annular reactors to evaluate the interactions between disinfectants and corrosion inhibitors within distribution biofilms. In each experiment, pretreated dilution water and associated chemical feeds were introduced to each reactor to produce a hydraulic residence time of 2 hours for the field type annular reactors and 90 minutes for the laboratory style annular reactors. Dilution water was pretreated by a granular activated carbon (GAC) filter to remove disinfectant residual followed by a biological active carbon (BAC) filter to remove available biodegradable organic matter (BOM) from Bozeman tap water. The dilution water also provided a continuous inoculum of adapted microorganisms to each reactor. Various chemical feeds were then introduced to the reactors by a series of chemical solutions of disinfectant, nutrients, and corrosion inhibitor. A typical schematic of the annular reactor system is presented in Figure 3.3. There are several features of the annular reactors that are problematic because the small liquid volume and corresponding high surface area leads to noisy bulk fluid measurements (both chemical and microbial) due to sloughing 4 AOC free water Carbon Substrate N itrogen and Phosphorus Annular ^ reactor □ Disin fectant Corrosion Control Figure 3.3 Typical Annular Reactor System 60 events. Another problem is the reactors (both the field and laboratory models) are constructed of polycarbonate and metal surfaces. Past research has shown that these materials will support different microbial populations under the same nutrient conditions (Chen, et al 1993; Camper, et al 1996; Olios, et al 1997; Delanoue1 et al 1997; Camper 1995), which means that they will have different effects on the performance of the annular reactor. 3.3.1 Bench-scale Startup Procedures Prior to the startup of the bench-scale reactors, each reactor was cleaned to remove any buildup of corrosion products from previous experiments. Field type reactors were bead-blasted and washed with a mild detergent, while the lab style reactors were hand cleaned and refurbished with new mild steel slides. Once clean, the reactors (excluding the experiments conducted using the laboratory style reactors) were inoculated with 1,100 ml of effluent from a 4.4 L chemostat containing five coliform cultures {Klebsiella pneumoniae, K. oxytoca, Enterobacter aerogenes, E. cloacae, and Escherichia coli) and allowed to operate in a chemostat mode for 24 hours. Conforms were added so that they could colonize the reactor surfaces prior to introduction of other non-coliform heterotrophic microorganisms present in the dilution water. After inoculation, dilution water and nutrients (carbon, nitrogen and phosphorus) were added to the reactors for one week to establish a healthy population of biofilm in each reactor. On the seventh day biofilm samples (collected from the removable coupons) and bulk fluid samples were collected 61 and assayed for coliforms and heterotrophic microorganisms. After the first sampling, the addition of disinfectant (free chlorine or monochloramine) and corrosion inhibitor were initiated. Bulk fluid and biofilm samples were collected the following day and periodically over the duration of the experiment to determine the effect of the disinfectants and corrosion control on bulk fluid and biofilms within the reactors. 3.4 Pilot-scale Studies Pilot-scale studies were conducted using facility at the City of Bozeman Water Treatment Plant (WTP). This system was designed to simulate physical conditions of drinking water distribution systems and has the capability of controlling flow rate, water temperature, and hydraulic residence time. The system also has the capability of simulating various chemical properties of a finished water system by the addition of organic substrates, nutrients (nitrogen and phosphorus), disinfectants (free chlorine or monochloramine), corrosion inhibitors (zinc orthophosphate or polyphosphate), and pH control. 3.4.1 Pretreatment Facilities The pretreatment facilities are designed to dechIorinate the finished water from the Bozeman WTP and remove trace organics and biodegradable organic matter (BOM). Pretreatment facilities consist of a granular activated carbon (GAC) filter for the removal of chlorine and trace organics, and two biological active carbon (BAC) filters to remove any available BOM from the finished water. I62 Once pretreated, the water is transported to an insulated 0.208 m3 (55-gallon) reservoir for temperature control prior to distribution to the five pipe loops. A schematic of the pretreatment system is presented in Figure 3.4. 3.4.2 Pipe Loop System The pipe loop system consists of five 12.18 m (40 ft) sections of 10.16 cm (4-inch) mild steel pipe and 18.41m (60.4 ft) of 3.81cm (1-1/2 inch) diameter mild steel pipe. Each pipe loop contains 80 flush-mounted coupons (with an 8.84 cm2 surface area) for biofilm sampling, an insulated and temperature controlled recycle tank, a 0.1514 m3/min (40 gpm) recycle pump and associated valves, flow monitor, and sample spouts. Each pipe loop also has chemical feed facilities for the addition of organic substrates, nutrients, disinfectant, and corrosion inhibitor. A pH controller (Chemcadet pH meter/controller, Cole- Parmer) and caustic feed pump were used on one pipe loop when pH control was used as a corrosion control method. A schematic of these facilities is presented in Figure 3.5. The pilot loop system operated in a recycle mode with a hydraulic residence time of 120 minutes at a temperature of 20°C. As with the annular reactor studies presented in Section 3.3, the short residence time minimizes microbial growth in the bulk fluid. As a result of this short residence time, any increase in microbial cell counts (either viable or inactivated) can be attributed to detachment of biofilm organisms from the pipe walls. Bozeman Tap Water AOC Free Water Head Tank GAC Filter Flash Heater Pipe Loop System / (Typ. of 5) Figure 3.4 Pretreatment Facilities for the Pilot-plant System Figure 3.5 Schematic of Pilot-scale Distribution System 65 In normal operation, water was delivered from the main reservoir to each recycle tank by a dedicated vane pump at a flow rate of 0.0026 m3/min (0.68 gpm). Carbon substrate, disinfectant, and corrosion inhibitor are then continuously added to the recycle tank by peristaltic pumps. A recycle pump circulates the water into the pipe loop at a flow rate of 0.1514 m3/m.in (40 gpm) to produce a flow rate of 0.305 m/s (1 ft/sec). An overflow outlet in the recycle tank regulates the hydraulic residence time of the system. 3.4.3 Pilot-plant Startup Procedures Prior to the startup of the pilot plant facilities, all coupons are removed and bead blasted to remove any corrosion products accumulated on the surface, chemical feed pumps are recalibrated, tubing is replaced, and the recycle pumps/valves are adjusted to provide the proper flow rates. Once calibrated, each pipe loop was operated with dilution water (at 20°C) and organic substrate and nutrient feed for 3 or 4 weeks to create a natural biofilm on all surfaces. Following this period, effluent from a 4.4 liter chemostat containing five coliform cultures {Klebsiella pneumoniae, K. oxytoca,. Enterobacter aerogenes, E. cloacae, and Escherichia coli) operated at a residence time of 20 hours was introduced to the main reservoir for 5 days. At the conclusion of the inoculation period, bulk fluid and biofilm samples were collected from each loop to quantify initial bulk fluid and biofilm density. After the first sampling, the disinfectant (free chlorine or monochloramine), corrosion inhibitor (zinc orthophosphate or polyphosphate), or pH adjustment (for 66 pH corrosion control method) were initiated. Bulk fluid and biofilm samples were collected the following day and periodically over the next 12 weeks to determine the effect of the disinfectants and corrosion control treatments on distribution biofilms. With a few exceptions, the mechanics of pilot-scale facilities are similar to the annular reactor system previously described. The pilot-scale system also provides a high surface area to volume ratio and a single material (mild steel) with the exception of cast-iron pumps and valves. The pilot-scale facilities are operated in a recycle mode with a mean hydraulic residence time of 120 minutes. The higher volumes and flow rates provide a significantly higher degree of control for chemical feeds such as carbon substrates, nutrients, corrosion inhibitors, and disinfectants. The short hydraulic residence time minimizes microbial growth in the bulk fluid. 3.5 Miscellaneous Laboratory Studies In addition to bench-scale and pilot-scale studies, several laboratory studies were conducted to determine the humic adsorption capacities of corrosion products and the disinfectant demand of disinfectants (free chlorine and monochloramine) corrosion products. 67 3.5.1 Batch-scale Adsorption Studies of GAC and Corrosion Products Adsorption isotherms were determined by adding known masses Of adsorptive material (GAC, magnetite, and corrosion products) to several media bottles containing a buffered humic substance solution. Magnetite (Fe3O4, Aldrich Chemical Company, Cat. No. 31006-9) and uncharacterized corrosion products were collected from a field style annular reactor. Corrosion products were used because siderite (FeCO3) and goethite (a-FeOOH) are not commercially available or practical to make in the laboratory. GAC was also used because of its known high adsorption capacity and was used as a base-line material. The humic substance solution was prepared from humic material supplied by the International Humic Substances Society and buffered using 200 mg of potassium bicarbonate per liter, of solution and the pH adjusted to 7.5 units using a sodium hydroxide and/or sulfuric acid solution. The humic solution was stored in a walk-in cooler. 100-ml media bottles (Corning Cat. No. 1395-100) were used to contain the corrosion product mass or GAC and humic substance solution. All glassware was acid washed and glass fired at 550°C for at least 8 hours to remove any carbon that may be present. Bottle tops were boiled in a 5% potassium persulfate solution to oxidize any surface carbon present. Bottle tops were rinsed in ultrapure water and wrapped in aluminum foil prior to use. 68 Adsorption isotherms were determined by placing known masses of material (GAC, magnetite, and corrosion products) into the media bottles filled with the buffered humic substance solution. Media bottles were then placed on a shaker table and vigorously mixed for 48 hours to allow the adsorptive media to reach equilibrium with the humic substances in the bulk fluid. One control (media bottle without any adsorptive material present) was utilized for each experiment. After shaking, each bottle was centrifuged at 10,000 rpm for 15 minutes to separate the particulate/colloidal material from the bulk fluid. Total organic carbon (TOC) samples were then collected and measured on a Dohrman DC-80 Total Organic Carbon Analyzer. 3.5.2 Disinfectant Demand Studies Disinfectant demands of corrosion products were determined using corrosion product samples collected from a mild steel annular reactor surface at the Bozeman Water Treatment Plant. Corrosion product samples were collected by scraping the attached corrosion products from the pipe surface. After collection, the corrosion products were air-dried and stored in an air tight media bottle at room temperature. Demand studies were conducted in 125-ml serum bottles (Wheaton No. 223748) with teflon-lined septa (Wheaton 224223-01). Serum bottles and septa were soaked in a 25 mg/L chlorine solution for 48 hours prior to the experiment to remove any disinfectant demand of the glass or teflon. Immediately prior to use, 69 all serum bottles and septa were rinsed with ultrapure water (TOC less than 40 pg/L) to remove any traces of disinfectant. Disinfectant solutions were prepared using ultrapure water buffered with the addition of 200 mg of KHCO3 per liter of solution in addition to household bleach (for free chlorine solution) or household bleach and ammonium chloride (for monochloramine solution). A 4:1 ratio of CbiNH3 was used for the monochloramine solution. Chlorine and monochloramine solutions were prepared to produce a disinfectant residual of approximately 5 mg/L and the pH adjusted to 7.5 units with the addition of sodium hydroxide or a sulfuric acid solution. Both disinfectant solutions were stored for at least 24 hours to remove any disinfectant demand of the water prior to use. 70 Chapter 4 Bench-seal]© Stedlies Bench-scale laboratory tests were conducted to determine the long-term effect of disinfectants and corrosion inhibitors on the formation of distribution biofilms on pipe materials. The experiments presented in this chapter were designed to provide insight into the numerous interactions between organics, disinfectants, and corrosion inhibitors on pipe materials. Bench-scale experiments presented in this section utilized both free chlorine and monochloramine as secondary disinfectants. These studies utilized low disinfectant residuals to simulate future trends in disinfection that will minimize the formation of various disinfection byproducts. These experiments also evaluated the use of polyphosphate and zinc orthophosphate as corrosion control methods. Although there are several other corrosion control methods used by the drinking water industry, these two inhibitors are the most commonly used methods. pH adjustment was not utilized during bench-scale experiments due to the difficulty in controlling pH at low flow rates. 4.1 Bench-scale Studies with Free Chlorine without Corrosion Control The purpose of this study was to determine the effects of various free chlorine residuals on the formation of distribution biofilms. This study is of particular interest because in many situations, the addition of free chlorine 71 actually increases biofilm levels even though a free chlorine residual is present in the bulk fluid (Camper 1995). I 4.1.1 Experimental Design To determine the effects of various levels of free chlorine on distribution biofilms, six laboratory style annular reactors were used, each receiving the same nutrient and hydraulic conditions but operating with different effluent and bulk fluid free chlorine residuals. To simulate worst case conditions, mild steel slides were used as a biofilm substratum. This experiment was conducted in three steps. The first step was the inoculation step in which microorganisms were allowed to attach and grow on the substratum without the presence of free chlorine for 30 days. The second step was the addition of free chlorine to five of the reactors (one reactor used as an non-chlorinated control) at various levels up to 0.3 mg/L with the last reactor maintained and effluent and bulk fluid chlorine residual of approximately 1.25 mg/L for a period of 52 days. Specific operating conditions for this step are identified in Table 4.1. At the conclusion of the second step, effluent and bulk fluid chlorine residuals were increased to approximately 0.75 mg/L with the last reactor again operated at a free chlorine residual of approximately 1.25 mg/L for a period of 76 days. Specific operating conditions for this step are identified in Table 4.2. The entire experiment was conducted during the winter months when water quality was nearly constant and not affected by spring runoff. Typical finished water 72 quality changes in Bozeman, MT are illustrated in Table 4.3 A schematic of the experimental setup is presented in Figure 4.1. Table 4.1 Operational Conditions of Bench-scale Study (Step 2) Using Variable Free Chlorine Residuals without Corrosion Control ■ '■■■'" - 1" ; Reactor' ■ Residence Time, minutes ; Influent Carbon . v supplement,' Influent Phosphate. and Nitrate added, x : jug/L each ' ' % Average Effluent Free Chlorine : Residual, mg/L 1 90 0.25 100 0.00 2 90 0.25 100 0.04 3 90 0.25 100 0.09 4 90 0.25 100 0.12 5 90 0.25 100 0.21 6 90 0.25 100 1.25 Table 4.2 Operational Conditions of Bench-scale Study (Step 3) Using Variable Free Chlorine Residuals without Corrosion Control f.v, Reactor 1 •*' . =' - : ! Residence ^ Time;: minutes -Carbon::.' supplement, Influent Phosphate . and Nitrate added, ..pg/Leach 1 ‘' i ; -i v.- ' ' : ' Average Effluent Free Chlorine Residual, mg/L '"''.-V1.' '.Vc 1 90 0.25 100 0.00 2 90 0.25 100 0.12 3 90 0.25 100 0.58 4 90 0.25 100 0.75 5 90 0.25 . 100 0.54 6 90 0.25 100 1.22 73 Table 4.3 Seasonal Finished Water Quality Changes in Bozeman, MT siisais W1Smm -^ifciardnessig-I JSiiWig/L'as-gg Alkalinity, mg/L as CaCO3 ; Sssi Total Dissolved Solids, mg/L ' i S ", ' - January - April 95 - 116 93-109 97-114 May - June 68 - 94 53 - 99 73-94 July - October 68-105 54-76 72-114 November - December 97-106 95-100 107-117 4.1.2 Results At the conclusion of each chlorination step, slides were removed from each reactor and analyzed for biofilm density and corrosion product density. As illustrated in Figure 4.2, under some conditions it is possible to have an increase in biofilm density with an increase in chlorine residual. This increase in biofilm density with an increase in chlorine residual can be attributed to: (a) the increased release of Fe+2 from the pipe surface resulting from increased chloride levels, (b) increased corrosion product formation associated with the presence of powerful oxidants such as free chlorine, and (c) an increase in bioavailability of organic substrates (both free and adsorbed) resulting from chemical oxidation of organics by chlorine. It can also be observed from Figure 4.2 that once the chlorine residual exceeds approximately 0.75 mg/L, the biofilm densities begin to decrease. This chlorine residual which results in a decrease in biofilm density can be defined as the threshold residual. The threshold residual will vary from Dilution Water - j - u Figure 4.1 Bench-scale Studies with Free Chlorine without Corrosion Control Process Flow Schematic Lo g( C FU /c m Not statistically different^ from control CurveA Curve B Note: Curve A is statistically different than Curve B (p < 0.001) All data statistically different from control, except as noted. Effluent Chlorine Residual, mg/L Figure 4.2 Biofilm Density as a function of Chlorine Residual 76 site-to-site and will likely change with the physical and chemical properties of the water. In addition to biofilm density comparisons, evaluations of corrosion product mass densities were performed. Results from these evaluations are presented in Figure 4.3. Biofilm density (CFU/cm2) and corrosion product density (mg/cm2) typically increased with increased chlorine residuals up to the threshold level (approximately 0.75 mg/L chlorine residual). Once the chlorine residual was maintained at levels above the threshold, biofilm densities began to decrease even though corrosion product density continued to increase. The increase in disinfectant efficacy can be attributed to bulk fluid chlorine residuals being high enough to overcome the reaction/diffusion limited mass transfer limitations of the corrosion product/biofilm matrix, hence, allowing for deeper penetration of the disinfectant into the corrosion product/biofilm matrix. In view of Figure 4.2 and Figure 4.3, one important observation is that the mass of corrosion products present on a surface influences the biofilm density that can be supported by a pipe surface at a given chlorine residual, provided that the chlorine residual in the bulk fluid is less than the threshold residual previously defined. The sensitivity of corrosion product mass at various chlorine residuals can be determined by evaluating the microbial yield (CFU/cm2) as a function of corrosion product mass as a function of chlorine residual. Data for the development of the reaction rates were obtained by combining data from statistically insignificant chlorine concentrations from the two chlorinated steps Co rr os io n Pr od uc t D en si ty , m g/ cm 2 an d Lo g (C FU ) 35 -I □ Corrosion Product Mass, mg/cm2 ■ Iog(CFU) 20 15 0.04 0.087 0.122 0.123 0.21 0.545 0.585 0.747 1.22 1.25 Effluent Chlorine Residual, mg/L Figure 4.3 Corrosion Product Mass and Biofilm Populations at various Chlorine Residuals 78 previously described. Data were then plotted and a yield (the slope of line of best fit) determined by the Least Squares Method. Results from these evaluations are presented graphically in Figure 4.4 and summarized in Table 4.4. As illustrated in Figure 4.4 and Table 4.4, corrosion product mass has an influence on the amount of microorganisms that can be supported by a pipe surface at a given chlorine residual. The significance of corrosion product mass was highest at the lower chlorine residuals and less significant at higher chlorine residuals. Based on these observations, it can be concluded that for distribution systems maintaining low chlorine residuals, it is essential that corrosion rates be held at a minimum if biofilm related water quality problems are to be minimized. 4.2 Bench-scale Studies Comparing Free Chlorine and Monochloramine without Corrosion Control. The use of monochloramine as a secondary disinfectant is rapidly becoming widespread in the water industry because it is less reactive with pipe materials and does not form as many undesirable disinfection byproducts as free chlorine (LeChevaIIier 1997; Montgomery 1985; Pontius 1990; Bryant, et al 1992). Although monochloramine is considered to be a weaker disinfectant, it has been demonstrated that it is more effective at inactivating distribution biofilms because it is less reactive with EPS and corrosion products (LeChevaIIier 1990; van der Wende and Characklis 1990; Chen, et al 1993), allowing it to penetrate deeper into the corrosion product/biofilm matrix (LeChevallier, et al 1996; LeChevaIIier 1997; Camper, etal 1997). 79 Cl2 Residual = 0.064 +/- 0.02 mg/L Yield = 714 log(CFU/cm ) u. 6 0.0005 0.001 0.0015 0.002 0.0025 0.003 Corrosion Product Mass, g/cm2 E u D Li. O O)O Cl2 Residual = 0.105 +/- 0.2 mg/L Yield = 66.3 Iog(CFUZcnf) 0.0005 0.0009 0.0013 0.0017 0.0021 0.0025 Corrosion Product Mass, gZcm2 Figure 4.4 Biofilm density as a function of corrosion products and Cl2 residual 80 Cl2 Residual = 0.122 +/- 0.001 mg/L _ 8 Yield = 51.5 log(CFU/cm ) g/cnv 0.005 0.01 Corrosion Product Mass, g/cm 0.015 2 0.02 Cl2 Residual = 0.565 +/- 0.02 mg/L Yield = 36.8 Corrosion Product Mass, g/cm Figure 4.4 Biofilm density as a function of corrosion products and Cl2 residual 81 Ci2 Residual = 1.22 +/- 0.03 mg/L O - r E 7 . £ 7 2 6 . o 6 O C - O 5 A - U I 0 . 0 0 5 0.01 Yield = 12.6 Iog(CFL)Zcm ) " g/cm4 0.015 0.02 0.025 0.03 0.035 Corrosion Product Mass, g/cnv Figure 4.4 Biofilm density as a function of corrosion products and Cl2 residual 82 Table 4.4 Biofilm Density Yields at Various Bulk Fluid Chlorine Residual Levels without the use of Corrosion Inhibitors l Og(sS j) = Y(-£) Free Chlorine Residual, S y s iE ii i lmmsm L " . - . . y : ^ Significance of Yield as compared to zero - I - ' ' ' ' -S''", •- .......> v - i . • • • - , 0.064 +/- 0.02 mg/L 714 Not significant, p = 0.075 0.105 +/- 0.2 mg/L 66.3 Not significant, p = 0.64 0.122 +/- 0.001 mg/L 51.5 Significant, p = 0.002 0.565 +/- 0.02 mg/L 36.8 Significant, p = 0.003 1.22+/-0.03 mg/L 12.6 Not significant, p = 0.156 Note: Yield will vary depending on water quality and pipe material 4.2.1 Experimental Design This experiment utilized three field type annular reactors and was conducted to provide a side-by-side comparison between a non-chIorinated control, monochloramine, and free chlorine. For the two disinfected reactors, the same influent disinfectant (free chlorine or monochloramine) and nutrient feeds were used in each reactor. A summary of the experimental design is presented in Table 4.5. 4.2.2 Results As illustrated in Figure 4.5, the monochloramine reactor produced the lowest biofilm density at the conclusion of the experiment. However, it must be 83 noted that the monochloramine feed solution was buffered with phosphorus, which likely produced results that would not be representative of monochloramine used in an actual water treatment facility. Table 4.5 Operational Conditions of Bench-scale Study Comparing Free Chlorine and Monochloramine without Corrosion Control Parameter ControlReactor NH2Cl Reactor Free Cl2 Reactor Residence Time, minutes 120 120 120 Carbon supplement, mg/L® 0.25 0.25 0.25 Influent Monochloramine feed, mg/L None 3.25 None Influent Free Chlorine feed, mg/L None None 3.25 ® In addition to background carbon present in dilution water As a result of the elevated phosphorus levels in the monochloramine solution, these results should not used to conclude that monochloramine is a superior disinfectant for the inactivation of distribution biofilms. However, this experiment did provide some insight about the significance of corrosion products on a pipe surface and the role of phosphorus used in most corrosion inhibitors. These observations became evident when the physical condition of the unlined cast-iron pipe reactors was inspected at the conclusion of the experiment. As illustrated in Figure 4.6, the reactor receiving monochloramine had some type of Lo g (C FU /c m 2) Free Chlorine MonochloramineControl Time, Days Figure 4.5 Biofilm Density Comparisons between Free Chlorine and Monochloramine Figure 4.6 Photographs of Reactor Walls after Experiment using Free Chorine and Monochloramine 86 chemical coating on the surface which minimized the formation of corrosion products on the pipe surface. In view of Figure 4.6, several other significant observations can also be made. Observational comparisons can be made between the amount of corrosion products present on the pipe surfaces and biofilm densities on each reactor. For example, the chlorinated reactor usually had the most corrosion products and likewise had the highest biofilm density of all reactors. The reactor receiving the phosphorus buffered monochloramine had the lowest amount of corrosion products and the lowest biofilm density. Another interesting observation from Figure 4.5 is the shape of the curve for the monochloramine-disinfected reactor. This graph indicates that there was a steady decrease in biofilm density after the 2nd week of operation, which is likely the result of the phosphorus passivation of the pipe surface. The steady decrease in biofilm density can be attributed to: (1) the reduction of disinfectant demand from the pipe surface, (2) reduction in habitat for microorganisms, and (3) changing.the surface chemistry of the pipe surface which makes microbial attachment less likely. 4.3 Bench-scale Studies with Free Chlorine and Corrosion Inhibitors As previously discussed in Chapter 2, the use of phosphorus based corrosion inhibitors is rapidly gaining popularity in the water industry as a method to control the leaching of lead and copper into the finished water. The use of 87 these inhibitors is of concern to water suppliers because phosphorus is an essential nutrient for microorganisms. It was therefore the purpose of this experiment to evaluate the impacts that two commonly used corrosion inhibitors (zinc orthophosphate and polyphosphate) have on distribution biofilms. 4.3.1 Experimental Design To determine the effects of zinc orthophosphate and polyphosphate on distribution biofilms, four field type (unlined cast iron) reactors were used, each operating under the same nutrient and hydraulic conditions. Reactor 1 served as a non-chlorinated control. without any type of corrosion control, while the remaining reactors each received the same disinfectant feed. In addition to nutrients and free chlorine, Reactors 3 and 4 also received zinc orthophosphate and polyphosphate respectively. Corrosion inhibitor influent feed dosages was determined based on water quality information using the OSCAR™ program by Calgon Chemical Corporation. Specific operating conditions associated with this experiment are presented in Table 4.6. 4.3.2 Results During the course of this experiment, numerous biofilm and bulk fluid samples were taken to evaluate the efficacy of the corrosion control/disinfection treatment for each reactor. As illustrated in Figure 4.7, the use of corrosion inhibitors can have an influence on the amount of biofilm supported by a ductile iron pipe surface. Long-term effects of the corrosion control/disinfection systems 88 were analyzed using Fisher’s Multiple Comparison with a 95% confidence level on the last four weeks of data. Results from this statistical analysis are presented in Table 4.7 and graphically in Figure 4.8. Table 4.6 Operational Conditions of Bench-scale Study Comparing Free Chlorine with Zinc Orthophosphate and Polyphosphate 5 Parameter ControlReactor Free Cl2 Only Reactor Free Cl2 and Polyphosphate Reactor Free Cl2 and Zinc Orthophosphate Reactor Residence Time, minutes 120 120 120 120 Carbon supplement, mg/LO 0.25 0.25 0.25 0.25 Influent Free Chlorine feed, mg/L None 3.25 3.25 3.25 Influent Zinc Orthophosphate feed, mg/L© 0 0 0 10.0 Influent Polyphosphate feed, mg/L© 0 0 5.0 0 O In addition to background carbon present in dilution water @ Inhibitor dosages determined by OSCAR program In review of Figure 4.7, Figure 4.8, and Table 4.7, it should be noted that the chlorinated control once again had the highest biofilm density and that the reactors utilizing corrosion inhibitors supported significantly less microorganisms than the chlorinated reactor. Another important observation is the reaction times of the two reactors that received corrosion inhibitors. As illustrated in Figure 4.7, Lo g (C FU /c m ') 9 T— Control CI2 Only CI2 + Polyphosphate CI2 + Zinc Orthophosphate Time, Days Figure 4.7 Biofilm Density Comparisons using Free Chlorine and Corrosion Inhibitors 9O CL ^ 6 O CM U -J o> E LL O O) O -J 8 7 - 5 - ^ 4 4 cIE o 2 - 1 - Control i$ $ CI2 Only ■ | B « CI2 +Zn-o-P04 Wk m # CI2 + PolyP04 □ Log(CFU/cm2) ■ Effluent Chlorine Residual, (mg/L X 10) D Effluent Phosphate, mg/L Figure 4.8 Summary Data from Bench-scale Experiment Using Free Chlorine and Corrosion Inhibitors 91 Table 4.7 Statistical Analysis of Biofilm Densities Using a 95% Confidence Level for Bench-scale Experiment Using Free Chlorine and Corrosion Inhibitors Log Removal or Gain Reactor Control Cl2 Only Cl2 and Zinc As Compared to ► Orthophosphate Cl2 Only 1.06 Log Cl2 and Zinc Orthophosphate -0.50 Log -1.55 Log Cl2 and -0.08 Log -1.14 Log 0.41 Log Polyphosphate Not Significant Not Significant Note: The Clz Only Reactor had 1.06 Log more biofilm density than the Control Reactor. All values are statistically significant unless noted otherwise. the zinc orthophosphate reactor had a rapid increase in disinfection efficacy while the polyphosphate reactor began to demonstrate increased disinfection efficacy and biofilm control after the 3rd week. As with the previous experiment, a visual inspection of the pipe reactors at the conclusion of the experiment also provided some interesting observations. As illustrated in Figure 4.9, the chlorinated control reactor once again had the highest corrosion product mass and also the highest biofilm density. The non- chlorinated control and the polyphosphate had approximately the same quantity of corrosion product mass and likewise did not have any significant differences in biofilm density. The zinc orthophosphate reactor had the lowest corrosion product mass and the lowest biofilm density. See Figure 4.6 for photograph o f reactor prior to experiment Figure 4.9 Photographs of Reactor Walls after Experiment using Free Chorine and Corrosion Inhibitors 93 4.4 Bench-scale Studies with Monochloramine with Corrosion Inhibitors The purpose of this experiment was to determine the affect of zinc orthophosphate and polyphosphate as corrosion inhibitors using monochloramine as a secondary disinfectant. As with previous studies, low disinfectant levels were used to reflect future disinfection practices of many water utilities that have problems with excessive levels of regulated disinfection byproducts. 4.4.1 Experimental Design The experimental design for this experiment is identical to the experiments presented in Section 4.3 with the exception that monochloramine is used as a secondary disinfectant. The buffering system for the monochloramine solution was also changed to a potassium bicarbonate buffer in stead of a phosphate buffer so that no additional phosphorus was added to the system. Specific operating conditions associated with this experiment are presented in Table 4.8. 4.4.2 Results As with the previous bench-scale experiment, biofilm and bulk fluid samples were routinely collected to evaluate the efficacy of the various corrosion control/disinfection treatments. As illustrated in Figure 4.10, the use of corrosion inhibitors did have an impact on the formation of distribution biofilms, but not as apparent as with the previous experiment that used free chlorine as a secondary disinfectant. An analysis of the long-term effect was again conducted by pooling 94 the last four weeks of data using Fisher’s Multiple Comparison with a 95% confidence level. Results from this analysis are presented in Table 4.9 and graphically in Figure 4.11. Table 4.8 Operational Conditions of Bench-scale Study Comparing Monochloramine with Zinc Orthophosphate and Polyphosphate 6 Parameter ControlReactor NH2CI Only Reactor NH2CI and Polyphosphate Reactor NH2CI and Zinc Orthophosphate Reactor Residence Time, minutes 120 120 120 120 Carbon supplement, mg/LO 0.25 0.25 0.25 0.25 Influent Monochloramine feed, mg/L None 3.25 3.25 3.25 Influent Zinc Orthophosphate feed, mg/L© 0 0 0 10.0 Influent Polyphosphate feed, mg/L© 0 0 5.0 0 O In addition to background carbon present in dilution water © Corrosion inhibitor dosages determined by OSCAR program In review of Figure 4.11, Table 4.9, and Figure 4.11, it should be noted that the monochloramine only reactor had 0.45 log more biofilm than the reactor that did not receive any disinfectant. However, in this experiment, polyphosphate did not significantly increase or decrease disinfection efficacy when compared to Lo g( C FU Zc ir i) 9 T— 5 - ----- Control NH2CI Only NH2CI + Polyphosphate NH2CI + Zinc Orthophosphate Time, Days Figure 4.10 Biofilm Density Comparisons Using Monochloramine and Corrosion Inhibitors 6 CL ± Z -J I D LL O U) O -J 8 7 - 6 ■ 2 - 1 - 5 - 4 - Si: I j Control NH2CI Only NH2CI + Zn o-P04 NH2CI + PolyP04 □ Log(CFU/cm2) ■ Effluent NH2CI Residual, (mg/L X 10) D Effluent Phosphate, mg/L Figure 4.11 Summary Data from Bench-scale Experiment Using Monochloramine and Corrosion Inhibitors 97 the monochloramine addition only reactor. The zinc orthophosphate reactor once again had a significant (1.79 log or 98 percent) reduction in biofilm density. It can also be observed that the response time for noticeable increases in disinfection efficacy took place after two weeks of treatment. Table 4.9 Statistical Analysis of Biofilm Densities Using a 95% Confidence Level for Bench-scale Experiment Using Monochloramine and Corrosion Inhibitors Log Removal or Gain Reactor ^ As Compared to ► Control NH2CI Only NH2CI and Zinc Orthophosphate NH2CI Only 0.45 Log Not Significant NH2CI and Zinc Orthophosphate -1.17 Log -1.55 Log NH2CI and Polyphosphate 0.62 Log -0.16 Log Not Significant 1.79 Log Note: The NHzCI Only Reactor had 0.45 Log more biofilm density than the Control Reactor. All values are statistically significant unless noted otherwise. These numerical results were also reflected in the visual inspection of the pipe reactors at the conclusion of the experiment as illustrated in Figure 4.12. As with the previous visual observations, the cleanest reactor surface once again produced the lowest biofilm density. and Corrosion Inhibitors 99 4.5 Bench-scale Studies with Free Chlorine and Polyphosphate The purpose of this study was to determine the effects of various levels of polyphosphate, under constant influent chlorine residuals, have on the chlorine residual and biofilm density. This experiment is of importance because it has been demonstrated that the addition of a corrosion inhibitor can reduce coliform occurrences (Lowther and Moser 1984) at dosages recommended to reduce leaching of lead and copper, but it is not known if lower or higher dosages of inhibitors are needed to reduce distribution biofilm densities. 4.5.1 Experimental Design To investigate this issue, six laboratory style annular reactors using mild steel coupons were employed. Each reactor received the same carbon supplement and flow throughout the experiment. As with previous bench-scale studies, each reactor was inoculated with heterotrophic bacteria for approximately 10 days. After the inoculation phase, free chlorine and polyphosphate feeds were initiated in accordance with Figure 4.13 and Table 4.10 and the reactors allowed to operate continuously for 81 days. 4.5.2 Results At the conclusion the experiment, four coupons were removed from each reactor and biofilm samples collected and processed as previously described in Chapter 3. Free chlorine and phosphorus samples were also collected daily Dilution Water Chlorinated Dilution Water Note: Carbon substrate, nutrients, and chlorine added upstream o f dilution water. Reactor I received unchlorinated dilution water. Figure 4.13 Bench-scale Studies with Free Chlorine without Corrosion Control Process Flow Schematic 100 101 during the last week of the experiment and analyzed in accordance with Table 3.1. Results from the biofilm, chlorine, and phosphorus analysis are presented in Figure 4.14 and Table 4.11. Table 4.10 Operational Conditions of Bench-scale Study Using Constant Free Chlorine Dosage and variable Polyphosphate Levels Reactor Residence Time, minutes Influent Chlorine dose, mg/L Influent Carbon supplement, mg/L and Phosphate and Sodium Nitrate supplements, pg/L Influent Polyphosphate Dosage, mg/L as PoIyphosphateO 1 90 0 0.25 mg/L and 100 pg/L 0.0 2 90 2.0 0.25 mg/L and 100 pg/L 0.0 3 90 2.0 0.25 mg/L and 100 pg/L 2.4 4 90 2.0 0.25 mg/L and 100 pg/L 4.0 5 90 2.0 0.25 mg/L and 100 pg/L 6.4 6 90 2.0 0.25 mg/L and 100 pg/L 8.0 O The OSCAR program recommended a polyphosphate dose of 5 mg/L. In addition to laboratory analysis, photographs were also taken of the collected coupons from each reactor. Photographs were taken before and after biofilm/corrosion product removal and are illustrated in Figure 4.15 and Figure 4.16 respectively. In review of Figure 4.14 and Table 4.11, there are several interesting facts that can be gained from this experiment. As with previous experiments, the chlorinated only reactor (Reactor 2) once again had a statistically higher biofilm density and corrosion product density than the control reactor (Reactor 1) even 35 T tIE £ O) E o T - X _l O) E CnT U L. O O I e § LL O O) O 30 - 25 - 20 - ■ Log(CFU/cm2) □ Chlorine residual, mg/L X 10 B Phosphate residual, mg/L X 10 □ Corrosion Product Mass, mg/cm2 Figure 4.14 Biofilm Density, Chlorine Residual, Phosphate Residual, and Corrosion Product Mass Comparisons 102 103 with an effluent free chlorine residual of approximately 1.0 mg/L. Another interesting observation is that the presence of a corrosion inhibitor immediately began to reduce biofilm densities, even when the corrosion product mass increased on most reactors. Reactor 6, which had the highest polyphosphate dose, also had the highest chlorine residual, lowest corrosion product mass, and lowest biofilm density of all chlorinated reactors. Table 4.11 Results from Bench-scale Study Using Constant Free Chlorine Dosage and variable Polyphosphate Levels Reactor Influent Polyphosphate Dose, mg/LO Effluent Free Chlorine Residual, mg/L© Effluent Phosphate residual, mg/L as PO4 Biofilm Density, CFU/cm2 Corrosion Product Density, (mg/cm2) 1 0 0.09 0.26 9.8 x 105 9.6 2 0 1.00 0.28 6.9 x 106 17.1 3 2.4 0.62 1.00 3.4 x 106 20.2 4 4.0 0.86 1.19 4.2 x 106 24.4 5 6.4 0.99 1.77 2.9 x 106 26.8 6 8.0 1.20 2.15 1.4 x 106 9.9 O The OSCAR program recommended a polyphosphate dose of 5 mg/L. © Reactors 2-6 each received an influent containing 2 mg/L of free chlorine. In view of the above, it can be concluded that the corrosion product mass formed in the presence of polyphosphate is less reactive with free chlorine, hence allowing the disinfectant to penetrate and inactivate microorganisms deeper in the biofilm/corrosion product matrix. This is apparent when comparing 104 Reactor 2 with Reactor 5 where both have essentially the same bulk fluid chlorine residual, but have significantly different biofilm densities and corrosion product densities. This observation is completely opposite to the findings found in experiment described in Section 4.1 of this chapter where an increase in corrosion product mass resulted in an increase in biofilm density. This data also indicates that the polyphosphate dosage recommended by the manufacturer for this project (5 mg/L) may not provide adequate passivation of the mild steel coupons. This observation is evident when comparing the coupon from Reactor 6 with the other chlorinated coupons in Figure 4.15 and Figure 4.16 which clearly illustrates that electrochemical cells are formed below each corrosion tubercle allowing the release of iron from the substratum. These types of observations have been made from each experiment conducted in this chapter, which confirms that an effective corrosion control program should not only minimize the leaching of lead and copper from plumbing materials, but also reduce the corrosion rates of unlined ferrous materials in the distribution system if microbially related water quality problems are to be minimized. 4.6 Key Findings o When compared to having no disinfectant, a low disinfectant residual on iron pipe surfaces can actually increase biofilm densities. o The presence of corrosion products has a significant impact on a surface’s ability to support a biofilm, particularly at low disinfectant levels. Reactor 105 Figure 4.15 Photographs of coupons prior to sample collection Reactor 106 4 ITT? 6 : ___ Figure 4.16 Photographs of coupons after sample collection 107 o If corrosion products are present, it is important to maintain disinfectant levels above the threshold residual level if a reduction in biofilm density is desired. © If the chlorine residual in a distribution system is maintained above the threshold residual level, a decrease in residual will likely result in an increase in biofilm density. o If the chlorine residual in a distribution system is operating below the threshold residual level, an increase in residual may result in increased biofilm density provided the residual remains below the threshold value. o The use of phosphorus based corrosion inhibitors can increase the disinfection efficacy of secondary disinfectants. o The reaction rates of corrosion inhibitors are slow. At least several weeks are needed before conclusions should be made. o Corrosion products formed in the presence of corrosion inhibitors are less reactive with free chlorine allowing for deeper biofilm/corrosion product penetration and increased biofilm inactivation. o Corrosion inhibitor dosages should be determined based on minimizing corrosion rates in the distribution system if microbially related water quality problems are to be minimized. 108 Chapter S Piioft=ScalIe Studies The purpose of the pilot-scale study was to determine the long-term effect of various corrosion control methods and disinfectants on distribution biofilms and to substantiate the results obtained in the bench-scale studies (Chapter 5). Pilot-scale facilities were conducted using the facilities described in Section 3.4. Pilot-scale studies utilized unlined mild steel as a substratum, while the bench- scale studies utilized unlined ductile iron as a substratum. These experiments were conducted using two types of corrosion inhibitors (zinc orthophosphate and polyphosphate) along with pH control as a corrosion control method with either free chlorine or monochloramine as a secondary disinfectant. Since the . presence of various disinfection byproducts will be of major concern to utilities in the near future, these experiments utilized low disinfectant residuals to simulate future trends in secondary disinfection. All pilot-scale studies were conducted using a pilot-scale distribution system located at the City of Bozeman Water Treatment Plant. The pilot-scale facilities have the capability of controlling pipe velocity, temperature, and hydraulic residence time, along with being able to regulate nutrients (carbon, nitrogen, and phosphorus), pH, disinfectants (free chlorine and monochloramine), and corrosion inhibitors (zinc orthophosphate and polyphosphate). Detailed descriptions of these facilities are given in Chapter 3. 109 5.1 Pilot-scale Studies with Free Chlorine and Corrosion Control The purpose of this experiment was to determine the effect of three commonly used corrosion control methods and free chlorine on distribution biofilms. As with the annular reactor experiments, all pipe loops were operated under identical flow rates, shear stresses, temperatures, nutrient loading and free chlorine feed dosages. Corrosion control methods used in this experiment included the use of two corrosion inhibitors (zinc orthophosphate and polyphosphate) and pH control. Corrosion Inhibitor feed rates were based on water quality information using the OSCAR™ program by Calgon Chemical Corporation, while pH levels were determined using the Langelier Saturation Index (LSI). The pH was maintained at levels that would produce a slightly positive LSI to insure that CaCO3 would precipitate on the pipe surface. 5.1.1 Experimental Design To determine the effects of the corrosion control methods, an experimental design had to be developed that would independently isolate each corrosion control method. For this experiment LOOP 1 utilized pH control as a corrosion control method, LOOP 2 served as a chlorinated control and did not receive any corrosion control treatment, LOOP 3 served as a non-chlorinated control, LOOP 4 utilized zinc orthophosphate as a corrosion control method, while LOOP 5 utilized polyphosphate as a corrosion control method. Specific operating conditions associated with this experiment are presented in Table 5.1. 110 Table 5.1 Operational Conditions of Pilot-scale Experiments using Free Chlorine and Corrosion Control Treatments Parameter LOOP 1 LOOP 2 LOOP 3 LOOP 4 LOOPS Operating Temperature, 0C 20 20 20 20 20 Pipe velocity, ft/sec 1.0 1.0 1.0 1.0 1.0 Residence Time, minutes 120 120 120 120 120 Influent Carbon supplement, mg/L® 0.25 0.25 0.25 0.25 0.25 Influent Free Chlorine feed, mg/L@ 1.1-1.5 1.1-1.5 None 1.1-1.5 1.1-1.5 pH Control® yes None None None None Influent Zinc Orthophosphate feed, mg/L© 0 0 0 10.0 0 Influent Polyphosphate feed, mg/L© 0 0 0 0 5.0 dD In addition to background carbon present in dilution water @ Chlorine feed varied to maintain desired residuals ® pH controlled to provide a slightly positive LSI © Corrosion inhibitor dosages determined by OSCAR program 5.1.2 Results and Discussion During the course of this experiment, numerous samples were taken to evaluate the efficacy of corrosion control/disinfection treatment on each pipe loop of the pilot-scale facilities. As with the annular reactor studies presented in 111 Chapter 4, bulk fluid chlorine residuals were maintained at low levels (0.40 to 0.75 mg/L) to simulate future disinfection trends that will minimize the formation of various regulated disinfection byproducts. Influent total organic carbon (TOC) measurements ranged from 3.0 mg/L at the beginning of the experiment to 1.5 mg/L at the end of the experiment. The variation of TOC can be attributed to water quality changes associated with spring runoff in beginning of the experiment. Although TOC values may appear to be high in the early stages of this experiment, it should be noted that TOC bioavailability was minimized due to the performance of the pretreatment facilities. The pilot-scale experiment produced similar trends to those found with the annular reactor studies presented in Chapter 4, even though pipe materials were different. Comparisons of biofilm densities with each pipe loop are presented in Figure 5.1. To determine if there were any statistical differences of biofilm densities associated with the corrosion control treatments, data from the last 4 weeks of the experiment were pooled and analyzed using Fisher’s Multiple Comparison ANOVA with a 95% confidence level. Results from this analysis are presented in Table 5.2 and graphically in Figure 5.2. As illustrated in Table 5.2, Figure 5.1, and Figure 5.2, the use of corrosion control methods can have a significant effect on a pipe surface’s capacity to support distribution biofilms. There was no statistical difference between the chlorinated only (LOOP 2) system and the system that utilized zinc orthophosphate (LOOP 4) or the system that did not utilize a disinfectant or Lo g (C FU /c m 2) 8 0 10 20 30 40 50 60 70 80 90 100 Time, days Figure 5.1 Biofilm Densities for Pilot-scale Experiments using Free Chlorine and Corrosion Control Treatments 112 Tf O O- TS C(0 CN o> E Eo LL O O) O - I CI2 + pH Adjustment CI2 Only Control CI2 + Zinc o-P04 CI2 + Poly-P04 D log(CFU/cm2) Effluent Chlorine Residual,( mg/L X 10) D Effluent Phosphate, mg/L Figure 5.2 Biofilm Density, Cl2, and PO4 Levels for Experiments using Free Chlorine and Corrosion Control Treatments 113 114 corrosion control technique (LOOP 3). It can also be noted that the pH control system (LOOP 1) and the polyphosphate system (LOOP 5) were able to support a significantly higher population of distribution biofilms under these water quality conditions. Table 5.2 Statistical Analysis of Biofilm Densities Using a 95% Confidence Level for Pilot-scale Experiments using Free Chlorine and Corrosion Control T reatments Log Removal or Gain LOOP ▼ As Compared to ► Cl2 + pH Control LOOP 1 Cl2 Only LOOP 2 Control LOOPS Cl2 + Zn O-PO4 LOOP 4 Cl2 Only LOOP 2 -0.69 Log Control LOOPS -0.53 Log 0.16 Log Not Significant Cl2 + Zn o- PO4 LOOP 4 -0.67 Log 0.02 Log Not Significant -0.14 Log Not Significant Cl2 + Poly- PO4 LOOP 4 -0.10 Log Not Significant 0.60 Log 0.44 Log 0.58 Log Note: LOOP 2 has 0.69 Log less biofilm density than LOOP 1 All values are statistically significant unless noted otherwise Another significant observation was that the free chlorine residual was higher in the pH-controlled system (LOOP 1) than in the remaining loops, even though they all received the same influent dose of free chlorine. This higher free 115 chlorine residual can be attributed to a shift in equilibrium between hypochlorous acid (HOCI) and the hypochlorite (OCP) ion. At pH values less than 7.5, HOCI is the dominant species while at pH values greater than 7.5 OCP is the dominant species (Snoeyink and Jenkins 1980; AWWA 1998; Stumm and Morgan 1996; Montgomery 1985; Pontius 1990; Bryant, et al 1992). Although the chlorine residual may be greater at higher pH values, the disinfecting power will be significantly lower because the hypochlorite ion (OCP) is a much weaker oxidant than hypochlorous acid (HOCI) (Pontius 1990; Bryant, et al 1992; AWWA 1998). In addition to statistical comparisons, several other observations and comparisons were made. These include observation of the presence of molds, fungi, and actinomyces like colonies on R2A plates and visual observations of corrosion products on the pipe loop coupons. As discussed in Chapter 2, the presence of molds, fungi, and actinomyces are of significance because these microorganisms are known to produce metabolic byproducts that cause numerous taste and odor complaints from consumers (LeChevallier, et al 1996; van der Wende and Characklis 1990; van der Kooij and Oorhuizen 1997). These microorganisms were routinely observed in biofilm and bulk fluid samples from the unchlorinated control loop (LOOP 3) through out the experiment, and were found less frequent in the four chlorinated loops. These microorganisms were also detected in the bulk fluid and biofilm samples in the chlorinated loops during peak spring runoff (Week 6 and 7 of experiment) but were not able to colonize in the chlorinated loops after peak runoff. These microorganisms were able to colonize in the unchlorinated loop (LOOP 3) and be detected in the biofilm and 116 bulk fluid samples for the duration of the experiment, even though they were not routinely observed in the influent bulk fluid samples. Of particular interest was the type and magnitude of corrosion products present on the coupons collected from each pipe loop. As expected, the type of corrosion products observed varied between pipe loops. Photographs of the coupons were taken at the last sampling and are presented in Figure 5.3. Observations of these coupons clearly reflect the effect of corrosion control treatments along with the evidence of sloughing events. The coupon from LOOP 1 shows a large corrosion tubercle that is likely to be siderite (FeCO3) with an overlaying black layer of magnetite (Fe3O4), a conditioning film on the coupon surface, along with a recent tubercle sloughing at the 1 o’clock position. The LOOP 2 coupon shows definite signs of corrosion with no visible conditioning film and an old tubercle-sloughing event at the 7 o’clock position. The LOOP 3 coupon shows uniform corrosion without any signs of large corrosion tubercles. The LOOP 4 coupon shows a definite passivating film, a small corrosion tubercle, and recent tubercle sloughing events at the 8 o’clock and 10 o’clock positions. LOOP 5 consistently had the highest mass of corrosion products and consequently had the highest biofilm density of all loops. 5.2 Pilot-scale Studies with Monochloramine and Corrosion Control The purpose of this experiment was to determine the effect that three commonly used corrosion methods and monochloramine, as a secondary disinfectant, will have on distribution biofilms. The use of monochloramine is Clean Coupon pH Adjustment + Cl2 LOOP 1 Cl2 Only LOOP 2 Control LOOP 3 LOOP 4 Cl2 + Polyphosphate LOOPS Figure 5.3 Photographs of Coupons after Experiment using Free Chlorine and Corrosion Control Treatments 117 118 rapidly gaining popularity because it is less reactive with organics that lead to the formation of various undesirable disinfection byproducts (Kirmeyer, et al 1993; Symons, et al 1982; LeChevaIIier 1997; AWWA 1998), Although monochloramine is considered to be less reactive than free chlorine, the low reactivity of monochloramine has been said to allow it to penetrate deeper into the biofilm/corrosion product matrix, hence, increasing biofilm inactivation (LeChevaIIier 1990; van der Wende and Characklis 1990). 5.2.1 Experimental Design The experimental design is identical to the previous experiment with the exception that monochloramine was used as a disinfectant instead of free chlorine. The monochloramine dose was prepared by mixing solutions of free chlorine (made from household bleach) with an ammonium chloride solution. These two solutions were stored separately, pumped and mixed to a 4 to I chlornerammonia ratio prior to application. Specific operating conditions associated with this experiment are presented in Table 5.3. 5.2.2 Results and Discussion As with the previous pilot-scale experiment, samples were taken to evaluate the efficacy of the various corrosion control/disinfection treatments on each loop. Influent water quality was exceptionally high during this experiment mainly because it was conducted during the extreme of winter when stream flows are minimal and surface runoff events were negligible. As a result of these water 119 quality conditions, the overall mineral content (alkalinity, hardness, etc.) was slightly higher than those experienced during the previous pilot-scale experiment, and the TOC was consistently in the 0.6 to 0.7 mg/L range. Total chlorine residuals were maintained in the 0.15 mg/L to 0.25 mg/L range and was mostly in the form of monochloramine. Table 5.3 Operational Conditions of Pilot-scale Experiments using Monochloramine and Corrosion Control Treatments Parameter LOOP I LOOP 2 LOOP 3 LOOP 4 LOOPS Operating Temperature, 0C 20 20 20 20 20 Pipe velocity, ft/sec 1.0 1.0 1.0 1.0 1,0 Residence Time, minutes 120 120 120 120 120 Influent Carbon supplement, mg/L® 0.25 0.25 0.25 0.25 0.25 Influent Monochloramine feed, mg/L 0.70 0.70 None 0.70 0.70 pH Control® yes None None None None Influent Zinc Orthophosphate feed, mg/L® 0 0 0 10.0 0 Influent Polyphosphate feed, mg/L® 0 0 0 0 5.0 dD In addition to background carbon present in dilution water @ pH controlled to provide a slightly positive LSI ® Corrosion inhibitor dosages determined by OSCAR program 120 As with the previous pilot-scale experiment, the use of various corrosion control methods had significant effects on the quantity of microorganisms that could be supported on the pipe surface. Comparisons of biofilm densities from each pipe loop are presented in Figure 5.4. Based on a statistical analysis of the last 4 weeks of data, there are several statistically significant differences between the pipe loop systems. Results from this analysis are presented in Table 5.4 and graphically in Figure 5.5. It should be noted that the results from this experiment should not be directly compared to the results obtained in the previous pilot-scale experiment using free chlorine, because there were statistically significant differences between the controls for these two experiments. These differences can be attributed to the changes in water quality between the two experiments. „ As illustrated in Table 5.4, Figure 5.4 and Figure 5.5, the use of corrosion control methods had significant effects on the disinfection efficacy on distribution biofilms. These results demonstrate that an effective corrosion control program can significantly enhance disinfection efficacy of distribution biofilms. For this experiment, the pH control system (LOOP 1), zinc orthophosphate system (LOOP 4), and the polyphosphate system (LOOP 5), were found to have significantly less biofilm than the system that only received monochloramine (LOOP 2). It can also be observed that a small residual of monochloramine (LOOP 2) increased biofilm density when compared to the control (LOOP 3). Also noted was the physical appearance of the coupons removed from the pilot-scale system on the last sampling event. As observed in previous 87 04 6 Eo 4 3 -- -------- -----------------------* . -♦ -NH2C I + pH Control - ti-NH2CL0nly - A - Control NH2CI + ZnoPCM -B-NH2CI + PoIy-PCM - 0 10 20 30 40 50 60 70 80 90 Time, days Figure 5.4 Biofilm Densities for Pilot-scale Experiments using Monochloramine and Corrosion Control Treatments pH Control NH2CI Only Control NH2CI + Zinc o- NH2CI + Poly- P04 P04 □ log(CFU/cm2) ■ Effluent Monochloramine Residual, (mg/L X 10) □ Effluent Phosphate, mg/L Figure 5.5 Biofilm Density, Cl2, and PO4 Levels for Experiments using Monochloramine and Corrosion Control Treatments 122 123 experiments (bench-scale and pilot-scale), coupons with large quantities of corrosion products typically had higher biofilm densities. Photographs of the coupons are presented in Figure 5.6. Table 5.4 Statistical Analysis of Biofilm Densities Using a 95% Confidence Level for Pilot-scale Experiment s using Monochloramine and Corrosion Control T reatments Log Removal or Gain LOOP v' As Compared to t> NH2C I+ pH Control LOOP 1 NH2CI Only LOOP 2 Control LOOP 3 NH2 Cl+ Zn 0 - PO4 LOOP 4 NH2CI Only LOOP 2 0.95 Log Control LOOPS 0.82 Log -0.13 Log NH2 Cl + Zn 0 -PO4 LOOP 4 0.72 Log -0.23 Log -0.10 Log Not Significant NH2CI + Poly-P04 LOOP 4 0.20 Log -0.75 Log -0.62 Log -0.52 Log Note: LOOP 2 has 0.95 Log more biofilm density than LOOP 1 All values are statistically significant unless noted otherwise Another significant observation between the previous pilot-scale experiment (free chlorine used as a secondary disinfectant) and this pilot-scale experiment (monochloramine used as a secondary disinfectant) was the performance of pH control system (LOOP 1) and the polyphosphate system Clean Coupon pH Adjustment + NH2CI LOOP 1 Control NH2CI + Zinc Orthophosphate LOOP 3 LOOP 4 NH2CI Only LOOP 2 NH2CI + Polyphosphate LOOPS Figure 5.6 Photographs of Coupons after Experiments using Monochloramine and Corrosion Control Treatments 124 125 (LOOP 5). These two corrosion control treatments had significantly higher biofilm densities than the chlorine only system (LOOP 2), but significantly less biofilm density than the monochloramine only system (LOOP 2). These observations clearly illustrate that both seasonal water quality changes and the type of secondary disinfectant can have significant effects on the performance of corrosion control treatments. 5.3 Pilot-scale Studies with Free Chlorine, Monochloramine and Zinc Orthophosphate The purpose of this experiment was to conduct side-toy-side comparisons between free chlorine and monochloramine, with and without zinc orthophosphate, on distribution biofilms. This experiment is of importance because the data collected from the previous two pilot-scale experiments can not be compared against each other, because the water quality was different during the two experiments. The first pilot-scale experiment (as described in Section 5.1) was conducted during the spring when the water quality is changing from spring runoff, resulting in a water that is lower in hardness and alkalinity, fluctuating organic carbon, and is slightly corrosive (see Table 4.3). The second pilot-scale experiment (as described in Section 5.2) was conducted during the winter months when the water supply is higher in alkalinity and hardness, lower in organic carbon and turbidity, and is less corrosive. The impact of the changing water quality can be observed by comparing the controls (LOOP 3) from the two previous pilot-scale experiments presented in Sections 5.1 and 5.2. This 126 experiment is of importance because the influent water quality will be the same for each test loop. 5.3.1 Experimental Design The experimental design for this experiment is slightly different from the previous pilot-scale experiments, because two types of secondary disinfectants (free chlorine and monochloramine) and a single corrosion control treatment (zinc orthophosphate used as a corrosion inhibitor) were used. This experiment did encounter several mechanical problems in the early stages of the experiment but were corrected prior to the 4th week of the experiment. Specific operating conditions associated with this experiment are presented in Table 5.5. 5.3.2 Results and Discussion As with the previous two pilot-scale experiments, samples were taken to evaluate the efficacy of the various combinations of disinfectants (either free chlorine or monochloramine) with and without the use of zinc orthophosphate. This experiment was conducted during the spring when the water quality is continuously changing due to snowmelt and frequent rain events. The majority of the snowmelt and rain occurred between days 30 and 50 and are reflected by increased biofilm densities as illustrated in Figure 5.7. Total organic carbon levels ranged from 2.4 mg/L at the beginning of the experiment to 0.9 mg/L at the end of the experiment. Alkalinity and hardness steadily corresponding to the reduction in the portion of water contributed by snowmelt and rain events. 127 Table 5.5 Operational Conditions of Pilot-scale Experiments using Free Chlorine or Monochloramine with and without Zinc Orthophosphate Parameter LOOP I LOOP 2 LOOP 3 LOOP 4 LOOPS Operating Temperature, 0C 20 20 20 20 20 Pipe velocity, ft/sec 1.0 1.0 1.0 1.0 1.0 Residence Time, minutes 120 120 120 120 120 Influent Carbon supplement, mg/LQD 0.25 0.25 0.25 0.25 0.25 Influent Free Chlorine feed, mg/L 0.70 0.70 0 0 0 Influent Monochloramine feed, mg/L® 0 0 0 0.70 0.70 Influent Zinc Orthophosphate feed, mg/L® 0 10.0 0 0 10.0 QD In addition to background carbon present in dilution water ® Part of monochloramine feed rate lost due to stripping activity in recycle tank ® Corrosion inhibitor dosage determined by OSCAR program As with the previous experiments, data from the last 4 sampling events were pooled together and analyzed using Fisher’s Multiple Comparison Tests with MINITAB™ software. Results from this analysis found no statistical differences between any of the disinfectant/corrosion control treatment systems. A summary of the phosphate, free chlorine, monochloramine, and biofilm densities are presented in Figure 5.8. Although the free chlorine residuals (Loops 1 and 2) were nearly twice as high as the monochloramine residuals Lo g( C Fl )Z cm 2) 8 7 6 5 -$-CI2 Only Control NH2CI + ZnP04 + CI2 + ZnP04 -S-NH2CI Only i i i i I l l l 0 10 20 30 40 50 60 70 80 90 Time, days Figure 5.7 Biofilm Densities for Pilot-scale Experiments using Free Chlorine and Monochloramine as Secondary Disinfectants with and without Zinc Orthophosphate 128 1.4 1.0 0.6 0.2 1 2 Loops I & 2 are free chlorine 3 4 5 J-oops 4 & 5 are monochloramine Figure 5.8 Biofilm Density, CI2Or NH2CI , and PO4 Levels for Experiments using Free Chlorine or Monochloramine as Secondary Disinfectants with and without Zinc Orthophosphate R es id ua l & P O 4 (m g/ L) 130 (Loops 4 and 5) there were no significant differences in biofilm densities. In view of this observation, it can be concluded that the monochloramine is more effective than free chlorine because the monochloramine residual was significantly lower and accomplished the same level of biofilm inactivation. In can also be concluded that the addition of corrosion control did not increase disinfection efficacy or increase biofilm densities. In addition to the above analysis, photographs of the coupons were also taken during the last sampling event and are presented in Figure 5.9. These photographs illustrate differences in disinfectant/corrosion treatments on the formation and types of corrosion products formed, but do not clearly identify significant differences in biofilm densities. 5.4 Key Findings o The use of some corrosion control practices can increase disinfectant efficacy by reducing the rate of leaching of Fe+2 from the pipe surface. Reducing the availability of Fe+2 decreases disinfectant demands, hence, raising disinfectant residuals downstream and allows the disinfectant to penetrate deeper into the biofilm/corrosion product matrix. o The reaction rates of some corrosion inhibitors are slow. Allow a couple of months before a final conclusion is made about the efficacy of a corrosion inhibitor. Clean Coupon Control LOOP 3 Cl2 Only LOOP 1 NH2CI Only LOOP 4 Cl 2+ Zinc Orthophosphate LOOP 2 NH2CI + Zinc Orthophosphate LOOPS Figure 5.9 Photographs of Coupons after Experiments using Free Chlorine or Monochloramine with and without Zone Orthophosphate 132 o In systems that have seasonal water quality changes, the most effective corrosion control program may differ from season-to-season. o Systems utilizing pH adjustment as a corrosion control method will likely experience an increase in free chlorine residual that results from a shift in equilibrium between HOCI and OCI'. Although the residual will be higher, the disinfection power of the residual will be lower, because OCI" is a weaker disinfectant. 133 Chapter 6 Miseellaraeoys Laboratory Stodlies Several miscellaneous experiments were conducted to evaluate specific interactions between corrosion products with organics and disinfectants. Additional studies were also conducted to determine the effect of high pH levels on Klebsiella pneumoniae, a coliform frequently found in distribution systems. 6.1 Adsorption and Bioavailability Studies of Humics and Corrosion Products As previously discussed in Chapter 2, many iron oxides are capable of adsorbing humic substances from the bulk fluid (Gu, et al 1994; Chang, et al 1997; Benjamin, et al 1993; Benjamin and Li 1997; Benjamin, et al 1990; Parfitt, et al 1977; Tipping and Cooke 1982; Characklis 1989). The adsorption properties of corrosion products are of interest because many older water systems are constructed of unlined ferrous materials that form iron oxides on the pipe surface. The presence of iron oxides (corrosion products) creates a system that can adsorb and concentrate humic substances on the pipe surface (van der Wende and Characklis 1990). Once adsorbed, the bioavailability of humics are increased by (a) uncoiling or collapsing on the surface (Gu, et al 1994; Chang 1992; Stumm and Morgan 1996) and by (b) removal of the loosely attached 134 humics by biosurfactants produced by microorganisms that are commonly found in distribution biofilms (Georgiou, et al 1992). 6.1.1 Experimental Design To investigate the humic adsorption and bioavailability characteristics of corrosion products, a two-step experiment was conducted. The first step was to allow corrosion products to adsorb humic substances, while the second step was to allow microorganisms to consume the adsorbed humics from the corrosion products. The first step of the experiment utilized a bench-scale upflow filter filled with corrosion products collected from one of the field type annular reactors described in Chapter 4. Once the filter was constructed, it was heat disinfected at SO0C for 24 hours to inactivate microorganisms that may be present and to minimize the chemical changes of the corrosion products. A sterile flow of ultrapure lab water (TOC less than 50 jug/L) was initiated to the filter unit to flush out any particulates. TOC measurements indicated that a significant amount of organics leached from the corrosion products. After a day of operation with ultrapure water, a humic solution with a TOC of approximately 2.9 mg/L was introduced continuously. TOC, total iron, and HPC samples were collected from the effluent over the next 7 days to assess humic adsorption, iron leaching, and biological activity within the filter. A schematic of the filter system is presented in Figure 6.1. Upflow Filter filled with Corrosion Products . . V V V?+*+*+I-++*+*+ + + + +• <• H * V'»+ t $ • f 4 ** t * * ' ' f t ■ - t t + t * - + » * * * > * » t * t - • a ~ Effluent Humic Substance Solution Figure 6.1 Experimental Setup for Corrosion Products Study 135 136 The second step of the experiment consisted of removing and air drying the corrosion products from the upflow filter. After air drying and UV disinfection, known masses of the corrosion products were added to 250-ml flasks containing 100 ml of AOC free water (collected from a BAC filter containing approximately 104 CFU/ml of heterotrophic microorganisms) and trace nutrients of phosphorus and nitrogen. The addition of phosphorus and nitrogen produced a water that was carbon limited, with the only carbon source originating from the adsorbed humics on the corrosion products. Each flask was then homogenized and placed in a shaker at room temperature. The flasks were then sampled over the next week for microbial growth. HPC populations from these flasks would then be compared to a control that did not contain corrosion products. 6.1.2 Results and Discussion Results from first step (adsorption step) indicate that corrosion products have humic adsorption capabilities that increase with time (approximately 30% removal on day 2 to approximately 60% removal on day 7). The increased TOC removal efficiencies may be attributed to the tails of adsorbed humic substances bonding with humic substances in the bulk fluid to further increase adsorption (Gu, et al 1994). The adsorption step of the experiment was terminated once the HPC counts in the effluent approached 900 CFU/ml. Since HPC counts were low, it can be assumed that the humics consumed by microbial activity was negligible. 137 The second step was used to determine if microorganisms, commonly found in distribution systems, could utilize the carbon adsorbed on the corrosion products. As illustrated in Figure 6.2 , the microbial growth was substantial after approximately 2 days for each flask containing corrosion products and minimal in the control flask (no corrosion products present). These results indicate that many microorganisms can readily utilize adsorbed humics found on corrosion products if sufficient trace elements (nitrogen and phosphorus) are present. 6.2 Batch-scale Adsorption Properties of Corrosion Products In addition to the adsorption/bioavailability studies presented in the previous section, adsorption studies were also conducted to quantify the adsorption isotherms of various corrosion products typically found in water distribution systems. 6.2.1 Experimental Design Since it has been previously found that the adsorption capacities of iron oxides increase with decreasing pH units (Gu, et al 1994; Parfitt, et al 1977; Chang, et al 1997; Benjamin and Li 1997; Chang 1992), all studies utilized a humic substance solution buffered with 200 mg/L of KHCO3 and adjusted to a neutral pH of 7.5 units. For this study, three surfaces were tested. These were magnetite (FesO^ Aldrich Chemical Company), samples of corrosion products collected from annular reactors, and virgin granular activated carbon (GAC) were used. Goethite (cc-FeOOH), the most common corrosion product found in 1.97 grams 2.02 grams 2.06 grams Control T im e, days Figure 6.2 Bioavailability Study of Adsorbed Humics on Corrosion Products 138 139 distribution systems (Singley and Ahmadi 1985; Smith, et al 1996), or siderite (FeCOs) were not directly evaluated, because they are not commercially available. Adsorption isotherms were determined by adding known masses of material to several glass fired media bottles (Corning Cat. No. 1395-100) containing a buffered humic substance solution. Each bottle, including a control that did not contain any adsorption material, was placed on a shaker table for 48 hours and allowed to reach equilibrium. Each sample was then centrifuged to separate particulate matter from the fluid prior to TOC analysis. TOC was then measured, using a Dohrman DC-80 Total Organic Carbon Analyzer, and the Freundlich adsorption isotherm estimated using the Least Squares Method. 6.2.2 Results and Discussion The virgin GAC had the highest affinity to adsorb humic substances. Although GAC is not a corrosion product, or found in distribution systems, it was used as a baseline measurement. Results from the GAC adsorption isotherm and for magnetite (FesO^ are illustrated in Figure 6.3 and Figure 6.4 respectively. Adsorption isotherms for siderite (FeCOs) and goethite (a-FeOOH) were not conducted because they are not commercially available or practical to make in the laboratory. In lieu of siderite and goethite, a collected sample of corrosion products from an unchlorinated annular reactor were collected and air-dried. The 0.9455 R = 0.83(XIM) = 0.3738C log (TOC, mg/L) Figure 6.3 Adsorption Isotherm for GAC and Humic Substances O6 7 17 2 (X/M) = 3 .1 7X 1 0 'C ' R =0.85 log (TOC,mg/L) Figure 6.4 Adsorption Isotherm for Magnetite (Fe3O4) and Humic Substances 142 adsorption test for this material was not successful, because the TOC in the bulk fluid increased with increasing corrosion product mass, indicating desorption. The desorption of organics indicates that the mass of concentration of carbon on the corrosion products was significantly higher than the TOC present in the initial bulk fluid (approximately 7.3 mg/L TOC). The release of TOC from the corrosion products indicates that adsorbed organics are in equilibrium with the bulk fluid, and that TOC desorption may occur when the TOC in the bulk fluid is less than the carbon concentration on the corrosion products. The release of TOC from corrosion products may explain why coliform events frequently occur when mineral levels are reduced (hardness and alkalinity) after a rain event (LeChevallier, etal 1996; LeChevaIIier 1990). 6.3 Disinfectant Demand Studies of Corrosion Products The reaction rates of corrosion products with disinfectants are of concern because it is thought that corrosion products rapidly consume disinfectants before they are able to penetrate to the deeper portions of the biofilm/corrosion product matrix (Geldreich 1996; Chen, et al 1993; van der Wende and Characklis 1990; LeChevallier 1990). Penetration of disinfectants (free chlorine or monochloramine) into a biofilm/corrosion product matrix is particularly important in distribution systems that contain large amounts of unlined ferrous pipes that contain copious quantities of corrosion products because niches that favor problematic microorganisms are found in the deeper portion of the biofilm/corrosion product matrix (Videla 1996; LeChevallier, et al 1993; 143 LeChevaIIier 1990; LeChevaIIier 1997; AWWA 1996; Costerton, et al 1987; Geldreich 1996). 6.3.1 Experimental Design Reaction rates of corrosion products were determined for both free chlorine and monochloramine. Procedures used to conduct the disinfectant demand studies are described in Chapter 3. Data collected.from the disinfectant demand studies were used to determine the first-order reaction rate using the Integral Method as described by Weber and DiGiano (1996). 6.3.2 Results and Discussion As indicated in Figure 6.5 and Figure 6.6, the first-order reaction rate for free chlorine was substantially higher than the reaction rate for monochloramine. These results indicate that free chlorine readily reacts with the corrosion products and the organics adsorbed on the corrosion products. This fast reaction rate may explain why free chlorine is less efficient at biofilm disinfection than monochloramine in pipeline systems that contain corrosion products on the pipe surface. The fact that the reaction rate of monochloramine is much lower than free chlorine enables monochloramine to penetrate deeper into the biofilm/corrosion product matrix, hence, achieving higher levels of biofilm inactivation. X = mass o f corrosion product, mg V = vo lume o f bottle, L C = ch lo rine concentration, mg/L 0 -0.5 -1 -1.5 -2 O C -2.5 -3 -3.5 -4 -4.5 0 0.5 1 1.5 2 Time, days Figure 6.5 Disinfectant Demand Study using Free Chlorine and Corrosion Products 144 In C /C v ' X = mass o f corrosion product, mg -------= —k — C V = vo lume o f bottle, L C = ch lo rine concentration, mg/L R = 0.9211 Time, days Figure 6.6 Disinfectant Demand Study using Monochloramine and Corrosion Products 145 146 6.4 pH Inhibition of Klebsiella pneumoniae As previously discussed in Chapter 2 (Literature Review) and Chapter 5 (Pilot-scale Studies), pH control is widely used in the water industry as a corrosion control method. A utility in Utica, New York utilized this corrosion control method and noticed that the percentage of positive coliform samples quickly reduced once a positive Langelier Saturation Index (LSI) was maintained (Schreppel and Geiss 1996; Schreppel, et al 1997). The converse was also true when the pH was maintained below the LSI, resulting in a reoccurrence of positive coliform samples. One interesting feature about these observations is that they occurred within a few days after a positive LSI was maintained. Considering the slow reaction times of corrosion control methods, it is highly possible that the decrease in positive coliform samples was not entirely related to the change of corrosion rates. A possible explanation to this sudden decrease in positive coliform occurrences is'that some microorganisms may be inhibited by high pH values. ,It has been suggested . (Martin, et al 1982) that Klebsiella pneumoniae is inhibited at pH values above 9 units. Considering that Klebsiella pneumoniae is one of the most commonly found conforms in water distribution systems (Geldreich 1996), the implementation of pH control as a corrosion control method may also inhibit Klebsiella pneumoniae, resulting in substantial reductions in positive coliform samples. 147 It is thought that most coliform occurrences result from cross connections within the distribution system (Geldreich 1996). Considering that Klebsiella pneumoniae are commonly found in soils (Alexander 1977), the probability of having groundwater infiltration are highest for systems with: (a) old leaky pipe joints, (b) high groundwater tables, and (c) systems with frequent problems with water hammer (LeChevaIIier 1998). Although systems normally operate under positive water pressure, the pressure drop associated with a hydraulic surge may produce a short duration negative (vacuum) condition that pulls contaminated water (possibly containing Klebsiella pneumoniae) into the distribution system (LeChevaIIier 1998). 6.4.1 Experimental Design To investigate if Klebsiella pneumoniae is inhibited by high pH values, chemostat experiments were conducted. For each study, pure cultures of Klebsiella pneumoniae were prepared as described in Chapter 3. Batch cultures and chemostats were operated in low carbon conditions (approximately 4 mg/L as carbon). Chemostats had a residence time of approximately 20 hours (growth rate of 0.05 hr"1). Low growth rates were used because previous research has shown that these types of cultures are more resistant to environmental changes (Camper 1995; LeChevallier, etal 1988; Camper, etal 1991). After culture preparation, each chemostat was inoculated and allowed to operate at a pH near 7 for approximately 4 residence times to establish steady- state growth conditions. The pH of the sterile nutrient feed was then changed to 148 produce an bulk fluid/effluent pH of 9.1 for the test chemostats, while the control chemostats continued to operate at a pH near 7 units. Since the chemostats are complete mix reactors, the pH change of the bulk fluid slowly increased until steady-state pH conditions were achieved. Bulk fluid colony counts (CFU/ml) and pH were monitored daily for 13 days. 6.4.2 Results and Discussion As illustrated in Figure 6.7, the cell density (CFU/ml) of Klebsiella pneumoniae was reduced by approximately 85 percent (0.9 log reduction) shortly after the pH exceeded 9. It is thought that high pH values cause the cytoplasmic membrane to dissolve, resulting in cell death (Brock, et al 1994). This finding is significant because many water systems operate at pH units slightly above 9 to obtain a slightly positive LSI. This finding may also explain why the Utica, New York system experienced significant reductions in positive coliform samples when the pH was maintained at levels that produced a positive LSI value. The experiment also found that pure cultures of Klebsiella pneumoniae are capable of producing acidic byproducts that resist changes of increasing pH units. In view of this, it is desirable to have finished water that has a high buffering capacity at this pH to insure that Klebsiella pneumoniae within the biofilm/corrosion product matrix can not locally resist pH increases. Lo g( CF UZ m I) an d pH Log(CFUZmI) of Control Reactor Effluent pH of Control Reactor Log(CFUZmI) of Test Reactor Effluent pH of Test Reactor 4 5 6 7 8 9 10 11 12 13 14 Time, days Figure 6.7 pH Inhibition of Klebsiella pneumoniae 149 150 6.5 Key Findings o Corrosion products can adsorb humic substances in the bulk fluid. o Microorganisms can readily use humic substances adsorbed to corrosion products. o The concentration of organic substances is substantially higher on corrosion products than in the bulk fluid. o Adsorbed organic substances on corrosion products are soluble in water. o The first-order reaction rate of free chlorine with corrosion products is significantly higher than the first-order reaction rate of monochloramine. o The lower first-order reaction rate of monochloramine with corrosion products probably enables monochloramine to penetrate deeper into the biofilm/corrosion product matrix providing a higher biofilm inactivation efficacy. o The numbers of Klebsiella pneumoniae, that may be present in distribution biofilms, can be reduced by increasing the pH above 9.0 units. 151 Chapter 7 Diseussoomi, ApplDcafiSomis and CorDelusiomis As shown in previous chapters, there are numerous factors that influence the growth and inactivation of distribution biofilms. This chapter will (a) discuss how the presence of corrosion products provides additional support mechanisms to distribution biofilms, and (b) identify several practical ways distribution biofilms can be minimized by process modifications and/or chemical additions at the water treatment plant or by chemical/disinfectant addition within the distribution system. 7.1 Biofilm Support Mechanisms Provided by Corrosion Products The presence of corrosion products on pipe surfaces has been shown by many researchers to strongly correlate with high biofilm densities (Camper 1994; LeChevallier, et al 1993; LeChevaIIier1 et al 1991; Martin, et al 1982; Crayton, et al 1997; McMath, et al 1997; van der Kooij and Oorhuizen 1997; Herson, et al 1991; Rice, et al 1991). This same observation has been made in the bench- scale testing (Chapter 4) and in the pilot-scale testing (Chapter 5) of this document. In view of these observations, the underlying questions are: o Why does the addition of chlorine sometimes increase biofilm densities on ferrous materials? 152 © Why does the implementation of a corrosion control program typically reduce biofilm densities? The answers to these questions are thought to lie in the physical and chemical properties of the corrosion products. It is therefore the purpose of this section to discuss the various properties of corrosion products and to explain how corrosion products support microbial biofilms. 7.1.1 Chemical Properties of Corrosion Products As previously discussed in Chapter 2, the formation of corrosion products result from the release of Fe+2 ions from the pipe surface that can react with various electron acceptors such as carbonate, oxygen, and free chlorine in the bulk fluid (Singley and Ahmadi 1985). Many publications indicate the reaction of Fe+2 with oxygen and carbonate, and acknowledge that chlorine reacts with Fe+2; but do not address the chemical pathways by which corrosion products are formed (Singley and Ahmadi 1985; Pontius . 1990; Benjamin, et al 1990). Balanced chemical reactions that form corrosion products along with their corresponding Gibbs Free Energy (AG0) were calculated and presented in Table 7.1. Although the possibilities of reactions are numerous, several may not be thermodynamically possible under typical drinking water conditions. For example, reactions with negative AG0 values indicate the reaction is thermodynamically possible, while reactions with positive AG0 values indicate the reaction is not thermodynamically possible at 25°C. However, it may be possible 153 Table 7.1 Partial List of Possible Reactions That Produce Corrosion Products. Equation Chemical Reaction AG 0 (Real) at25°C 1 Fe -> Fe2+(aq) + 2e -18 .8 2 2 Fe2+ + HOCI + H t -> 2 Fe3+ (aq) + H2O + Cl' -33 .5 3 2 Fe2* + OCI' + 2H + -> 2 Fe3* (aq) + H2O + Cl' -43 .8 4 2 Fe2* + CO 3- ^ F e C O 3 (siderite) -28 .6 5 4Fe2* + 4 P 0 43" + 0 2 + 4H * -» 4 F e P 0 4 + 2H 20 -196.2 6 Fe3* + P 0 43'+ 2 H 20 -» FePO4+ 2H20 -38 .5 7 2 Fe2* + O SO2 + 40H" -+ 2FeOOH (goethite) + H2O -104.0 8 2 Fe2* + HOCI + 50H" -> 2FeOOH (goethite) + 2H 20 + Cl' -133.4 9 2 Fe2* + OCI' + 40H " -> 2FeOOH (goethite) + H2O + Cl' +71.1 10 3 Fe2* + HOCI + 7 0 H -> Fe3O4 (magnetite) + 4H20 + Cl' +131.5 11 3Fe2* + OCI' + 60H " -> Fe3O4 (magnetite) + SH2O + Cl' +140 .3 12 3Fe2* +O SO2 + 60H " -» Fe3O4 (magnetite) + SH2O +160.9 13 2 F e C 0 3 + O SO2+ H2O -> 2FeOOH (goethite) + 2 C 0 2 -44.8 14 SFeCO3 + O SO2 -+ Fe3O4 (magnetite) + SCO2 -43.6 15 2 F e C 0 3 + HOCI + OH" -> 2FeOOH (goethite) + 2 C 0 2+ Cl' -74 .0 16 2 F e C 0 3 + + OCI" + H2O -> 2FeOOH (goethite) + 2 C 0 2 + Cl' -65 .2 17 2 Fe3O4+ O SO2+ SH2O -> GFeOOH (goethite) -46.7 18 2 Fe3O4+ HOCI + 2H 20 + OH" -> GFeOOH (goethite) + H* + Cl' -76.1 19 2 Fe3O4+ O C I'+ SH2O -+ GFeOOH (goethite) + Cl" -67 .3 20 FeOOH + S H * - ) . Fe3* (aq) + 2H 20 +2.2 AG0 = (ZyAG0)prOducts - (ZvzAG0)reaCtsnts. AG0 values obtained from (Snoeyink and Jenkins 1980; Stumm and Morgan 1996) 154 for reactions with a slightly positive AG0 to be thermodynamically favorable at temperatures other than 25°C or under extreme concentrations of various ions found in the bulk fluid. In these situations, the actual AG should be determined using the following equation: AG = A G " + f i r m ™ where the chemical reaction has the form of aA + bB cC + dD , AG0 is the value listed in Table 7.1, R is the gas constant, T is the absolute temperature, and [C] is the molar concentration of a specific ion. As identified in Table 7.1, there are many types of corrosion products that can be formed with carbonate (COz=), oxygen (Ga), and chlorine (HOCI and OCI ). Many of the reactions are pH dependent which can have a significant impact on the thermodynamic feasibility of the reaction or influence the reaction rate. A summary of the various corrosion product pathways is presented in Figure 7.1. In consideration of the various chemical pathways forming corrosion products, a corrosion tubercle will typically have a layered structure as illustrated in Figure 7.2 (Singley and Ahmadi 1985). Since systems with high corrosion rates are more likely to have coliform and other microbially related water quality problems (LeChevaIIier 1990; LeChevaIIier, et al 1996), it is necessary to investigate the physical and chemical properties of corrosion tubercles to understand how they support microorganisms. + 0 . or HOCI Pipe Surface oA O O O + U 2 Siderite (FeCO3) --------- ► Magnetite (Fe3O4) Figure 7.1 Corrosion Product Pathways 155 Direction of Flow_____ Pipe Surface Disinfectant or O2 Magnetite (Fe3O4) Goethite (a-FeOOH) Siderite (FeCO3) Figure 7.2 Typical Structure of a Corrosion Tubercle 156 157 The physical and chemical properties of corrosion products provide many benefits to microbial biofilms. The first benefit of corrosion products is the large surface area available for microbial attachment. Another physical property is that these corrosion products are very dense, which limits the diffusion of oxygen and disinfectants to the inner core of corrosion tubercles or to the original pipe surface. This diffusion limitation may lead to an anoxic condition, which promotes the formation of siderite, and produces a unique niche for coliform bacteria and sulfate reducing bacteria (SRBs) (LeChevaIIier 1990; LeChevaIIier 1997; van der Wende and Characklis 1990; AWWA 1996; Costerton, et at 1987; Geldreich 1996; LeChevaIIier, etal 1993). Corrosion products are reactive with oxygen and chlorine as presented in Table 7.1. The consumption of oxygen by corrosion products may produce an anoxic region within the biofilm/corrosion product matrix as described in the preceding paragraph. The reaction of corrosion products and the organics that they adsorb with chlorine (HOCI and OCI') can quickly consume the chlorine leading to a reduction in residual, or absence of chlorine or chloramines in the deeper regions of the biofilm/corrosion product matrix. When this conditions exists, microorganisms near the surface of the pipe wall are free from chlorine exposure and flourish without the possibility of inactivation. As illustrated above, the terminal pathway of corrosion products in a distribution system will typically lead to the formation of goethite. When chlorine is added as a disinfectant, the pathways of goethite formation are significantly 158 increased. The addition of chlorine, or other powerful disinfectants, is known to break down complex organics to a more bioavailable form of carbon, hence, increasing the potential of biological activity. The increased bioavailability of carbon substrate resulting from disinfection, -coupled with benefits of the physical and chemical (adsorption) properties of goethite, creates an environment that is well suited for biofilm survival. 7.1.2 Electrostatic Charges of Corrosion Products The electrostatic properties of the surface can explain why some materials adsorb nutrients along with supporting microorganisms. The. concept is simple: oppositely charged particles attract and like charged particles repel. The electrostatic charge of a surface can be determined by comparing the pH at the surface-to-water interface and the point of zero charge (PZC) of the surface. If the pH is above the PZC, the net surface charge is negative while a pH below the PZC results in a positive charge (Chang 1992; Montgomery 1985). The relative position of the PZC for corrosion products, CaCO3, humic substances, and microorganisms is presented in Figure 7.3. As illustrated in Figure 7.3, the type of corrosion product or passivating film can have a significant effect on the electrostatic attraction of microorganisms or humic adsorption capacity of goethite. For example, if the pH of the water is near a neutral pH (7.0 - 7.5), goethite will have a positive charge and humics and bacteria will have a negative charge, resulting in humic adsorption to goethite along with an attraction for microorganisms. Likewise, if the pH is adjusted to pH CaCO --}— [ BACTERIAHUMIC ACID <11IU r c r v / 4 Figure 7.3 pH of Zero Charge(PZC) for selected materials 159 160 approximately 9.0, CaCOs, humics, and microorganisms will all have negative charges, resulting in reduced humic adsorption capacity and biofilm density. It should also be noted that FeP04, a corrosion product formed by the use of phosphorus based corrosion inhibitors, has a PZC of approximately 3, hence, minimizing the electrical attraction of both humic substances and microorganisms. 7.2 Distribution System Biofilm Control Strategies Since it has been demonstrated that complete removal/inactivation of distribution biofilms is not possible, the challenge in the future will be how to control biofilms with the added constraints of maximum disinfectant levels and disinfection byproducts. It is therefore the purpose of this section to identify numerous factors that may reduce microbially related water quality problems caused by distribution biofilms. As discussed in previous chapters, there are numerous factors that influence the growth or control of distribution biofilms. These factors can be categorized in three basic categories: (1) factors beyond human control, (2) factors within human control, and (3) factors within human control but with substantial physical or financial constraints. The first category includes factors that are not financially practical or can not be controlled by human action. These include controlling the water temperature to below 15°C, and controlling events of nature such as rainfall 161 events and spring runoff. Since it is not possible to regulate these factors on a large-scale level, they are not considered to be a feasible method of biofilm control and not discussed further. However, the impacts of increased nutrient levels resulting from runoff can be minimized by optimizing the water treatment and disinfection processes as discussed in the next two categories. Categories 2 and 3 include the factors that are within human control, but may or may not be physically or financially feasible. These two categories will change from system to system based on the physical limitations of the water treatment plant or the physical limitations of the distribution system. Factors that fall in these two categories include: o Changing water supplies o Enhanced coagulation p Changing primary disinfectants o Treatment plant optimization o Advanced oxidation and biological treatment o Membrane treatment o Changing secondary disinfectants o Implementing a corrosion control program o Replacement or rehabilitation of unlined cast iron pipes within the distribution system o Distribution flushing programs 162 As identified above, there are numerous alternatives that can be implemented at a water treatment plant that will increase the biostability of the finished water. The first six items represent treatment alternatives that may increase biostability by reducing the nutrient (mainly organic carbon) availability to microorganisms within the distribution system, or by removing small particulate matter that is known to protect microorganisms from disinfection. The last four alternatives are typically made downstream of filtration. 7.2.1 Changing Water Supplies Most water utilities do not have the luxury of having more than one raw water source. For utilities with multiple raw water supplies, it may be possible to utilize the water source that has the highest biostability to minimize the growth of distribution biofilms. Although this practice may not be possible for some utilities due to water permitting constraints, it may be possible on a seasonal or daily basis depending on the characteristics of the watershed or aquifer. This alternative is particularly attractive to utilities with multiple surface water sources originating from different watersheds. These utilities can change water sources in response to increased nutrient concentrations and turbidities resulting from runoff events. 7.2.2 Enhanced Coagulation Enhanced coagulation is the optimization of coagulation to increase the removal of disinfection byproduct precursors by increasing the coagulant dosage, lowering the pH at which coagulation occurs, or both (AWWA 1998; Bryant, et al 163 1992). The goal of enhanced coagulation is to increase the removal of precursors. Precursors of disinfection byproducts are mainly humic substances (Geldreich 1996), which are also the major carbon source (food) for heterotrophic microorganisms. If enhanced coagulation is practiced at a water treatment plant, the resulting finished water will have lower disinfection byproduct formation potential and increased biostability. 7.2.3 Treatment Plant Optimization The practice of optimizing the performance of a water treatment plant to produce the highest quality of water at any given time should be the goal of every treatment plant operator and utility manager. The elements of plant performance optimization can include every process from rapid mix to . filtration. As with enhanced coagulation, the goal of treatment plant optimization should be to maximize organics and particulate removal in a manner that optimizes each physical and chemical process in the water treatment plant. Treatment plant optimization options include but are not limited to any single or combination of the following: o Avoiding prechlorination of raw waters o Increasing the performance of rapid mixing by changing coagulants, coagulant injection locations, and adjusting mixing intensities for optimum mixing. 164 o Installing streaming current monitors to continuously monitor coagulation efficacy. o Adding powdered activating carbon (PAC) to the rapid mix basin to enhance organic removal. o Optimizing flocculation by reducing short-circuiting and mixing intensities. © Optimizing sludge blanket composition (addition of PAC) and depth to maximize biological activity in upflow clarifier units. o Optimize particulate removal by installing tube settlers, reducing short- circuiting, reducing surface overflow rates, or by efficient sludge removal in conventional sedimentation basins. o Reduce or eliminate chlorination prior to filtration facilities. o Replacing anthracite with GAC to obtain long-term benefits of increased biological activity in filtration beds. o Optimize filtration rates to obtain maximum organic and particulate removal. o Increase monitoring of filter effluents by the addition of continuous on­ line of turbidity and/or particle counters. 165 7.2.4 Advanced Oxidation and Biological Treatment The use of advanced oxidants such as ozone and chlorine dioxide in water treatment has become increasingly popular in the past decade due to their lower formation of regulated disinfection byproducts and their ability to quickly inactivate microorganisms such as Giardia and Cryptosporidium (Geldreich 1996; Bryant, et al 1992; Pontius 1990; DeMers L.D. and Renner 1992). Although these disinfectants are more powerful than free chlorine and monochloramine, they are known to transform complex organic molecules into smaller more bioavailable molecules (Geldreich 1.996; Symons, et al 1982; Pontius 1990). As a result, the bioavailability of organic carbon will increase allowing for elevated microbial activity in downstream processes or within the distribution system. To prevent excessive microbial growth in the distribution system, the bioavailable carbon should be reduced prior to distribution. Biological filtration is an attached growth process in which microorganisms are allowed to colonize and grow on and within the filter media. As water passes through the media, the attached microorganisms consume a large portion of the bioavailable carbon from the water. After biological filtration, the water is disinfected and transported to the distribution system. The concept of biological filtration is simple; allow the microorganisms to grow in a controlled environment with controlled downstream disinfection. In the event that the microorganisms are not allowed to grow in a controlled environment, the substrate will enter the 166 next attached growth reactor (the pipe walls in the distribution system) and grow in an environment that can only be inhibited by a regulated disinfectant residual. The use of biological treatment is used extensively in Europe, and is gradually gaining acceptance in the United States (Geldreich 1996). The use of biological filtration downstream of advanced oxidation processes is needed to biologically consume nutrients (such as bioavailable organic carbon) prior to primary and secondary disinfection (Pontius 1990). Since the majority of bioavailable carbon is in the form of humic substances(Geldreich 1996; Kaplan, et al 1994), the removal of bioavailable carbon by biological filtration will not only increase the biostability of the finished water, it will also reduce the TTHM formation potential of the water. 7.2.5 Membrane Treatment The use of membrane technologies for water treatment is also gaining in popularity in the water industry. Membrane processes are attractive today because they can remove organics, bacteria, viruses, Giardia, Cryptosporidium and inorganic chemicals such as sulfates, nitrates, calcium, magnesium, and chlorides (Pontius 1990; AWWA 1998). The use of membranes enables utilities to remove organic precursors for disinfection byproducts as well as substrates for microorganisms in the distribution system. As a result, the membrane-produced water is very low in organics, hardness, alkalinity, total dissolved solids, and it’s biostability is very high. Since membranes remove numerous ions, including 167 alkalinity and hardness, finished water from membranes may have unusually high corrosive properties. 7.2.6 Changing Secondary Disinfectants Changing disinfectants in the distribution system can be a very effective biofilm control strategy (Kirmeyer, et al 1993). In the United States, surface water systems must use either free chlorine or monochloramine as a secondary disinfectant. As previously discussed in Chapter 2 and Chapter 6, monochloramine has been found to be a more effective disinfectant for inactivating distribution biofilms, because it is less reactive with corrosion products and is not known to react with EPS (LeChevaIIier 1990; Geldreich 1996; van der Wende and Characklis 1990). As a result of monochloramine’s lower reactivity, it is able to penetrate deeper into the biofilm/corrosion product matrix. Another major advantage of monochloramine is that it does not form as many regulated disinfection byproducts as free chlorine (Kirmeyer, et al 1993; Geldreich 1996; DeMers L.D. and Renner 1992; Symons, et al 1982; LeChevaIIier 1997). The conversion from free chlorine to monochloramine will require the installation of ammonia feed facilities at the water treatment plant (AWWA 1998). These facilities are significantly more complicated than typical chemical feeds and require moderate capital expenditures. The use of monochloramine is also known to cause nitrification problems in the distribution systems if excessive ammonia levels are present in the finished water or cause sporadic changes in 168 pH if the finished water is low in alkalinity (Kirmeyer, et al 1993). Nitrifying bacteria may be controlled by periodically changing over to free chlorine as a secondary disinfectant (AWWA 1996). 7.2.7 Implementation of a Corrosion Control Program In addition to reducing the leaching potential of lead and copper, the implementation of an effective corrosion control program has been found to increase disinfection efficacy within the distribution system by reducing the leaching of ferrous iron from the pipe surface (Geldreich 1996; Pontius 1990). . The benefits of corrosion control are numerous and include: (a) modifying the surface chemistry of the pipe surface, (b) reducing the habitat for microbial colonization, (c) reducing the disinfectant demand of the pipe surface, (d) reducing the carbon adsorption properties of corrosion products, and (e) minimizing the transformation of adsorbed humic substances to bioavailable carbon substrates. Corrosion control programs are very effective in distribution systems that contain significant quantities of unlined cast-iron pipes or unlined mild steel pipes. A corrosion control program will vary depending on water quality conditions, and the type of corrosion control mechanism utilized. Utilities that utilize pH adjustment will be required to install caustic soda feed facilities or lime slaking facilities. These facilities require moderate capital expenditures and can be difficult to maintain and operate. Process control especially can be a problem when low alkalinity waters are being treated. 169 Utilities that utilize corrosion inhibitors are required to install chemical storage and feed facilities. These facilities are not difficult to install or operate and require minimal capital expenditures. 7.2.8 Distribution Pipe Replacement and Rehabilitation As previously discussed, systems with unlined ferrous pipe materials may have significant microbially related water quality problems (LeChevaIIier1 et al 1996; LeChevaIIier 1990; Geldreich 1996). Systems with large quantities of unlined ferrous pipe (mostly unlined cast-iron pipe) were constructed prior to the 1950’s and are reported to be about 22 percent of all water pipes in use today (LeChevallier, et al 1996). Many of these systems are approaching or have already passed their useful lives, and should be replaced with modern pipe materials such as cement lined ductile iron pipe, pre-stressed concrete cylinder pipes, and PVC pipes. There are also ways to rehabilitate these unlined ferrous pipes with cement linings (Geldreich 1996) or synthetic coatings that significantly reduce the corrosion rates and increase the hydraulic performance of the distribution system. The implementation of a replacement or rehabilitation program is expensive and does not have favorable responses from the public. As a result, this strategy is seldom physically or financially feasible. In the event that it is not a desirable alternative, one or more of the previous listed strategies should be considered. 170 7.2.9 Distribution Flushing Programs Flushing programs have been implemented in numerous utilities to remove loosely attached corrosion products and sediments that accumulate in distribution systems (Geldreich 1996). 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