Pathogen transport and capture in a porous media biofilm reactor

Abstract

Drinking water distribution systems pose the potential to transport biological and chemical contaminants to the consumers' tap that can be responsible for widespread waterborne disease outbreaks (WBDO). A need exists to improve the ability to monitor contaminants that can attach to the distribution system's interior surfaces and to obtain samples for diagnosing both the cause of a WBDO and the extent of contamination within the system. In this study, a porous media reactor colonized with a mixed-species drinking water biofilm was used to study the capture of Salmonella typhimurium as a model pathogen. Parallel reactors were operated under constant flow (CF) and constant head (CH) to compare flow-regime induced spatial variations in biofilm accumulation and the resulting pathogen capture. Parallel test reactors were operated with 0.5 mg/L supplemental carbon until the accumulation of biofilm in the CH reactor reduced the flowrate to the target sampling point (CF flowrate). Both test reactors were then inoculated with slug doses of approximately 3x109 CFU S. typhimurium. Effluent water samples were collected for five pore-volumes, followed by the destructive sampling of the reactor.

Plate counts were used to enumerate S. typhimurium present in effluent samples and captured within the reactor. Cell counts in effluent samples displayed an accelerated breakthrough compared with a non-reactive tracer. Compared with uncolonized control reactors (0.13%), colonized reactors (0.96%) captured significantly more cells. Despite spatial variations in biofilm accumulation, colonized CH and CF reactors captured comparable amounts of S. typhimurium. Increasing sampling duration to twenty pore volumes demonstrated greater retention of captured cells in the colonized reactors over the control reactors. S. typhimurium transport and capture was also observed in a 0.9 mm square flowcell packed with 100 mm beads using a confocal microscope. Interception and straining were responsible for capture on clean beads while biofilm accumulation narrowed pore throats sufficiently to allow for mechanical filtration to occur. This study demonstrates that using biofilm colonized porous media may be an effective tool to capture pathogens for monitoring drinking water distribution systems.