Influence of pollutant loading rate on seasonal performance of model constructed wetlands

Abstract

Constructed wetlands (CW) are a viable alternative wastewater treatment technology for many wastewater types. However, recommended loading rates vary widely between regulatory agencies. A greenhouse experiment was carried out for approximately 19 months to study the effect of loading rate, plant species selection, temperature and season on pollutant removal in bench-scale constructed wetlands. The wetlands were operated in batch mode at batch lengths of 3, 6, and 9 days, corresponding to loading rates of 210, 105, and 70 kg COD/ha·d, respectively. Greenhouse temperature cycled from 4°C to 24°C. Treatments included plant species Carex utriculata, Schoenoplectus acutus and Typha latifolia and unplanted controls. Water and air temperature, redox potential, COD, SO4 2-, NH4 +, PO4 3- and pore volume were monitored throughout the study. Data from the current research is compared with a previous study performed under similar conditions, but with a 20 day batch length resulting in a loading rate of 32 kg COD/ha·d. Performance of all treatments and loading rates was compared on the basis of percent COD and SO4 2- removal, redox potential, and remaining NH4 + and PO4 3- concentration.

There were strong interactions between loading rate, plant species and temperature. Within species, pollutant removal typically decreased with an increase in loading rate at all temperatures. Planted treatments generally improved pollutant removal at all loading rates and typically removed more NH4 + and PO4 3- at 24°C than at 4°C. However at lighter loading rates Carex and Schoenoplectus showed little temperature effect for COD removal, and had more SO4 2- remaining and increased redox levels at 4°C. However, as loading rate increased these species tended to have poorer COD removal at colder than warm temperatures. These results indicate that the ability of some plant species to increase aerobic respiration due to increased oxygenation in winter, and thus mitigate typical temperature effects on COD removal, is limited by higher organic load rates. Although not the focus of this study it was observed that wetland column porosity varied with season and with wetland age. Column porosity was lower for older columns and during winter.