How Effective is SRT Control for Eliminating Microconstituents?

Control of the solids retention time in secondary treatment has been proposed as a strategy for enhancing biodegradation to improve removal of microconstituents (MCS) such as pharmaceuticals and personal care products. The purpose of this study was to evaluate, using fate modeling, the effectiveness of SRT control for eliminating a variety of MCs during wastewater treatment, using three calibrated wastewater treatment plant configurations from Ontario, Canada. A suite of five target MCs, exhibiting a range of hydrophobic and biodegradable characteristics was selected for testing the effect of SRT on their elimination during wastewater treatment using the TOXCHEM predictive fate model. Mixed liquor suspended solids, waste activated sludge (WAS) suspended solids and WAS wasting rates were modified in the existing model layouts to provide the appropriate HRT and SRT values.The modeling results indicate that increasing SRT alone will not always be effective for enhancing biodegradation to eliminate MCs. Because the kinetics of biodegradation are not only dependent on the biomass concentration and population, as reflected by SRT, but also on the time available for biodegradation to occur, both hydraulic retention time and SRT in the biological reactor need to be assessed to maximize the contribution of biodegradation to overall contaminant removal. For some MCs, a higher HRT for the biological process may be as important as the SRT. The relative importance of SRT and HRT for eliminating MCs from treated effluents appears to depend on the physical, chemical and biological properties of the individual MCs.