Abstract
Return Activated Sludge (RAS) is the cornerstone of activated sludge treatment and research presented herein demonstrates the considerable impact it has on BPR process stability. Data collected at Moscow’s full-scale and scale model Water Resource Recovery Facilities (WRRFs), as well as a process model demonstrate that increasing the RAS ratio, in turn increases the RAS nitrate load through decreased secondary clarifier denitrification and increased RAS flow containing nitrate. Impacts of the increased nitrate load are apparent within the anaerobic basins in terms of decreased phosphorus release and PHA synthesis. The results indicate that RAS rate is an important process control parameter that can be used to increase BPR process stability and success. Additionally, the BPR Operator Survey responses gathered as part of this research indicate many highly regarded WRRFs practicing BPR are beginning to use novel RAS control techniques to enhance their systems. Further discussion with the operators of these WRRFs would undoubtedly lead to valuable insight that could then be analyzed and disseminated to benefit the BPR community as a whole. The findings from this research will hopefully shine light on an otherwise largely neglected process control parameter, the RAS rate.