Comparison of methods for solids retention time determination and control

Solids retention time (SRT) is the basic and most important process parameter in design, operation and control of activated sludge (AS) systems, especially with nitrification and biological nutrient removal. However, determination of SRT at the full-scale wastewater treatment plant (WWTP) may not be straightforward and the significance of proper SRT control is often underestimated by plant operators, which may lead to severe operational problems, including complete nitrification failure. This paper presents different methods for SRT determination and control, including simple hydraulic method and phosphorous mass balance method. The comparison is based on an extended data set from a large WWTP in south-west Poland, both in stable and transient operational conditions. The results of the comparison show that during stable operational conditions, all the SRT determination methods give similar results, thus the simple hydraulic method can be applied during standard operation, even at small WWTPs, which lack laboratory facilities. Hydraulic method is based on continuous on-line flowrate measurements, therefore it gives opportunity to implement an algorithm to automatically control SRT and sludge wastage from the system.