Using Probabilistic Methods to Estimate the Risk of Nitrifier Washout and Set the Design SRT

Design safety factors are normally based on industry experience for developing a design robust enough to accommodate future variability and uncertainty in operating conditions and treatment requirements. The nitrification safety factor (NSF) is a widely applied heuristic (“rule of thumb”) safety factor used to set the design SRT of a nitrying activated sludge system. When the NSF is set at a “rule of thumb” value, the sources of uncertainty and variability are only qualitatively accounted for, potentially leading to an unnecessarily conservative and expensive design. Clean Water Services (CWS) chose to use a more rigorous approach to setting the NSF to determine the wet weather solids retention time (SRT) needed for nitrification. A probabilistic analysis was used to set the wet weather design SRT by quantitatively estimating the risk of nitrifier washout. The recommended wet weather design SRT was determined to be 8.0 days, compared to a design SRT of 8.5 days that would result from a “rule of thumb” NSF. The lower design SRT maximizes infrastructure investment because it increases the rated capacity of the secondary process while providing CWS with confidence that the system will perform under critical wet weather conditions.