Why SRT Control Is Not More Widely Practiced and Three Ways to Fix It: Making SRT Control a Reliable Tool for Operators to Use

Solids retention time (SRT) is the most important variable in determining the performance of an activated sludge system. Controlling SRT should therefore be one of the most important tasks of plant operators. However, one often finds that operators abandon SRT control in favor of sub-optimal parameters, such as mixed liquor suspended solids concentration or food to micro-organism ratio. Reasons for this relate to the common practice (in North America) to draw waste activated sludge (WAS) from the return activated sludge (RAS) conduit, which has the benefit of reducing the flow of WAS. To calculate the WAS flow for a given target SRT, it is common to use the ratio of mixed liquor suspended solids (MLSS) to RAS total suspended solids (TSS) concentration. Unfortunately, the RAS TSS concentration can vary significantly from day-to-day, resulting in highly variable WAS flow estimates. Using averaging to reduce variability adds significant lag to the control loop and typically achieves limited success.Three different strategies for overcoming this problem are discussed: (1) wasting mixed liquor, (2) using probes to continuously track MLSS and RAS TSS and (3) using a different calculation procedure which reduces daily variability significantly. The first has the benefit of providing simple, but accurate control, at the expense of increased WAS flow. The second allows for automation with the associated risk of probe failure. The third strategy can be implemented at most plants without changing the source for wasting or installing probes, but it does require accurate influent and RAS flow metering. All three strategies have been successfully employed to provide stable SRT control and improve secondary treatment performance while simplifying operation.