Control of Solids Retention Time (SRT) in the activated sludge process is critical for ensuring effective wastewater treatment. SRT sets the growth rate of microorganisms in the activated sludge process, thereby selecting the microbial composition of the mixed liquor and its settling
and treatment properties. The industry has developed various methods to control activated sludge. This study evaluated seven (7) methods to calculate SRT and implemented a simple, straightforward hydraulic control of SRT method. The study was conducted at the 25 mgd Seneca WWTP in Eagan,
MN, which operates a plug-flow nitrifying activated sludge process with reduced air-flows in initial zones to enhance phosphorus removal. Hydraulic control of SRT is based on a solids mass balance over the aeration tanks and clarifiers. Specifically, it depends on the influent wastewater
forward flow rate into the aeration tanks, the clarifier underflow rate, the waste sludge flow rate and aeration tank volume in service. It eliminates the need for costly TSS sampling at various points in the process, along with its associated measurement variation and time lags. During this
study, hydraulic control produced an SRT that agreed with conventional estimation methods, i.e. mass under aeration divided by mass flow rate wasted, when the conventional methods were corrected for solids inventory in the clarifiers. Hydraulic control of SRT provides a simple, real-time
calculation for adjusting return sludge waste flow rates to maintain a target SRT and a stable microbial population. At the Seneca WWTP, hydraulic control of SRT, combined with gently air-mixed inlet aeration zones, simplified process control, reduced O&M costs, and produced an excellent
effluent quality that averaged 4.6 mg/l TSS, 3.6 mg/l BOD, 0.58 mg/ l total phosphorus and 0.14 mg/l ammonia.
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