Constant Hydraulic SRT Improves BNR Activated Sludge Performance

摘要

Control of Solids Retention Time (SRT) in the activated sludge process is critical for ensuringeffective wastewater treatment. SRT sets the growth rate of microorganisms in the activatedsludge process, thereby selecting the microbial composition of the mixed liquor and its settlingand treatment properties.The industry has developed various methods to control activated sludge. This study evaluatedseven (7) methods to calculate SRT and implemented a simple, straightforward hydraulic controlof SRT method. The study was conducted at the 25 mgd Seneca WWTP in Eagan, MN, whichoperates a plug-flow nitrifying activated sludge process with reduced air-flows in initial zones toenhance 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, theclarifier underflow rate, the waste sludge flow rate and aeration tank volume in service. Iteliminates the need for costly TSS sampling at various points in the process, along with itsassociated measurement variation and time lags. During this study, hydraulic control producedan SRT that agreed with conventional estimation methods, i.e. mass under aeration divided bymass flow rate wasted, when the conventional methods were corrected for solids inventory in theclarifiers.Hydraulic control of SRT provides a simple, real-time calculation for adjusting return sludgewaste flow rates to maintain a target SRT and a stable microbial population. At the SenecaWWTP, 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 thataveraged 4.6 mg/l TSS, 3.6 mg/l BOD, 0.58 mg/ l total phosphorus and 0.14 mg/l ammonia.