Model-based analysis for step response characteristics of the anaerobic/anoxic/aerobic process

An anaerobic/anoxic/aerobic (AAO) process model was developed and applied to obtain step response signals of process variables including influent flow rate (Q_f), step feed ratio (SF), dissolved oxygen (DO) in the aerobic zone, waste activated sludge (WAS), returned activated sludge (RAS) and mixed liquor recirculation (MLR). Through sensitivity analysis and response time tests, the effects of these variables on process performance, as well as the dynamic characteristics of these variables were studied. Pollutants concentrations in the effluent were highly sensitive to Q_f, WAS, RAS and MLR. WAS demonstrated slow response speed and therefore was suitable for long-term adjustment of the process. DO in the aerobic zone was suitable for short-term adjustment of ammonia nitrogen concentration in the effluent. Total nitrogen concentration in the effluent was sensitive and fast responding to both RAS and MLR. Total phosphorus in the effluent was sensitive to RAS but insensitive to MLR. The functional biomass of the AAO system were greatly affected by Q_f. The most effective variables for heterotrophic and autotrophic biomass concentrations were WAS and RAS. The order of sensitivity of phosphorus accumulating organisms to variables was WAS > MLR> RAS > DO.