In the current study the influence of six operating parameters including impeller num- bers, hydraulic retention time (HRT), mixed liquor volatile suspended solids (MLVSS), influent COD concentration, glucose fraction, and diffuser diameter were investigated on organic removal and sludge settleability in an aerobic sequencing batch reactor (ASBR). Full factorial design (FFD) was used as an initial stage to screen the key parameters. FFD showed that two variables were insignificant and the curvature eliminated the possibility of further implication of this method. In the second stage, response surface methodology (RSM) was utilized to model, interpret the effects, and also optimize variable for achieving target conditions. Various tables, plots, and contours validated the model and implied the impact of variables on responses. All considered parameters of the RSM demonstrated a marked effect on COD removal and sludge volume index (SVI) as process responses. And the interaction between influent COD and HRT exerted a significant effect to both dependent variables. Ultimately, several optimization strategies were performed, resulting in diverse optimum conditions for different purposes. For influent COD of 9000 mg/L, MLVSS concentration of 5000 mg/L, HRT of 12 h, glucose fraction of 0.8, maximum COD removal of nearly 93% and SVI of approximately 57 mL/g were obtained. This optimum conditions were further proved using synthetic and real wastewater.

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