Synthetic wastewater containing wheat starch was treated using sequencing batch reactor (SBR) system working with bio sludge extracted from cow dung. In the first stage, full fac- torial design was used to investigate the impact of four hydrodynamic variables on process responses and optimum condition was detected. In the second stage, the most important hydrodynamic factor together with four different factors comprised a new design based on the response surface methodology and central composition design. Removal efficiency of starch (COD removal), effluent, and sludge quality were assessed as process responses. Impeller diameter and reactor geometry did not show strong impact on responses. Both designs showed superior results in predicting SBR performance. The analyses revealed that interactions between variables are crucially important and should be accounted in wastewa- ter management. Moreover, it was attained that independent factors have diverse effects on responses which intensifies the necessity of optimization on all dependent parameters. In the optimum condition of the second design, COD removal of 92%, TSS of 38 mg/L, and SVI of 57 ml/g were obtained.

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