Application of the central composite design for the treatment of soft drink factory wastewater in two-stage aerobic sequencing batch reactors combined with ozonation.

In current study, a two-stage aerobic sequencing batch reactor (SBR) at laboratory scale was used to investigate the removal of chemical oxygen demand (COD) of a soft drink factory wastewater. The experiment was settled in series and has been combined with ozonation unit as a middle step. We evaluated the effect of three significant independent variables on the process including: mixed liquor suspended solids in both SBRs (MLSS1 and MLSS2) and feed-gas/off-gas ozone concentrations. MLSS was limited to the 3,000, 4,500, and 6,000 mg/L, for both SBRs. Central composite design matrix (CCD) was used for the experimental design and response surface methodology (RSM) was employed to optimize the experiments. The results showed that using ozonation as a middle stage between two-stage SBR significantly affects COD removal of the soft drink factory effluent. In addition, multistage biological process revealed that MLSS in the first-stage SBR (i.e. MLSS1) influences the COD removal efficiency. We also found that due to a reduction in organic matter of effluent in first SBR, MLSS in the second stage SBR (MLSS2) is less important than MLSS1. Maximum COD removal efficiency (up to 95%) was obtained for the following conditions: MLSS1 6,000 mg/L, MLSS2 4,500 mg/L, and employing ozone. Based on a very good matching between predicted values of the RSM model and the experimental values (R² = 0.989 and Adj-R² = 0.981), we suggest that our model can be considered for future optimization of the COD removal. [ABSTRACT FROM AUTHOR]