Simultaneous removal of carbon and nutrients (CNP) in a single laboratory-scale bioreactor is advantageous in terms of reactor volume and energy consumption. In this study, an innovative up-flow anaerobic/anoxic/aerobic (A2O) single bioreactor with continuous feed and intermittent discharge (CFID) regime equipped with a movable aerator in the reactor height for simultaneous removal of CNP from soft drinks wastewater was successfully designed, fabricated and operated. The effects of four independent variables, i.e. hydraulic retention time (HRT), aerator height, biomass concentration and nitrogen/soluble chemical oxygen demand (N/sCOD) ratio at three levels in the range of 4–8 h, 37–55.5 cm, 4,000–6,000−1, and 0.05–0.2, respectively, on eight process responses were investigated. The central composite design (CCD) and response surface methodology (RSM) were applied to design the experimental conditions, model the obtained data, and optimize the process. The bioreactor provides three conditions with different dissolved oxygen (DO) (anaerobic, anoxic and aerobic) in a single bioreactor by placing the aerator in the middle of the reactor. As a result, the maximum sCOD, total nitrogen (TN) and total phosphorus (TP) removal were about 100, 92 and 41%, respectively. The optimum region obtained was an HRT of 5–11 h, a mixed liquor suspended solids (MLSS) concentration of 4,000–4,700 mgL−1, and an aerator height of 46.25 cm, at the N/sCOD ratio of 0.1.

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