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In this experimental research, the continuous-upflow lab-scale combined A/O Moving-Bed Biofilm Reactors (MBBRs), with 50% A/O volume ratio and 100% internal NO3− recycling ratio, were constructed to treat 60 L/day of domestic wastewater in Basra Province (Southern Iraq) under full nitrification-denitrification processes. The A/O treatment system consists of Plexiglas square anoxic reactor with effective volume (15 litters), Plexiglas square aerobic reactor with effective volume (30 litters), and 250-litter Plexiglas square primary and final settling tanks. Cylindrical K1 plastic Kaldnes carriers with density of 0.93 g/cm3 were added to both reactors with 60% filling ratio to achieve the process of biofilm attaching. The A/O treatment system was operated under the batch mode for four weeks for development of biofilm; then the system was operated in continuous-upflow mode with total hydraulic retention time (HRT) equal to 18 hours and five different gas : water ratios (5 : 1,7 : 1,10 : 1,15 : 1, and 20 : 1) in order to investigate the effect of gas : water ratio on the total performance of A/O system. The study results illustrated that gas : water ratio has no effect on the removing of NH4+-N and COD, while it significantly affects the removing of TP and TN. The optimum value of gas : water ratio is 7 : 1, with the average value of removal efficiency (R%) of TP, TN, NH4+-N, and COD being 84.49%, 78.67%, 97.27%, and 95.56%, respectively. Under this value of gas : water ratio, the average values of dissolved oxygen (DO) in both aerobic MBBR and anoxic MBBR are 3.96 mg/L and 0.181 mg/L, respectively.

Performance of Combined A/O Moving-Bed Biofilm Reactors for Biological Nutrients Removal from Domestic Wastewater under Different Gas/Water Ratios