Nitrogen removal from domestic wastewater in a novel hybrid anoxic-oxic biofilm reactor at different reflux ratios

A lab-scale hybrid anoxic-oxic biofilm reactor (HAOBR) with one anoxic compartment and two oxic compartments in sequence was constructed to treat domestic wastewater in this study. Performance of the HAOBR was evaluated at 25°C and hydraulic retention time 12 hr with reflux ratio increased from 100% to 300% by stages. The results showed that COD, (Formula presented.), and TN removal presented an increasing trend with the increase of reflux ratio by stages. At the optimal reflux ratio of 300%, removal of COD, (Formula presented.), and TN averaged 83.9%, 99.0%, and 80.8%, respectively. Analysis about pollutant concentration in each compartment of the HAOBR revealed that the excellent pollutant removal was mainly achieved by the cooperation of nitrification in 3rd oxic compartment and denitrification in 1st anoxic compartment. Denitrification in the anoxic zone of 2nd oxic compartment would also contribute to the nitrogen removal. The higher nitrogen removal of the HAOBR at the reflux ratio of 300% is attributed to the presence of the anammox in the 1st anoxic compartment, which is mainly due to the lower COD concentration in the compartment at the higher reflux ratio. Practitioner points: A hybrid anoxic-oxic baffled reactor was built to treat domestic wastewater. Effect of reflux ratio and mechanism of nitrogen removal were investigated. Reflux ratio 300% was favorable for COD, (Formula presented.) and TN removal. The removal of COD, (Formula presented.) and TN averaged 84.4%, 99.0% and 80.8%, respectively. Cooperation of nitrification, denitrification and anammox dominated the high nitrogen removal at the higher reflux ratio of 300%.