Effect of reflux ratio on COD and nitrogen removals from coke plant wastewaters

A laboratory-scale anaerobic-anoxic-aerobic-moving bed biofilm reactor (A1-A2-O-MBBR) system was undertaken to treat coke plant wastewaters from two different factories (wastewater A and B). Wastewater B had higher BOD₅/COD ratio and COD/TN ratio than wastewater A. The effects of reflux ratios on COD, TN and NH₃-N removals were studied. Results indicated that, with the reflux ratio increased from 2 to 5, COD removals of wastewater A and wastewater B increased from 57.4% to 72.6% and 78.2% to 88.6%, respectively. Meanwhile, TN removals were also increased accompanying reflux ratio rise, from 53.1% to 74.4% for wastewater A and 64.2% to 83.5% for wastewater B. At the same reflux ratio, compared with wastewater A, higher COD and TN removal efficiencies were observed in wastewater B, which had higher BOD₅/COD and COD/TN ratio. Reflux ratio had no significant influence on NH₃-N removal; 99.0% of the overall NH₃-N removal efficiency was achieved by the system for both coke plant wastewaters at any tested reflux ratio. MBBR was effective in NH₃-N removal, and about 95% of the NH₃-N was removed in the MBBR.