Treatment effect of rural domestic sewage in rural area using biofilm with two-stage reflux

Abstract: A new suit of equipment combined two-stage reflux continuing aeration with a biofilm process was developed to treat the rural distributed domestic sewage with the characteristics of low carbon source and comparative high nitrogen and phosphorus concentration. The treatment effects and operating mechanism were studied. The effective volume of this equipment was 270 m3, including the volumes of 207, 24, 14, 7 and 18 m3 for the sewage adjusting tank, the anaerobic hydrolysis tank, the contact oxidation tank, the secondary sedimentation tank, and the effluent circulation tank, respectively. The reflux sludge pump and the circulating water pump were located on the bottom of the secondary sedimentation tank and the effluent circulation tank, respectively. There was one gate for sludge discharge at the end of the water outlet of the anaerobic tank. The depth of the packed and combined filler was 1.0 m. The treating capacity of the equipment was 90 t sewage daily, with the hydraulic retention time (HRT) of 3.0 d. The returning rate is 2:1 and the packing ratio of bio-balls in the hydrolyzation-acidification stage and the contact oxygenation stage was 60% and 75%, respectively. After the sewage was once lifted by the water pump, it could be completed for all procedures by the way of self-flow. This equipment had kept stable running for over twelve months. The results showed that the removal rate of chemical oxygen demand (CODcr), biological oxygen demand (BOD5), ammonia nitrogen (NH3-N), total nitrogen (TN), total phosphorus (TP) and suspension solid (SS) were 75.7%, 84.8%, 69.2%, 68.0%, 58.9% and 87.9%, respectively. The average effluent concentration of CODcr, BOD5, NH3-N, TN, TP and SS were 51, 15.8, 1.8, 16.5, 2.3 and 15.7mg/L, respectively, which reached the criterions of the effusion for the rural sewage treatment of Tianjin. We found that oxygenation stage and hydrolyzation-acidification stage were mainly controlled by nitrification and denitrifying, respectively. The treatment process was more stable with less land occupation, lower cost, and easier maintenance and management. In conclusions, this procedure greatly improves the treatment efficiency and is suitable for treatment of domestic sewage in the rural areas.