Effect of A²O-MBBR+CWs combined technology in treating rural domestic sewage

The rural domestic sewage is characterized by significant fluctuations in both quality and quantity. The traditional A²O (Anaerobic-Anoxic-Oxic) process faces challenges such as susceptibility to load shocks, instability in biomass concentration within the facilities, fluctuations in effluent quality, and poor operational stability. The A²O-MBBR+CWs combined process was developed using the A²O-MBBR process, subsurface flow constructed wetlands, and ecological ponds. In this study, we examined the actual operational treatment performance of the combined process, evaluated the impact of seasonal water temperature variations on its treatment efficiency, and analyzed the economic viability of the process. The results indicated that the maximum sewage treatment capacity of the test device was 180 L/d. Under the conditions of an air-to-water ratio of 6∶1-10∶1 and a mixed liquor recirculation ratio of 50%-150%, the combined process achieved average removal rates of TN, \mathrmNH_4^+ -N, COD and TP were 68.40%, 89.45%, 93.94% and 94.02%, respectively. The corresponding average effluent concentration was 11.69, 3.50, 26.90 and 0.22 mg/L, respectively. The effluent quality reached Grade A of the Discharge Standard of Water Pollutants for Rural Domestic Sewage Treatment Facilities (DB 34/ 3527-2019). The A²O-MBBR unit contributed the most to pollutant removal. Specifically, the removal rates of TN, \mathrmNH_4^+ -N, COD and TP achieved by baffled anaerobic-anoxic contact tank reached 44.25%, 59.46%, 43.38% and 32.61% respectively. Meanwhile, the aerobic membrane tank achieved removal rates of 9.55%, 24.24%, 14.69% and 59.69%, respectively. The removal of TP and COD was not affected by seasonal water temperature variations. Low water temperature in winter only impacted the removal of TN and \mathrmNH_4^+ -N. When the water temperature exceeded 12 ℃, the average removal rates for TN and \mathrmNH_4^+ -N were 75.61% and 95.70%. However, when the water temperature dropped below 12 ℃, the average removal rates for TN and \mathrmNH_4^+ -N decreased to 58.56% and 80.40%, respectively. But even when the water temperatures dropped to 0 ℃, TN and \mathrmNH_4^+ -N removal rates remained at 51.38% and 74.77%, respectively. The process can efficiently utilize organic matter in sewage as carbon source and ensure nitrogen removal performance at low water temperatures. The treatment cost of the combined process was approximately 0.46 yuan RMB per ton of wastewater during the study period.