Comparison of effect and cost of five-compartment purification tank reactor and three-compartment septic tank in domestic sewage treatment

Three-compartment septic tank (TcST) with high concentrations of effluent COD, nitrogen and phosphorus would cause local water pollution or black odor when tailwater was directly discharged or overflowed. In view of the above problems, an equal-volume five-compartment purification tank reactor (FcPTR) was constructed and, taking actual domestic sewage as the treatment object, the operating characteristics and unit pollutant removal cost were compared and analyzed. The results revealed that when FcPTR operated continuously for 120 days, average concentrations of effluent COD, TN, \rm NH_4^+ \text-N, TP were 74, 26, 20, and 1.6 mg/L, respectively, which could reach Class Ⅱ of local standards for water pollutants discharge from rural domestic facilities that had been issued by 11 provinces (autonomous region, municipality directly under the Central Government) in China when applied in rural areas where the quality of the influent water was similar. The removal of nitrogenous and phosphorus-containing materials was promoted by the growth and metabolism of Proteobacteria on the filler and sludge in the anaerobic baffle zone and aerobic zone. Compared with TcST, FcPTR could significantly enhance the degradation of organic matter and denitrification and phosphorus removal, and improve the shock load resistance. According to cost accounting, when operating for 10 years with the same design volumes and operating scenarios, FcPTR could reduce the cost of 1 kg COD and 1 kg \rm NH_4^+ \text-N removal both by 19.8% compared with TcST.