Characteristics of rural sewage discharge and a case study on optimal operation of rural sewage treatment plant in Shaanxi, China

Abstract

Rural wastewater treatment has become an important issue in China. This study investigated 76 rural wastewater treatment plants (WWTPs) in Shaanxi over a one-year period. We found that although rural sewage is suitable for biological treatment (BOD5/COD = 0.38–0.41), it has an obvious intermittent flow cutoff (5–19 h/d) characteristic, resulting in low sewage loading and poor pollutant removal in rural WWTPs. In order to improve operation efficiency and increase economic benefits of the existing rural WWTPs, a pilot-scale A2/O bioreactor was established, and the effect of rural sewage discontinuity (cutoff duration 10 h/d) on A2/O process was studied. An intermittent aeration and dissolved oxygen (DO) regulation operation strategy was developed. The results demonstrated that ammonia oxidizing bacteria (AOB), nitrite oxidizing bacteria (NOB), and polyphosphate accumulating bacteria (PAOs) were all enriched when Taeration:Tstop = 5 h:5h and DO = 2.0–3.0 mg/L. During the subsequent process of reducing DO concentration to 0.5–1.0 mg/L, AOBs (2.43 %) and PAOs (1.91 %) were enriched and NOBs (1.61 %) were panned. The transformation of functional bacteria in the system stimulated partial nitrification and denitrification phosphorus removal (DPR) (nitrite accumulation rate = 46.56 ± 6.88 %, DPR rate = 43.15 ± 3.54 %), which improved the nutrient removal rates. Finally, the optimal operation strategy was applied to a typical town-level WWTP in Ankang, China. The removal rates of NH4+-N, TN, and TP increased from 85.85 ± 1.31 %, 59.77 ± 2.86 %, and 23.91 ± 1.77 % to 94.17 ± 2.02 %, 65.32 ± 4.75 % and 61.05 ± 2.34 %, respectively, and the operating cost was reduced by 118,600 CNY (17,541 USD) per year.