Performance analysis and evaluation of the 146 rural decentralized wastewater treatment facilities surrounding the Erhai Lake

Abstract The decentralized wastewater treatment is considered as an effective way to control the water pollution in rural areas and it has been rapidly developed in many countries. However, there is still a lack of analysis and evaluation of the operation of treatment facilities. Herein, rural decentralized wastewater treatment facilities of 146 sets distributed in ten villages and towns of the Dali City surrounding the Erhai Lake, were analyzed and evaluated for one whole year. The running status, treatment technologies including anaerobic-anoxic-oxic membrane bio-reactor (A2/O-MBR) facultative membrane bio-reactor (FMBR) and soil purification tank (SPT) were identified, and only 94 sets were in normal running status during the whole year of four times evaluation, with a total yearly serviceability ratio of 64.4%. The 94 sets facilities in normal running status, including 55 sets of A2/O-MBR, 33 sets of FMBR and 6 sets of STP, exhibited annual average removal efficiency of 73.7% total nitrogen (TN), 87.6% ammonia nitrogen (NH3-N), 72.0% chemical oxygen demand (COD), 83.6% biochemical oxygen demand (BOD5) and 56.9% total phosphorus (TP). The annual average standard-achieving ratio of 94 sets facilities for COD and BOD5 reached more than 70%, whereas it was 44.68% and 55.32% for TN and NH3–N. Particularly, it was merely 4.26% for TP, suggesting that how to improve TP treatment efficiency is the primary target in future research. The A2/O-MBR in normal running status could achieve an annual removal efficiency of 75.5% TN, 89.4% NH3-N, 64.1% TP and 72.8% COD, which demonstrated a significant advantage in organics degradation, denitrification, and dephosphorization. In contrast, the FMBR demonstrated a weaker capability for removing phosphates, whereas the SPT showed a lower nitrogen removal. On this basis, some suggestions in design, operation, and management are put forward.