Black water collected from the septic tank treated with a living machine system: HRT effect and microbial community structure

Abstract

In this study, the performance of a living machine (LM) system was evaluated for use in the treatment of black water collected from septic tanks with hydraulic retention times (HRTs) of 6, 5, and 4 days. We found that the HRT had little effect on the removal efficiency of chemical oxygen demand (COD). However, the removal rates of total nitrogen (TN) and ammonium nitrogen (NH4+-N) decreased with the reduction of HRT, whereas the removal efficiency of total phosphate (TP) was consistently low because of the long sludge retention time. The working conditions of #1 achieved the highest removal efficiency of COD (85%), NH4+-N (75%), and TN (47%), although the removal efficiency of TP (11%) was slightly lower than that of #2 (12%). The microbial communities in each tank of the LM system were characterized by high-throughput sequencing, which showed that the LM system successfully created more favorable conditions for fermentative bacteria than traditional systems, with relative abundances of 13% (#1), 13% (#2), and 15% (#3) compared to that of the anaerobic/anoxic/oxic (A2O) system (<3%). Smithella was the dominant fermentative bacteria, accounting for 9% (#1), 7% (#2), and 10% (#3) of total bacteria in the LM system. The relative abundances of ammonia oxidizing bacteria (AOB) (12%) and anaerobic ammonium oxidizing bacteria (AnAOB) (7%) in the LM system were much higher than that in the A2O system. Overall, the LM system offered a more sustainable and economical solution for treating black water.