Influence of inoculation ratio on the performance and microbial community of bacterial‐algal symbiotic system for rural wastewater treatment

Abstract In this study, co‐culture of microalgae and activated sludge in photobioreactors (PBRs) was investigated at different inoculation ratios (0:1, 0.3:1, 0.7:1, and 1.3:1 sludge wt./algae wt.) for rural domestic wastewater treatment under direct solar radiation. Effluent qualities (such as pH, NO 2 − , PO 4 3− , and NH 4 + ‐N concentrations) were assessed; bacterial and microalgal communities in co‐culture system were compared. The microalgal and bacterial biomass fraction played a significant role in the performance and microbial community structure of the treatment system. In reactors with inoculation ratio of 0.3:1 and 0.7:1, the pH exceeded 9 or 10 under solar radiation, which led to some functional bacteria being missing. In the reactor with inoculation ratio of 1.3:1, activated sludge effectively prevented excessive increase in pH in the reactor. Similar observations were made for reactors with inoculation ratios below 1.3:1 by adding sludge halfway through the process. The results show that activated sludge can inhibit excessive increase in pH caused by algal photosynthesis, maintain the activity of nitrite‐oxidizing bacteria in PBR, and reduce algae loss with the effluent. Practitioner Points Appropriate fraction of activated sludge can effectively inhibit the excessive increase in pH caused by algal photosynthesis in PBR. Adding activated sludge could maintain the activity of nitrite‐oxidizing bacteria in PBR, and reduce microalgae loss with the effluent. Considering the stability of operation and biodiversity in PBR, a 1.3:1 inoculation ratio of activated sludge and microalgae is preferred.