Promising Membrane Photobioreactor for continuous flow treatment of Low C/N wastewater: treating process, community structure, and cooperation mechanism

Low carbon/nitrogen (C/N) wastewater treatment is an emerging issue for conventional treatment technologies. To achieve this goal, a novel membrane photobioreactor (MPBR), integrating microalgae consortium (MC) and activated sludge (AS) phases, was investigated. The separation of the MC and AS phases could improve the removal of pollutants in low C/N wastewater, which could achieve the rapid denitrogenation through NH4+-N assimilation in MC phase, and effectively reduce organic matter concentration in AS phase, resulting in better nutrient recovery, lower costs, and stable pH. The results in phase I showed that the pollutants, such as chemical oxygen demand (COD), ammonium (NH4+-N), and total phosphorus (TP), met the Environmental Quality Standards for Surface Water (GB3838-2002) by MPBR. The AS phase with low DO, which could enrich heterotrophic bacteria (e.g., Bacteroidetes vadinHA17, Spirochaeta and Ahniella) and nitrogen cycling bacteria (e.g., Mycobacterium, Bacillus and Ellin6067), could promote the removal of COD and NO3--N in the continuous effluent. Based on the co-occurrence pattern analysis, the main influencing factor of MC and AS phases was the reduction of COD, which also illustrated the synergistic removal of organic matter by the cooperative interaction of microalgae and heterotrophic bacteria. From a genetic perspective, the internal circulation mode also enhanced microbial nitrogen metabolism in the MPBRs, with higher abundances of nitrification- and denitrification-related functional genes in the MC phase, and higher abundances of nitrification, dissimilatory nitrate reduction, and denitrification genes in the AS phase. This study provided an experimental and theoretical basis for MPBR in low-C/N wastewater treatment.