Microalgal-bacterial granular sludge for municipal wastewater treatment under simulated natural diel cycles: Performances-metabolic pathways-microbial community nexus

In this study, a eukaryotic microalgae dominated microalgal-bacterial granular sludge process was explored for municipal wastewater treatment under simulated natural diel cycles. Different from previous studies with constant illumination, the excellent performances were obtained in terms of organics, ammonia-N and phosphate-P removal under simulated natural day-night conditions. It was found that about 94.9% of organics, 69.5% of ammonia-N and 90.6% of phosphate-P could be removed on average over a day-night cycle at a shorter hydraulic retention time of 2 h in the daytime, while 93.1% of organics, 62.5% of ammonia-N and 80.8% of phosphate-P removed at a hydraulic retention time of 4 h during the nighttime. Chlorophyceae were identified as the dominant microalgae, with Alphaproteobacteria and Sphingobacteriia being the major bacteria in MBGS. It was further revealed that microbial assimilation by microalgae and bacteria could serve as the main mechanism for the observed removal of organics, ammonia and phosphate, while the corresponding key metabolic pathways were also elucidated. Chlorophyceae were found to be the main player for the phosphorus removal via H+-exporting ATPase and H+-transporting two-sector ATPase, while Alphaproteobacteria were potentially responsible for the observed ammonia removal mediated by glutamine synthetase and glutamate dehydrogenase. It is expected that this study can offer an environmentally sustainable option for municipal wastewater treatment.