Enhancement of aerobic granulation and nutrient removal by an algal-bacterial consortium in a lab-scale photobioreactor

This study aimed to evaluate the properties, nutrients removal and microbial community dynamics of algal-bacterial granules in a lab-scale photobioreactor treating synthetic domestic wastewater. Two identical bioreactors were operated under non-phototrophic (Rc, the control) and phototrophic (Rp) conditions (12 h/12 h dark-light cycles). The results showed that aggregation of algal and bacterial biomass occurred rapidly in Rp, possibly as a granule nucleus, which favored aerobic granulation. Mature algal-bacterial granules had a compact structure and large size as well as exhibited excellent settling properties. In addition, increased biomass production was facilitated in Rp compared to Rc. Notably, increased production of extracellular polymeric substances (EPS) was induced in Rp, especially aromatic and tryptophan-like substances, implying that proteins significantly contributed to the formation and stability of the algal-bacterial granules. Further investigations revealed that algal-bacterial granules provided resistance to temperature fluctuations and enhanced the removal efficiencies of organics and nutrients (nitrogen and phosphorus), probably attributable to the enrichment of the Betaproteobacteria, Gammaproteobacteria and Flavobacterium bacterial classes, and Chlorophyta, Trebouxiophyceae and Bacillariophyceae algae classes, whereas Cyanobacteria only occupied a small proportion. These findings demonstrated that the algal-bacterial consortium was conducive to accelerating aerobic granulation, enhancing system stability and improving nutrient removal efficiency.