Inoculation ratio effects on a self-sustaining microalgal-bacterial membrane photobioreactor: Mechanistic insights...

Self-sustaining microalgal-bacterial membrane photobioreactors (SSMB-MPBRs) have attracted growing attention due to energy conversation and carbon reduction potential. However, microalgae proliferation, which is essential for pollutant removal, exacerbates membrane fouling, and the underly mechanisms remain unclear. In this study, pollutant removal efficacy and membrane fouling in a SSMB-MPBR process were investigated at different inoculation microalgae to bacteria (M/B) ratios. The intrinsic mechanisms were examined based on algae-bacteria cooperative and competitive interactions. The results showed that pollutant removal efficacy and chlorophyll-a productivity increased, and decreased with the inoculation M/B ratio from 1:1 to 3:1, in line with the rate of algae-bacteria interaction. The highest pollutant removal efficiency (93.65 % of chemical oxygen demand (COD), 85.42 % of total nitrogen (TN), 91.65 % of total phosphorus (TP)) and chlorophyll-a productivity (0.82 mg/(L·day)) were achieved at inoculation M/B ratio of 2:1 due to the selective enrichment of microalgae and their competitions with predominant functional bacteria, which increased the upregulation of functional enzymes and relative metabolism. Similar trends were observed for the fouling behavior, which corresponded to the algae-bacteria competitive interaction. The increased involution stress promoted biosynthesis and hydrophobicity of extracellular polymeric substances (EPS), which, combined with small free microalgae, contributed to a compact bio-cake, and the most significant membrane fouling. These findings provide mechanistic insights into pollutant removal and membrane fouling and highlight the need to optimize algae-bacteria cooperative and competitive interactions to increase the efficiency and sustainability of the SSMB-MPBR process in treating municipal wastewater.