Enhancing granule formation: The role of chitosan and chitosan nanoparticles in microalgal-bacterial granular sludge development

Microalgal-bacterial granular sludge (MBGS) is an emerging wastewater treatment technology that enhances nutrient removal through the synergistic interactions of microalgae and bacteria. However, the slow granulation process remains a significant limitation. Therefore, this study investigates the effects of chitosan (Ch) and chitosan nanoparticles (Ch NPs) on accelerating MBGS formation and enhancing system performance. Three photo-sequencing batch reactors (PSBRs) were operated for 70 days, where R1 served as a control, R2 received chitosan, and R3 received chitosan nanoparticles at 5 mg L-1. The results revealed that Ch and Ch NPs significantly reduced granulation time, with granules forming in R2 by day 28 (33%) and in R3 by day 14 (67%) compared to R1 (day 42). Biomass retention was enhanced, with final MLSS concentrations of 6.27 g L-1 (R2) and 6.65 g L-1 (R3), surpassing R1 (5.83 g L-1). The extracellular polymeric substances (EPS) content was also notably higher in R2 (170.4 mg g-1 MLVSS) and R3 (180.6 mg g-1 MLVSS), contributing to improved sludge stability and flocculation. Nutrient removal performance was enhanced, with COD, NH4+-N, and PO43--P removal efficiencies reaching 98%, 99%, and 98%, respectively. The microbial community analysis indicated that Ch and Ch NPs selectively promoted Nakamurella, increasing its abundance from 0.26% (control) to 20.91% (R2) and 48.18% (R3), while also stimulating the growth of EPS-producing Micropruina and filamentous Streptomyces, further facilitating granulation. This study demonstrates that chitosan-based biopolymer addition significantly accelerates MBGS formation and enhances sludge stability, providing a promising strategy for optimizing wastewater treatment processes.