Optimizing microalgal-bacterial granular sludge performance with solar-LED nighttime light supplementation for improved wastewater treatment.

The natural diurnal cycle induces nocturnal metabolic decline in microalgal-bacterial granular sludge (MBGS) systems due to photosynthetic cessation, leading to pollutant accumulation. This study implemented solar-LED supplemental lighting during dark phases to address this limitation. Results showed significantly increased abundance of functional genes (glk, scrK, glnA, ppa, atpA) encoding key enzymes, coupled with enhanced dominance of Proteobacteria (from 52.6% to 89.3%). These adaptations drove improved chemical oxygen demand, total nitrogen, and total phosphorus removal efficiencies by 20.4%, 48.2%, and 55.2%, respectively, in real wastewater. Biochemical analyses revealed elevated superoxide dismutase activity indicating compensatory antioxidant defense against photo-oxidative stress, increased lactate dehydrogenase activity supporting enhanced glycolytic metabolism, and sustained ATPase activity maintaining energy flux and cellular integrity to collectively stabilize granular function. Solar-powered operation potentially reduces operational costs by 31% versus the conventional activated sludge process. These findings suggest that nocturnal light supplementation is a promising strategy for enabling sustainable MBGS-based wastewater treatment.