CO₂-driven algae-bacteria granulation for biological nutrient removal from different treated sewage effluents: role...

Residual nitrogen and phosphorus in treated sewage effluent (TSE) remain a critical barrier to safe water reuse in water-stressed regions. Algae-bacteria granular sludge offers a promising polishing technology, yet no prior study has demonstrated its feasibility using CO₂ as the sole externally supplied inorganic carbon source for treating real TSE. This study evaluated CO₂-driven algae-bacteria granulation in photo-sequencing batch reactors for biological nutrient removal from three real TSE types: ammonium-rich effluent from an anaerobic electrochemical membrane bioreactor and nitrate-rich effluents from an aerobic membrane bioreactor and an oxidation ditch, using activated sludge (AS) and eutrophic lake biomass (ELB) as inocula. Both systems achieved mixed liquor suspended solids of 4 g/L and sludge volume index at 5 min < 20 mL/g by day 80. In ammonium-rich TSE, both granule types achieved total inorganic nitrogen <10 mg/L via nitrification and algal assimilation, with ELB granules showing superior phosphate-P removal (effluent PO₄^3--P below 1-2 mg/L vs. 2-4 mg/L for AS). In nitrate-rich TSE, both granule types reduced NO₃^--N below 7 mg/L by the fourth solid retention time through partial denitrification and algal assimilation. Phosphorus removal was consistent with algal photo-assimilation and intracellular polyphosphate storage and normal bio-P removal for cellular homeostasis rather than classical enhanced biological phosphorus removal. Microbial community composition was strongly shaped by inoculum type and TSE nitrogen form. These findings provide a scientific foundation for developing CO₂-driven algae-bacteria granulation as a sustainable tertiary treatment technology for water reuse applications in water-stressed regions.