Ionic response of algal-bacterial granular sludge system during biological phosphorus removal from wastewater

Biological phosphorus removal (BPR) from wastewater can be generally realized through alternative non-aeration and aeration operation to create anaerobic and aerobic conditions respectively for P release and uptake/accumulation by polyphosphate accumulating organisms (PAOs), with P removal finally achieved by controlled discharge of P-rich sludge. In this study, the response of algal-bacterial aerobic granular sludge (AB-AGS) during BPR to main ions including Ac⁻ (acetate), Cl⁻, SO₄²⁻, NH₄⁺, K⁺, Mg²⁺, Ca²⁺ and Na⁺ in wastewater was investigated with conventional bacterial AGS (B-AGS) as control and acetate as the sole carbon source. Results show that BPR process mainly involved the changes of Ac⁻, K⁺, Mg²⁺, and Ca²⁺ rather than Cl⁻, SO₄²⁻, NH₄⁺ and Na⁺. The mole ratio of ΔP/ΔAc kept almost unchanged during the non-aeration (P release) phase in both B-AGS and AB-AGS systems (ΔP_B-AGS/ΔAc_B-AGS > ΔP_AB-AGS/ΔAc_AB-AGS), and it was negatively influenced by the light in AB-AGS systems, in which 62% of acetate was not utilized for P release at the high illuminance of 81 k lux. During the entire non-aeration/aeration period, both ΔK/ΔP and ΔMg/ΔP remained constant, while ΔK_AB-AGS/ΔP_AB-AGS > ΔK_B-AGS/ΔP_B-AGS and ΔMg_AB-AGS/ΔP_AB-AGS ≈ ΔMg_B-AGS/ΔP_B-AGS. The presence of algae seemed not beneficial for PAOs to remove P, while more K⁺ and P uptake by algae in AB-AGS suggest its great potential for manufacturing biofertilizer.