Response of algal-bacterial aerobic granular sludge to wastewater pH fluctuation: Focus on extremely acidic and alkaline shocks

• Algal-bacterial granules have strong adaptability to influent pH 5–11. • Limited C utilization with substantial P/active components release occurred at pH 3. • No nitrification was detected at pH 3. • Algal-bacterial granules demonstrated gradual adaptability to pH 12 shock. • Microalgae play a crucial role in N removal under pH 12 shock. Algal-bacterial aerobic granular sludge (AGS) has shown great potential for sustainable wastewater treatment due to its compact structure and synergistic microbial activity. However, its tolerance to varying pH levels remains unclear. Since pH can directly influence microbial metabolism, pollutant removal efficiency, and system stability, understanding the impact of extreme pH fluctuations is crucial for assessing the feasibility of algal-bacterial AGS in practical wastewater treatment applications. This study systematically assessed algal-bacterial AGS under short-term influent pH shocks from 3 to 12. Nutrient removal performance, sludge characteristics, and environmental parameters (pH, dissolved oxygen, ion concentrations) were monitored to elucidate functional mechanisms. Dark-phase experiments were conducted to clarify microbial contributions. Granules maintained stable performance under pH 5–11. At pH 3, substantial loss of active components led to 66% phosphorus release and 36% organic matter loss, indicating severe functional suppression. In contrast, granules retained partial bioactivities including 22% PO 4 3- -P and 28% NH 4 + -N removal in the final 4 h when exposed to pH 12, reflecting some adaptability to alkaline conditions. Microalgae contributed to the system nitrogen removal and buffer capacity, while extracellular polymeric substances and Ca 2+ /Mg 2+ precipitation played vital buffering roles. These findings provide theoretical guidance for applying algal-bacterial AGS in pH-fluctuating wastewater treatment.