The neglected ammonia leaching calcium in anaerobic granular sludge

Previous researches have primarily emphasized the deleterious impacts of NH₄⁺ on anaerobic granular sludge due to its biotoxicity. Despite this, the role of NH₄⁺ as a monovalent cation in leaching multivalent Ca²⁺, thereby hindering granule formation and undermining its stability, remains underappreciated. This study investigated the potential of NH₄⁺ to leach Ca²⁺ from anaerobic granular sludges. The results indicated that a shock loading of NH₄⁺ at a concentration of 900 mg/L caused a Ca²⁺ leaching of 57.1 mg/L at pH 7.0. In an acidified environment (pH 5.0), the shock loading resulted in a Ca²⁺ release of 127.3 mg/L, a magnitude 5.24 times greater than the control group. The leaching process modestly affected granular sludge activity and size but markedly compromised granular strength due to calcium loss. Subsequent to the NH₄⁺ shock, the granular strength manifested a significant reduction, as evidenced by a 15-fold increase in protein release from the granules compared to the intact ones. Additionally, NH₄⁺ shock altered the calcium partitioning within the granular sludge, resulting in a decrease in residual calcium and a concomitant increase in bound calcium, further affecting granular strength. This study underscores the overlooked significant phenomenon of NH₄⁺ shock-leaching Ca²⁺ in anaerobic granular sludge, which warrants significant attention given to its rapid and deleterious effects on granular strength and the shift in calcium state.