Strategy to prevent calcification by restricting surface adhesion of Ca2+: Reduced affinity of extracellular polymeric substances for Ca2+ by mild acidic conditions

Controlling CaCO3 precipitation within anaerobic granular sludge (AnGS) is crucial for the anaerobic treatment of paper recycling wastewater. A viable strategy was proposed to control calcification by adjusting a mild acidic condition in an anaerobic reactor without hindering organic degradation. The results indicated that lowering the bulk pH (6.5 to 6.8) reduced calcium precipitation by 60.1 % in calcium-rich influent (Ca2+ 1200 mg/L) and eradicated CaCO3 deposition on AnGS. Extracellular polymeric substances (EPS) have proven to be crucial participants in Ca2+ migration. The acidic solution weakens the interactions between EPS and Ca2+ and then diminishes the EPS adsorption capacity and affinity for Ca2+. The mild acidic environment goes beyond reducing CaCO3 formation in wastewater. EPS protonation reduced the probability of Ca2+ adhering to the AnGS surface, which halted calcium transportation from bulk liquid to granule. This work offers a feasible strategy to prevent AnGS calcification in high-calcium wastewater.