Review of the mechanism and control methods of anaerobic granular sludge calcification

During the anaerobic treatment of high‑calcium (>800 mg/L) organic wastewater, calcium ions, carbonate ions, phosphates, and other substances combine on the biofilm surface to form calcium carbonate or calcium phosphate precipitates (two kinds), which affect anaerobic granular sludge (AnGS) mass transfer and occupy microbial breeding space, causing biomass reduction and seriously affecting the activity of AnGS, thereby affecting the treatment effect of anaerobic reactors. In this study, by analyzing the effect of calcium ions on physical properties, microorganisms, and extracellular polymeric substances (EPS), we elucidated the mechanism of AnGS calcification and the migration process of calcium precipitation into the interior of the AnGS, that is, the processes of surface adsorption of calcium ions, induced internal migration, crystal maturation, and formation of calcium deposition nuclei. Meanwhile, the control method of AnGS calcification, traditional physicochemical methods, the technique of microorganism-induced calcium precipitation to remove calcium, and methods for the removal of calcified AnGS are costly and cannot fundamentally solve the problem. By optimizing the types of flora and improving sludge activity, this type of method can fundamentally solve the calcification problem, and offers a new approach for resolving AnGS calcification.