Retardation of sludge calcification by blocking the transportation of Ca2+ into anaerobic granular sludge

Calcification of anaerobic granular sludge causing a decrease in reactor performance has been the focus of many industries for decades, especially the paper industry. But effective strategies for calcification retardation are still in demand. This study aimed to verify the strategy to alleviate calcification by neutralizing granular surface charge with H+ and removing extracellular polymeric substances (EPS) bonded calcium by phosphate assisted chelation. The validity of the proposed strategy was confirmed by the calcium interception of three UASB reactors, which were operated under a high calcium level of 1200 mg·L−1 for 97 d. The result shows that calcium interception was restricted below 100 mgCa2+·L−1 at the controlled bulk pH at 6.7. The ash content in the sludge was controlled at approximately 60%. COD removal ratio maintained above 92%, and calcium interception strongly depended on protons (H+) rather than phosphate in bulk liquid. The unique three-layered structure (“surface biofilm-deposition layer-biomass”) of the granules was observed by scanning electron microscopy and micro-CT scanning. In addition, amorphous calcium carbonate (ACC) was found extensively in this particular structure. Compared to calcite, the higher solubility of ACC would be a new starting point to figure out Ca2+ ion diffusion in granular sludge.