Biogas recirculation for simultaneous calcium removal and biogas purification within an expanded granular sludge bed system treating leachate

Biogas, generated from an expanded granular sludge bed (EGSB) reactor treating municipal solid waste (MSW) leachate, was recirculated for calcium removal from the leachate via a carbonation process with simultaneous biogas purification. Batch trials were performed to optimize the solution pH and imported biogas (CO2) for CaCO3 precipitation. With applicable pH of 10–11 obtained, continuous trials achieved final calcium concentrations of 181–375 mg/L (removal efficiencies ≈ 92.8–96.5%) in the leachate and methane contents of 87.1–91.4% (purification efficiencies ≈ 65.4–82.2%) in the biogas. Calcium-balance study indicates that 23–986 mg Ca/d was released from the bio-system under the carbonized condition where CaCO3 precipitating was moved outside the bioreactor, whereas 7918–9517 mg Ca/d was trapped into the system for the controlled one. These findings demonstrate that carbonation removal of calcium by biogas recirculation could be a promising alternative to pretreat calcium-rich MSW leachate and synergistically to improve methane content.