Contradictory effects of calcium on biological and membrane treatment of municipal solid waste leachate: A review and process optimization via biogas recirculation

Municipal solid waste (MSW) leachate is one of highly polluted wastewaters and requires pollution control for advanced treatment. However, high concentrations of calcium in the leachate would always affect the treatment performance. This review first summarized contradictory effects of calcium mainly on granule-based biological process and membrane fouling. From previous results, a calcium concentration of 1–100 mg/L was recommended to stimulate microbial growth and 10–150 mg/L to promote sludge granulation, while high-strength calcium (e.g., higher than 1000 mg/L) would severely suppress microbial activity or cause sludge mineralization. With respect to membrane treatment, 40–400 mg/L of calcium was found to mitigate membrane fouling with a lower or higher calcium content reported to aggravate the fouling. Excessive calcium in the MSW leachate needs to be removed and the carbonatation-based approach for calcium carbonate precipitation appears to be the most effective for the leachate. The carbonating techniques with diverse carbon sources were compared to remove calcium from waste streams, and in-situ biogas recirculation was demonstrated as a promising alternative. Through recirculating biogas into the MSW leachate, an optimized treatment was thus proposed by regulating appropriate calcium content to achieve a sustainable operational performance. Simultaneously, biogas purification by carbon dioxide capture and ammonia stripping by biogas sparging could be expected for better bioenergy recovery and reduced carbon dioxide emission. This novel process would be both economical- and environmental-effective, which supports a sustainable and energy-positive treatment for the MSW leachate.