Mitigating calcium inhibition in anaerobic digestion of organic sludge through injection of carbonate-forming additives.

Organic sludge (OS) contains high concentration of calcium, which can inhibit anaerobic digestion (AD) by limiting mass transfer between microbes and organics. In this study, we investigated the effects of carbonate-forming additives injection on methane production in AD of OS by mitigating calcium inhibition and clarified the underlying mechanisms. CO2, urea, and sodium bicarbonate were evaluated with focus on floc structure of OS and calcium dynamic, and energy analysis considered biogas-derived CO2 recycling. CO2 injection achieved highest methane production (113.9 mL-CH4/g-VS) and net energy gain (269 MJ/tonne-OS), representing 29 vol% and 9% increase, respectively, compared to control, while urea and sodium bicarbonate showed negligible effects. These improvements with CO2 injection resulted from dissolution of internal calcium decreasing particle size by 54% and increasing specific surface area by 8%. This study demonstrates the energetic feasibility of recycling biogas-derived CO2 to enhance AD of calcium-rich sludge, supporting the sustainability of waste-to-energy systems.