Integrated anaerobic digestion and CO_2 sequestration for energy recovery from waste activated sludge by calcium addition: Timing matters

摘要

Anaerobic digestion (AD) with in-situ biogas upgrading is a technology that integrates methane production and selective CO_2 sequestration in a one-pot process. This study investigated the effect of Ca~(2+) addition timing on thermophilic AD of waste activated sludge (WAS). CaCl_2 solution was added to the digester on day 0, 2, 4, 6 and 8 at concentration of 3000 mg·L~(-1). Ca~(2+) addition on day 4, after which exponential methane production started, achieved the best AD performance with cumulative CH_4 production, maximum CH_4 potential and maximum CH_4 production rate increased by 20.8%, 20.8% and 50.2%, respectively. This treatment also remarkably reduced free ammonia inhibition and enhanced Methanosardna abundance in the digester. A substantial increase (up to 58.4%) in the gas-phase CH_4 to CO_2 molar ratio was observed in the Ca~(2+)-supplemented digesters, demonstrating the effectiveness of Ca~(2+) addition for CO_2 sequestration. The relative change in the CH_4 to gross CO_2 ratio indicated the promoted hydrogenotrophic methanogenesis, possibly as a pertinent microbial mechanism behind the improved methane production.