Integrated anaerobic digestion and CO2 sequestration for energy recovery from waste activated sludge by calcium addition: Timing matters

Abstract

Anaerobic digestion (AD) with in-situ biogas upgrading is a technology that integrates methane production and selective CO2 sequestration in a one-pot process. This study investigated the effect of Ca2+ addition timing on thermophilic AD of waste activated sludge (WAS). CaCl2 solution was added to the digester on day 0, 2, 4, 6 and 8 at concentration of 3000 mg∙L−1. Ca2+ addition on day 4, after which exponential methane production started, achieved the best AD performance with cumulative CH4 production, maximum CH4 potential and maximum CH4 production rate increased by 20.8%, 20.8% and 50.2%, respectively. This treatment also remarkably reduced free ammonia inhibition and enhanced Methanosarcina abundance in the digester. A substantial increase (up to 58.4%) in the gas-phase CH4 to CO2 molar ratio was observed in the Ca2+-supplemented digesters, demonstrating the effectiveness of Ca2+ addition for CO2 sequestration. The relative change in the CH4 to gross CO2 ratio indicated the promoted hydrogenotrophic methanogenesis, possibly as a pertinent microbial mechanism behind the improved methane production.