Performance and stability of sewage sludge digestion under CO₂ enrichment: A pilot study

Carbon dioxide (CO₂) injection in anaerobic digestion has recently been proposed as an interesting possibility to boost methane (CH₄) recovery from sludge and organic waste by converting a greenhouse gas into a renewable resource. This research assessed the effects of exogenous CO₂ injection on performance and process stability of single-phase continuous anaerobic digesters. Two pilot scale reactors treating sewage sludge were operated for 130days. One reactor was periodically injected with CO₂ while the other acted as control. Two injection frequencies and injection devices were tested. The results indicated that CO₂ enrichment allowed an increase in CH₄ production of ca. 12%, with a CH₄ production rate of 371±100L/(kgVS_fed·d) and a CH₄ concentration of ca. 60% when dissolved CO₂ levels inside the test reactor were increased up to 1.9-fold. Results also indicated an improvement in process resilience to temporary overloads and no impacts on stability parameters.