Lab-scale experiment and model study on enhanced digestion of wastewater sludge using bioelectrochemical systems

Abstract

Anaerobic digestion is the slowest process in municipal wastewater treatment, requiring at least 15 days of SRT (solids retention time). Here, we implemented microbial electrolysis cells (MECs) in anaerobic digesters to shorten the long SRT requirement. The MEC bioanode oxidizes acetic acid while the cathode produces H2 gas. The electrode reactions can expedite acetic acid decomposition and thus enhance the rate of biosolids destruction because acetoclastic methanogenesis is known to be the rate-limiting step in conventional anaerobic digestion. A lab-scale electrically-assisted digester (EAD) with the MEC reactions was operated under a continuous fed-batch mode using raw wastewater sludge. Additionally, a steady-state model was developed by incorporating the MEC reaction in ADM1 (Anaerobic Digestion Model No.1 by International Water Association). In experiments, the EAD achieved 55% VSS (volatile suspended solids) removal and 61% COD (chemical oxygen demand) removal at a 6-day SRT while the control digester (built with the same electrode components but no MEC reactions induced) showed only 47% VSS removal and 50% COD removal. This result indicates that the SRT requirement can be substantially reduced by implementing the MEC reactions in mesophilic anaerobic digestion. Under a 14-day or 2-day SRT condition, however, the EAD did not show meaningful improvements on the COD and VSS removal compared to the control digester. Hydrogenotrophic methanogenesis was sufficiently rapid as H2 gas was not detected in produced biogas. The mathematical simulation results demonstrated that the MEC reactions substantially reduce acetic acid concentration and thus supplement the slow acetoclastic methanogenesis reaction.