Effect of hydraulic retention time on microbial electrolysis cell treatment of high-strength primary sludge-based blackwater

Microbial electrochemical cell (MXC) technology shows potential for decentralized blackwater treatment in remote areas, such as space missions. MXCs use anaerobic microorganisms to break down waste organics into volatile fatty acids, which are converted to electrical current by anode-respiring bacteria. To optimize MXCs for blackwater treatment without chemical additions or pH adjustments for inhibition of methanogens, this study assessed the effects of hydraulic retention time (HRT) on high-strength synthetic blackwater treatment in microbial electrolysis cells (MECs). MECs were tested at HRTs of 12, 9, 6, and 3 days, with corresponding organic loading rates of 1.1, 1.3, 2.0, and 3.7 g COD/day-L. Current density increased from 34.5 A/m³ to 41.5 A/m³ as HRT was decreased, but COD conversion to electricity decreased from ∼20 % to less than 10 %. Lower COD conversion at shorter HRTs was due to insufficient propionate degradation, as propionate-degrading microorganisms were scarce. The microbial community in the suspension was more diverse at longer HRTs, but became specialized at shorter HRTs, with an increased presence of hydrolytic bacterium Bacteroides. Biofilm communities tolerated low pH, and included electroactive microorganisms such as Geobacter and an unidentified genus in the Geobacteraceae family.