Primary sludge-based blackwater favors electrical current over methane production in microbial electrochemical cells

Abstract

Microbial electrochemical cells (MXCs) represent a technology that could potentially be used for decentralized blackwater treatment and resource recovery in remote locations, such as space. One challenge for blackwater treatment in MXCs is achieving sufficient conversion of complex organics to electrical current, especially by preventing electron diversion to dead-end products, such as methane. In this study, we evaluated the effect of source separation (treating feces and urine separately versus combined) on electron diversion to methane in MXCs. We operated five microbial electrolysis cells (MECs) in batch mode, two treating a synthetic primary sludge-based blackwater (PS-BW) with total ammonia nitrogen (TAN) concentration of 475 mg N/L representing combined blackwater (urine and feces), two treating PS-BW with a TAN concentration of 40 mg N/L representing a source separated blackwater (feces only), and one control fed only inoculum, which was anaerobic digester sludge. Chemical oxygen demand (COD) conversion to electrical current in the low and high TAN PS-BW fed MXC was 22% and 21%, respectively, while 1% or less was to methane. Biochemical methane potential assays demonstrated possible methane production from the PS-BW fed to the MXCs. Dysgonomonadaceae and Rikenellaceae dominated the suspension microbial communities of PS-BW fed MECs compared to the inoculum. Bacteria belonging to family Geobacteraceae, known anode-respiring bacteria, dominated the biofilms. Overall, PS-BW, whether combined or source-separated, appeared to limit electron diversion to undesirable methane likely due to the complexity of the PS-BW and the pre-establishment of a robust anode biofilm.