Context. Decentralized highstrength wastewater treatment can be accomplished in microbial electrochemical cells (MEC), in which anode-respiring bacteria (ARB) degrade organic matter to respire electrons to an electrode (anode), generating an electrical current that can be used for producing electricity or other beneficial chemicals.
Gap. Maximizing electrical current production from oxidized organics and preventing diversion to undesirable end products, such as methane, via understanding and engineering the associated microbial consortium, is important for MEC application.
Aim. To study the conversion of primary sludge-based synthetic blackwater (PS-BW) to quantify the proportion of electrons from COD recovered as electrical current vs. methane.
Methods. We operated five, two-chamber MEC; two were fed a 475 mg N/L ammonia PS-BW to simulate a combined blackwater (human excrement and urine combined, referred to as high-ammonia), two were fed a 40 mg N/L ammonia PS-BW to simulate a source separated blackwater (referred to as low-ammonia), and one was an inoculum-only control. We measured current, methane, and volatile fatty acid concentrations over time and performed 16S rRNA sequencing to identify the taxa contributing to the suspension and biofilm communities.
Findings. COD conversion to electrical current was 22.6% for the high-ammonia condition and 22.8% for the low-ammonia condition. Volatile fatty acids did not accumulate. Methane production was limited, less than 1% COD conversion to methane, but in simultaneously studied biochemical methane potential tests COD conversion to methane was 21.4% and 27.2% in the high- and low-ammonia conditions, respectively. These findings suggest the PS-BW inherently limited methane production in the MEC. The suspension microbial community consisted of a mix of fermentative bacteria and hydrogenotrophic methanogens, including Bacteroides, Bellilinea, Clostridium, and Methanobrevibacter. The biofilm communities were dominated by Geobacter.
Utilization. Maximizing electron recovery from blackwater will enable widespread implementation of MEC for decentralized treatment.
