H<sub>2</sub>O<sub>2</sub> Production in Microbial Electrochemical Cells Fed with Primary Sludge

We developed an energy-efficient, flat-plate, dual-chambered microbial peroxide producing cell (MPPC) as an anaerobic energy-conversion technology for converting primary sludge (PS) at the anode and producing hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) at the cathode. We operated the MPPC with a 9 day hydraulic retention time in the anode. A maximum H<sub>2</sub>O<sub>2</sub> concentration of ∼230 mg/L was achieved in 6 h of batch cathode operation. This is the first demonstration of H<sub>2</sub>O<sub>2</sub> production using PS in an MPPC, and the energy requirement for H<sub>2</sub>O<sub>2</sub> production was low (∼0.87 kWh/kg H<sub>2</sub>O<sub>2</sub>) compared to previous studies using real wastewaters. The H<sub>2</sub>O<sub>2</sub> gradually decayed with time due to the diffusion of H<sub>2</sub>O<sub>2</sub>-scavenging carbonate ions from the anode. We compared the anodic performance with a H<sub>2</sub>-producing microbial electrolysis cell (MEC). Both cells (MEC and MPPC) achieved ∼30% Coulombic recovery. While similar microbial communities were present in the anode suspension and anode biofilm for the two operating modes, aerobic bacteria were significant only on the side of the anode facing the membrane in the MPPC. Coupled with a lack of methane production in the MPPC, the presence of aerobic bacteria suggests that H<sub>2</sub>O<sub>2</sub> diffusion to the anode side caused inhibition of methanogens, which led to the decrease in chemical oxygen demand removal. Thus, the Coulombic efficiency was ∼16% higher in the MPPC than in the MEC (64% versus 48%, respectively).