Effect of high concentrations of ammonium-N on biofilm anode fed with a fermentable substrate

We tested the selective inhibition of methanogens at high NH ₄ ⁺-N concentrations (> 4 g/L), which led to an increased Coulombic efficiency (CE) in the anode of a continuous ethanol-fed (12mM influent) microbial electrolysis cell (MEC). We obtained optimum results using 6 g/L NH ₄-N by doubling the CE from 10% to 20%, compared to the control tests (0.4 g/L NH ₄ ⁺-N), at a short HRT of 5.4 hours. The increase in CE at 6 g/L NH ₄-N was accompanied by accumulation of acetate (7 mM versus 2 mM in the control), which suggests a positive syntrophic role of homo-acetogens. Inhibitory effects of NH ₄ ⁺-N (at 8 g/L) to a batch homo-acetogenic culture was minimal, confirming their positive role as H ₂ scavengers in the MEC anode. This work supports that high NH ₄ ⁺-N wastes provide an additional benefit to manage anode microbial communities fed with fermentable substrates for higher CE.