Our study evidently proves that alkalinity effect on current density is not important in MXCs treating domestic wastewater. Instead, the biodegradability of the wastewater is significant for current density in the MXCs. At Run 5 (acetate and 5 mM bicarbonate buffer), the current density was recovered from 0.30 ± 0.1 A/m2 to 1.7 ± 0.2 A/m2. However, the current density learn more sharply dropped to 0.4 ± 0.15 A/m2 again at Run 6 (76% reduction against 1.7 ± 0.2 A/m2 at Run 5) in which SS collected from the domestic wastewater was added to acetate medium;

SS concentration was 230 ± 28 mg/L in the anode chamber, which is close to the average SS concentration in the domestic wastewater (260 ± 15 mg/L). This substantial reduction of current density at Run 6 shows that particulate

matters seriously prevent ARB from generating current in anode biofilm. Particulate substances can attenuate current generation find more via several routes. Particulates readily accumulate on biofilm anode [1,29], and the accumulated particulates can alleviate substrate diffusion from bulk liquid to biofilm anode, accentuating substrate limitation. In addition, particulates can mitigate the opportunity of ARB to proliferate on the surface of the anode or expel existing ARB from the biofilm due to space competition. The growth of non-ARB (e.g., fermenters or methanogens) present in SS can compete with ARB for substrate, and as a result current density can be decreased [4] and [28]. Particulates can also limit extracellular electron transfer, since their inert fractions accumulated on biofilm anode can deteriorate the conductivity of anode biofilm

matrix or bother the diffusion rate of shuttling compounds between ARB and the anode [30]. It is interesting to observe the slight increase of current density from 0.4 ± 0.15 A/m2 at Run Adenosine 6 (acetate with particulates) to 0.5 ± 0.15 A/m2 at Run 7 (raw domestic wastewater). This current density at Run 7 is even higher than 0.30 A/m2 observed at Run 3 and 4 (filtered wastewater with and without 50 mM bicarbonate buffer). The small increase of current density at Run 7 is not meaningful in terms of energy recovery, but seems to provide a clue on how to improve current density in MXCs treating domestic wastewater. Particulates added to the anode chamber at Run 6 mainly worked as physical/chemical barriers to ARB metabolism in anode biofilm or extracellular electron transfer, as discussed above. It is expected that air exposure during SS collection (30–45 min) would suppress the activity of anaerobic microorganisms present in SS, so the syntrophic interactions between ARB and non-ARB (fermenters and methanogens) would not be promoted well.