## 3 Results
### Performance of TB-MES with Domestic Wastewater  
Domestic wastewater was employed as feed to evaluate the viability of TB-MES for real wastewater treatment. With an adaption period of 19 days, its current generation gradually increased from 4.2 A m\({}^{-3}\) to 10.6 A m\({}^{-3}\), and finally stabilized at \(10.8\pm 2.2\) A m\({}^{-3}\) during the following 26 stable-operation days. Figure 3: Variation of DO concentration on different part of the bioachode at each recirculation rate. According to the polarization curve, a maximum power density of \(3.2\pm 0.2\) W m\({}^{-3}\) was achieved by the TB-MES (Figure 4), decreasing by 15.8% in comparison with that using artificial medium (\(3.8\pm 0.3\) W m\({}^{-3}\)). The reduction in power generation was probably because of the relatively low conductivity of domestic wastewater (\(850\pm 118\) s cm\({}^{-1}\)), which was much lower than the artificial medium of 6.8 mS cm\({}^{-1}\). Furthermore, due to the unbuffered condition in domestic wastewater, it might be difficult to maintain the pH in the range suitable for the growth of microorganisms [34]. Though the power production was reduced as the influent changed from artificial medium to real wastewater, as compared to some works, the TB-MES obtained the same or even higher magnitude of maximum power density with the benefits of saving energy input for aeration [35; 36; 37]. During the 19-day adaption period, COD removal gradually increased from 64.4% to 89.9%, resulting in the effluent COD concentration decreasing from 126 mg L\({}^{-1}\) to 37 mg L\({}^{-1}\). During the stable operation period, an average COD removal of \(90.7\pm 1.1\%\) was obtained by the TB-MES, with the COD concentration decreased from \(297\pm 30\) mg L\({}^{-1}\) in the influent to \(27\pm 4\) mg L\({}^{-1}\) in the effluent (Figure 5a). Coulombic efficiency of TB-MES was 57.1%, demonstrating more than a half of the organic pollutant (equivalent to 214.6 mg\(\cdot\)d\({}^{-1}\) COD) in domestic wastewater was used for electricity generation. The COD consumption related to electricity process might include COD recovery in the form of electricity and COD metabolized by exoelectrogens. About \(72.8\pm 3.5\%\) of the COD was removed in the anode chamber, resulting in an anode effluent COD concentration of \(78\pm 9\) mg L\({}^{-1}\). COD concentration at this relatively low level was suitable for biocathodes to function well in terms of electricity production, since direct contact with solutions containing high concentrations of organic matter would make the heterotrophic bacteria out-compete the cathodic autotrophic bacteria (using electrons from the cathode), which would lead to the decrease in electricity generation [38]. The nitrogen mainly existed in the form of ammonium nitrogen in domestic wastewater, with its concentration fluctuated around \(36\pm 5\) mg L\({}^{-1}\). Nitrate nitrogen was also detected in domestic wastewater, but the concentration was only \(2\pm 0.8\) mg L\({}^{-1}\). A stable ammonium nitrogen removal efficiency of \(84.6\pm 2.4\%\) was achieved by the TB-MES after being fed with domestic wastewater for nine days, indicating that microbial communities for ammonium degradation were well established (Figure 5b) The ammonium nitrogen concentration in anode effluent was \(29\pm 4\) mg L\({}^{-1}\), demonstrating that majority of the ammonium nitrogen was removed in cathode chamber. The ammonium loss in anode chamber was most probably due to ammonium diffusion through the cation exchange  
Figure 4: Power density and polarization curve of the TB-MES treating domestic wastewater. membrane connecting the anode and cathode chambers, although some losses may occur through biological nitrification and denitrification [39]. The total nitrogen (TN) removal achieved by TB-MES was \(70.1\pm 4.2\%\), resulting in the accumulation of ammonium, nitrate and nitrite nitrogen in the concentration of \(4.8\pm 0.8\) mg L\({}^{-1}\), \(5.7\pm 0.9\) mg L\({}^{-1}\) and \(0.6\pm 0.4\) mg L\({}^{-1}\) in the effluent.