## 3 Results  
### Effects of Catholyte Recirculation Rate  
The TB-MES was operated at a fixed HRT of 48 h during the start-up phrase, when the external resistor was gradually reduced from 500 \(\Omega\) to 10 \(\Omega\) (500, 200, 100, 50, 10 \(\Omega\)). Functional microbialcommunities on anode and bioachode were accumulated, and the current density stabilized at \(5.5\pm 1.8\) A m\({}^{-3}\) after an acclimation period of 500 h. As the anode effluent sprinkling to the cathode chamber, oxygen in the air could be carried into the cathode chamber by the anode effluent. In order to enhance this reoxygenation process, as well as to improve the agitation and mixing of substrates, the catholyte was circulated back to the spray nozzle by a peristaltic pump at a flow rate of 50, 200, and 400 mL min\({}^{-1}\). Data was recorded after the current generation became relatively stable, and results showed that current density gradually increased from \(6.2\pm 2.8\) A m\({}^{-3}\) to \(19.6\pm 4.6\) A m\({}^{-3}\) as the catholyte recirculation rate increased from 50 mL min\({}^{-1}\) to 400 mL min\({}^{-1}\). Polarization curves were conducted after a stable open circuit voltage was obtained, and the system achieved a maximum power density at a 10 \(\Omega\) resistor at each recirculation rate. Based on the polarization curve, the maximum power density increased gradually as the recirculation rate increased, following the same trend as the current generation. At a recirculation rate of 50 mL min\({}^{-1}\), the maximum power density of TB-MES was \(2.6\pm 0.1\) Wm\({}^{-3}\), which increased to \(3.8\pm 0.3\) Wm\({}^{-3}\) at 200 mL min\({}^{-1}\) (Figure 2) As the recirculation rate was further increased to 400 mL min\({}^{-1}\), a maximum power density of \(6.7\pm 0.5\) Wm\({}^{-3}\) was achieved by the TB-MES, which was 1.6 times higher in comparison with that at 50 mL min\({}^{-1}\). The improved performance in power generation at higher recirculation rate was probably due to the following reasons. Firstly, recirculation could generate shear effects, which facilitated the formation of thicker and denser biofilm, thus resulting in a better performance in TB-MES [30]. It is reported that, in a upflow MES, the maximum power density increased from \(3.43\pm 0.04\) Wm\({}^{-3}\) to \(7.11\pm 0.24\) Wm\({}^{-3}\) as the anolyte recirculation rate increased from 0 to 500 mL min\({}^{-1}\)[31]. In the TB-MES, the higher catholyte recirculation rate was beneficial to the acclimation of cathodic microbial community, which could strengthen cathodic oxygen reduction reaction (ORR) and, thus, improved the power output of the TB-MES. Secondly, the increased dissolved oxygen (DO) concentration that was available for the ORR might be another reason for the improved power generation of TB-MES. To investigate the effects of the catholyte recirculation rate on reoxygenation performance of the trickling-bed biocachode, the DO concentration on different parts of the biocathode was monitored at each recirculation rate (Figure 3). The DO concentration at each sampling point gradually increased as the recirculation rate  
Figure 2: Power density and polarization curves of the TB-MES at different catholyte recirculation rate (V: voltage, P: power density, 50, 200, 400: 50 mL min\({}^{-1}\), 200 mL min\({}^{-1}\), 400 mL min\({}^{-1}\)). increased. Taking the upper sampling point for example, the DO concentration at Point A increased from \(0.9\pm 0.3\) mg L\({}^{-1}\) to \(4.5\pm 0.8\) mg L\({}^{-1}\) as the recirculation rate increased from 50 mL min\({}^{-1}\) to 400 mL min\({}^{-1}\), indicating that higher recirculation rate could help larger amounts of oxygen dissolve in the catholyte. At each recirculation rate, the DO concentration gradually decreased from the upper sampling point (Point A) to the lower sampling point (Point D).