External resistance as a potential tool for bioelectricity and methane emission control from rice plants in hydroponic microbial fuel cell

Recently, hydroponic plant microbial fuel cell (H-PMFC) was incorporated into cultivation medium to decrease methane (CH4) production while generating bioelectricity. However, no information is available about the effect of the external/internal resistance (Rext/Rint) ratio on the performance of H-PMFCs. To study the possibility of controlling biomass production, bioelectricity generation, and CH4 emissions from hydroponic rice plants (Oryza sativa L.) under different Rext/Rint ratios. In this study three H-PMFCs were operated as follows: H-PMFC-A had external resistance (Rext) lower than internal resistance (Rint); H-PMFC-B had Rext equal to Rint, and H-PMFC-C had Rext higher than Rint. Results showed that H-PMFC-B (Rext = Rint) has the highest biomass yield. H-PMFC-C (Rext > Rint) displayed the highest average power density (PDmax), which was 4.77 and 1.23 times higher than H-PMFC-A and H-PMFC-B over 131 days of operation. The PDmax was obtained from H-PMFC-C of 536.79 mW/m3 when Rext/Rint = 150 %. The highest current with the lowest CH4 was produced by H-PMFC-A (0.14 mA and 453.52 ± 0.28 g/m2/h). The CH4 emission exhibits an inverse trend with current production, which decreasing with the Rext/Rint decreased from 50 % to 5 %. The total CH4 emission flux in H-PMFC-A was 1.42 and 2.07 times lower than H-PMFC-B and H-PMFC-C, respectively. Furthermore, the COD removal rate in H-PMFCs negatively correlates with Rext/Rint, and the Rext does not affect the COD removal rate when 50 % ≤ Rext/Rint ≤ 150 %. The above results indicated that the performance of H-PMFCs can be controlled by changing the Rext/Rint ratio of H-PMFC according to actual needs. If the goal is to achieve higher biomass production, then Rext = Rint; to obtain the highest PDmax, then Rext > Rint; to obtain the highest current and lowest CH4 generation, then Rext < Rint.