The generation of biological wastes such as cow dung and aloe vera waste (AVW) causes a serious ecological pollution. The microbial electrolytic cell coupled with anaerobic digestion (MEC-AD) system can make a rational utilization of these biodegradable organic wastes, which is of vital importance for alleviating environmental deterioration and reducing resource waste. Electrode materials and accelerants are the two major factors that affect methane production in the MEC-AD system. They affect microbial attachment and electron transfer in the MEC-AD system. Bio-based carbon materials are carbon materials prepared from biomass as raw materials. They have characteristics such as a rich pore structure, good chemical stability, biocompatibility, and controllable surface properties, which can be used as accelerants and electrodes in the MEC-AD system to optimize its performance. This study was to investigate the influence of biomass-derived carbon as an electrode and accelerant on the performance of the MEC-AD system, and the mechanism for increasing the production of biogas and methane was also analyzed, thus providing a basis for the multifunctional application of biomass-derived carbon in the MEC-AD system.

A series of experimental methods were adopted to study the MEC-AD system. Two types of bio-based carbon, i.e., aloe vera waste derived spherical carbon (AVW-SC) and porous carbon (AVW-PC), were synthesized via hydrothermal carbonization. The raw AVW material was washed with water, dried, ground, and subjected to hydrothermal treatment to obtain AVW-SC. After activating AVW-SC with KOH, it was carbonized in a tube furnace to obtain AVW-PC.

In the preparation of the electrodes, bio-based carbon (AVW-SC and AVW-PC) was mixed with 5% polytetrafluoroethylene powder in ethanol and deionized water, and then ground in a ball mill for 4 h to form a slurry. The slurry was evenly sprayed on the Ti mesh, dried and sintered in N₂ atmosphere at 360 ℃ to obtain Ti-SC and Ti-PC electrodes.

Four groups of experiments were conducted to determine the optimal voltage, compare different carbon electrodes, and explore the optimal coating amount. The MEC-AD reactor adopted 500 mL wide-mouthed glass bottles with a working volume of 400 mL. Each MEC-AD system received a co-substrate mixture of cattle dung and aloe vera waste and inoculum of sewage sludge in a mass ratio of 3∶7. Afterward, they were placed at (36±1) ℃ for 35 d. The biogas was collected by a water displacement method.

The materials were analyzed by characterization techniques such as X-ray diffraction (XRD) and scanning electron microscopy (SEM), and electrochemical tests were conducted on different electrodes. The composition, pH, TS, VS, TCOD and nutrient content of biogas were analyzed by standard chemical methods. Microbial community analysis was conducted using high-throughput sequencing technology. The modified Gompertz model was adopted to predict the kinetic parameters, and the coulombic efficiency and methane recovery rate were calculated according to a specific formula.

The result shows that AVW-SC is spherical and closely aggregated, while AVW-PC has a three-dimensional network structure, with average pore diameter of 9.77 nm. The electron exchange capacity (EEC) of AVW-PC (i.e., 0.75 μmol·e^-/g) is higher than that of AVW-SC (i.e., 0.15 μmol·e^-/g), indicating a better electron exchange capacity. These results indicate that AVW-PC provides more substrate and bacteria accumulation sites, and has better electron-donating and electron-accepting ability, thus improving the digestion efficiency.

In the MEC-AD system, using Ti mesh as an electrode, the effect of different voltages (i.e., 0, 0.4, 0.6, 0.8 V and 1.2 V) on the system performance is investigated, obtaining the optimum biogas production and organic matter degradation rate at 0.8 V. AVW-SC and AVW-PC are respectively coated on Ti mesh as electrodes. The results show that the MEC-AD system with AVW-PC coated Ti mesh as the electrode has a better performance. The electrochemical analysis shows that the electrode coated with AVW-PC has a larger specific capacitance and a smaller charge transfer resistance, indicating that AVW-PC can improve the electrochemical properties and electron transfer ability of MEC-AD system. The influence of coating amount (i.e., 0.025, 0.05, 0.10, 0.15, and 0.20 g) of AVW-PC on the MEC-AD system is investigated. At a coating amount of AVW-PC of 0.1 g, the cumulative biogas production and methane content of the Ti_0.8-PC_0.1 group both reach the maximum values. Different doses of AVW-PC (i.e., 0.10%, 0.15%, 0.20%, and 0.25%) are added as accelerants in Ti_0.8-PC_0.1. At the addition amount of AVW-PC of 0.20%, the Ti_0.8-PC_0.1/PC_0.2 group performs the optimum biogas production (i.e., 633.63 mL/g VS), methane content (i.e., 65.85%), and total nutrient content of biogas residue (i.e., 42.30 g/kg). In Ti_0.8-PC_0.1/PC_0.2, Bacteroidales, Pseudomonadales, Oscillospirales, Methanobacteraceae, Methanospirillaceae, Methanosarcinacea and Methanosaetaceae significantly increase. The increase in microbial diversity promotes interspecific hydrogen transfer (IHT), interspecific acetic transfer (IAT), and direct interspecific electron transfer (DIET), thereby enhancing methanogenic efficiency.

AVW-SC and AVW-PC were utilized as electrodes and accelerants to enhance methane yield in MEC-AD system. The Ti mesh electrode coated with different concentrations of AVW-PC achieved the optimal biogas production at 0.8 V. Specifically, the Ti_0.8-PC_0.1 combination could generate the maximum total amount of biogas and methane proportion. The Ti_0.8-PC_0.1/PC_0.2 combination exhibited the optimum performance (i.e., biogas yield of 633.63 mL/g VS, methane content of 65.85% and total nutritional content of 42.30 g/kg). High abundances of Bacteroidales, Pseudomonadales, Oscillospirales, Methanobacteraceae, Methanospirillaceae, Methanosarcinaceae, and Methanosaetaceae appeared in the Ti_0.8-PC_0.1/PC_0.2 group, compared to other groups. In addition, an increased microbial diversity led to an enhanced methane production through processes like DIET. This research could highlight the potential significance of AVW-PC as both electrode and accelerator for increasing methane production and provide a perspective for improving MEC-AD performance through multiple applications of biomass-derived carbon.