A hybrid system of microbial electrolysis cell and anaerobic digestion for biomethane production

Abstract

Food waste (FW) poses a significant global challenge due to the increasing population. Anaerobic digestion (AD) was a commonly used process to convert organic waste into biogas, primarily methane (CH4). However, CH4 production in AD was often low. To upgrade CH4 production, a hybrid microbial electrolysis cell (H-MEC) system combining AD was used. In this study, two types of fungal strains (TNAF-1 to TNFA-3 and TNBC-1 to TNBC-3) were isolated from animal feed and compost. The enzyme activities such as cellulase, and amylase of 300U/mL and 400U/mL, respectively were produced by the selected strains. Optimization using the face centered central composite design (FCCCD) under the response surface methodology (RSM) was conducted to increase the reducing sugar production to 162 mg/mL under the optimized conditions of pH 5, Total solids (TS) of 12.5%, and enzyme loading of 80 U/mL. The biogas production was optimized using one factor at a time (OFAT) method with the parameters of inoculum of 25%, pH 7, digestion times of 29 days, 500mL of hydrolysate food waste, and temperature at 30°C (±2), resulting in a biogas composition of 3% H2, 57% CH4, and 40% CO2. To address energy efficiency and sustainability, the H-MEC system was designed based on electromethanogenic microbes (EMMs) for enhanced biogas upgrading. EMM strains (TNFW-1 to TNFW-3 and NTAS-1 to NTAS-3) were isolated from AD using food waste and anaerobic sludge samples, respectively. The EMMs demonstrated the efficient CO2 conversion in a dual-chamber method. The strain, TNFW-2 produced supernatants with a 92% BioM (biomethane) production rate from the direct gas phase (CO2/H2). Optimal growth conditions were determined, yielding a 92% BioM yield with substrate dose of 100mL, inoculum dose of 10mL, flow rate of 5L/hour, H2/CO2 ratio of 50:50%, pH 7, applied potential of 900mV, and 36 hours of incubation. The SEM (spell out) images revealed irregular EMM structures with netted texture, and their activity was associated with a recorded potential value of 900mV. The strain, TNFW-2 was identified as the same species and the chemical composition of the extracellular EMMs was Methanobacterium formicicum of 98% sequence similarity. EMMs exhibited stability within a pH range of 4.5-8, with maximum CH4 production at a temperature of 28±2⁰C and pH 7 for at least 36 hours. The optimized H-MEC system demonstrated 92% CO2 conversion at an organic CO2 flow rate of 5 L/h and 36 hours of incubation time. The optimal H-MEC conditions for EMM-based biogas upgrading included two chambers with strainless steel (SS) with graphite (SS+GF) electrodes, an applied voltage of 900mV, pH 7, and an EMM dose of 10mL. Under these conditions, 92% CO2 removal in terms of CH4 production was achieved. Kinetic analysis revealed growth-associated BioM production, with an estimated specific growth rate (µ) of 0.207h-1 and maximum specific rate of product formation of approximately 0.239h-1. These findings highlight the potential of the new EMMs for non-toxic and biodegradable biogas upgrading, which may show a potential solution for future applications in the wastewater treatment plants.