Effect of different graphite sizes on biogas production in anaerobic digestion

Abstract

High concentrations of organic pollutants have become a critical environmental challenge. Furthermore, the growing demand for energy, alongside the depletion of non-renewable fuels, has led researchers to investigate alternative energy sources. Biodegradable organic wastes are renewable and economic resources for producing biogas through anaerobic digestion (AD). AD suffers various operational problems, including low tolerance to changing temperatures, pH, and ammonium concentration, leading to poor biogas recovery. Therefore, improving AD performance by mitigating operational issues is crucial. The recent application of conductive materials (CMs) in AD matrix discovered the possibility of direct interspaces electron transfer (DIET). Therefore, the research aimed to improve AD efficiency by integrating CMs into the AD matrix. The type, size and specific surface area of CMs could influence microbial growth, attachment and electrical communication, hence, the study determined the optimum sizes of graphite granules for the best performance. Three sizes of graphite, i.e., G1 4-5 mm, G2 2-2.3 mm, and G3 0.85-1 mm were used in this study to (i) determine the effect of CM sizes on methane (CH4) recovery from the co-digestion of cow dung slurry with food waste; (ii) determine the percentage of CMs retrieval from the digestate discharge. All ADs showed approximately shorter lag-phases compared to the conventional AD without CM. The highest CH4 accumulation was 532 mL of AD volume with G2, while 452 with G3 and 405 with G1, indicating CM size influences microbial DIET and consequently the CH4 production. G2 possessed a higher recovery of CH4, i.e., 15.04% and 23.8% compared to G3 and G1, respectively. the results indicated that G2, the medium-size particles showed the best performance in terms of CH4 production and CMs retrieval from AD of co-digested feed. 34 Further studies are needed to determine the optimum doges of graphite particles in the 35 thermophilic AD matrix.