Review of performance enhancement of anaerobic digestion with the aid of biochar and future perspectives

Anaerobic digestion (AD), one of the most promising routes for producing clean energy from biodegradable wastes, encounters difficulties at operational and mechanistic levels impacting its overall efficiency. Along with other solutions to overcome this problem, biochar has arrived as a worthwhile additive to enhance the performance of the process. This review aims to discuss various perspectives of biochar addition to an anaerobic digester. It summarizes the mechanism of biochar action during AD. A direct inter-species electron transfer offers a synergistic mechanism specifying different aspects of biogas production. The importance of characteristics of biochar (ion-exchange capacity, surface properties, electrical conductivity, and hydrophobicity) has been explained in enhancing AD performance. Studies indicate that a surface area >150 m2/g and a particle size <1 cm can be referred to as benchmark along with its supporting properties apt for AD. Researchers have recommended varying amounts of biochar (up to 15 g/l) for optimizing biogas production. The applicability of biochar for upgrading the biogas with respect to significant impurities (CO2, H2S, and NH3) has been discussed in detail. Along with the beneficial effects of biochar, its limitations in AD have also been identified, which need to be addressed wisely in order to optimize the process and make an unbiased shift toward validating the candidature of biochar in AD. The whole discussion indicates that consciously planned scheme utilizing biochar may be helpful in enhancing the effectiveness of AD for biogas generation, which may prove to be an impactful technology in the world of renewable energy.