Multiple roles of humic substances in anaerobic digestion systems: A review

Humic substances (HS) is one of the main organic components of biowaste with high molecular weight and aromaticity. There is a growing concern that HS will hinder the recovery of biomass energy during anaerobic digestion in consideration of its recalcitrance and inhibitory effects. However, some studies have come to the exact opposite conclusion. This study for the first time comprehensively summarized the transformation mechanism, effect mechanism and inhibition mitigation strategies of HS in anaerobic digestion systems, as well as corresponding knowledge gaps, research defects and future perspectives. The content and aromaticity of HS are dynamic during anaerobic digestion followed a three-step process of release, decomposition and repolymerization, which is influenced by operation time and temperature, organic loading rate, precursor concentration and the type of substrates. In consistent with conflicting understandings of the role of HS in biomass energy recovery, HS is possible to positively or negatively affect the hydrolysis, acidification and methanogenesis processes of anaerobic digestion by acting as surfactant, adhesion agent, terminal electron acceptor, electron shuttle, organics binder or pH buffer. However, the boundary and threshold of different roles of HS are unable to be defined so far, due to the knowledge gap about the correlation between dynamic evolution of natural HS characteristics and its influencing behaviors. It is also found that HS could indirectly affect anaerobic digestion by regulating the bioavailability, biotransformation and bioactivity of exogenous pollutants (heavy metals, organohalides, microplastics, antibiotics, etc), but related knowledge is severely lacking. The existing HS inhibition mitigation strategies can be classified to passivator addition, co-digestion, pretreatment and integrate process, among which microbial electrolysis cell assisted anaerobic digestion is recommended as the most promising.