Challenges from pretreatment to sludge anaerobic digestion: A review on the influence of humic substances

Abstract Anaerobic digestion (AD) is a sustainable and cleaner technology for sludge treatment and reutilization, by which, waste organics in sludge can be converted into energy or added-value chemicals. Humic substances (HS) is one of the main organic compounds in sludge, but it is mainly bonded with solids and its influences are often neglected during conventional AD processes. However, currently as sludge pretreatments are widely employed, the enhanced release of HS generates substantial influences on AD of sludge. This review presents an extensive literature overview and decipherment on the influence of HS release on AD of pretreated sludge. Potential risks were evaluated by analyzing the abundance, distribution and characteristics of HS in sludge. The change laws of HS profiles during pretreatment and digestion were summarized in aspects of concentration, structural characteristics and composition, which are closely relative to its influences on AD. Results from literature survey show the enhanced concentration and shifted structural characteristics of soluble HS induced by pretreatment indeed influence AD of sludge. Moreover, this kind of influences are resulted from joint contributions of several mechanisms, including binding, electron shuttling, microbial community regulation and metal ion complexation, and the weight of individual mechanism shifts with operational conditions. Therefore, the topology of the mechanisms in practical application was proposed.