Abstract
Humic acid (HA) is an important active compound formed during anaerobic digestion process, with a complex structure and dynamic electron transfer capacity (ETC). However, the mechanisms by which these macromolecular organic compounds dynamically interact with the microbial anaerobic digestion process at different operating temperatures are still unclear. In this study, the link between the evolution of the ETC of HAs and the microbial community under mesophilic and thermophilic conditions was investigated. The results showed an increasing trend in the ETC of HAs in both mesophilic (671–1479 μmol gHA−1) and thermophilic (774–1506 μmol gHA−1) anaerobic digestion (AD) until day 25. The ETC was positively correlated with the bacterial community of hydrolytic and acidogenic phases, but negatively correlated with the archaeal community of the methanogenic phase. Furthermore, the relationship between ETC and key enzyme activity was explored using a co-occurrence network analysis. HAs revealed a high potential to promote key enzyme activities during hydrolysis (amylase and protease) and acidification (acetate kinase, butyrate kinase, and phosphotransacetylase) while inhibiting the key enzyme activity in the methanogenic phase during the anaerobic digestion process. Moreover, HAs formed under thermophilic conditions had a greater influence on key enzyme activities than those formed under mesophilic conditions. This study advances our understanding of the mechanisms underlying the influence of HAs on anaerobic digestion performance.
Highlights
Anaerobic digestion (AD) is one of the effective treatments for the bioconversion of organic wastes to produce bioenergy of methane (Tang et al, 2018)
The electron transfer capacity (ETC) of humic acid (HA) increased from 671 ± 15 and 774 ± 22 μmol gHA− 1 to 1479 ± 26 and 1506 ± 13 μmol gHA− 1 under mesophilic (Fig. 1 a) and thermophilic (Fig. 1 b) AD until day 25, respectively
The electron donating capacity (EDC) of HAs initially increased from 299 ± 17 to 762 ± 20 μmol gHA− 1 and gradually decreased to 718 ± 25 μmol gHA− 1
Summary
Anaerobic digestion (AD) is one of the effective treatments for the bioconversion of organic wastes to produce bioenergy of methane (Tang et al, 2018). Recalcitrant macromolecular organics, such as humic substances, are formed during the organic matter degradation and transformation processes (Wang et al, 2021a). Humic acid (HA), as one of the most important fractions of recalcitrant macromolecular organics, accounts for 14.6% of total solids and 27.0% of organic matter in the anaerobic digestion of sewage sludge (Tang et al, 2018). The content of HA ranged from 3.9 to 7.5 and 4.6–8.4 g L− 1 during the mesophilic AD process with corn stover and chicken manure, respectively. Under the thermophilic con dition, the content of HA ranged from 4.1 to 6.8 and 4.0–7.1 g L− 1 during the corn stover and chicken manure, respectively
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