From feedstock to digestion: Unraveling the impact of humic acid composition on anaerobic digestion

In the anaerobic digestion (AD) process, the effects of humic acid (HA) derived from different feedstocks on AD are influenced by the variations in their structural composition and oxygen-containing functional groups. Thus, clarifying the structural differences of HA obtained from different feedstocks is crucial for understanding their impact on AD. In this study, the structure of five humic acids (HAs) derived from liquid digestate, food waste, silage corn straw, lignite and commercial HA, and their effects on AD were investigated. The study found that HA from food waste had more carboxyl groups, while straw-derived HA had more phenolic hydroxyl groups. Both types of HA had higher aromaticity and humification degree and showed significant inhibition effect on AD. HA from food waste had an average methanogenic inhibition rate of 43.5 % with 1 g/L HA added. In addition, commercial HA and HA derived from lignite had similar functional group types and aromaticity, with an average methanogenic inhibition rate of about 20 %. The study revealed that HAs with more carboxyl groups exhibited greater effectiveness in inhibiting AD, thereby confirming the influence of HA structures derived from different feedstocks on AD. In conclusion, this study provides valuable insights into the mechanism of HA effect on AD and offers guidance for future research focused on enhancing AD efficiency.