Novel insights into liquid digestate-derived hydrochar enhances volatile fatty acids production from anaerobic co-digestion of sludge and food waste

The challenges in treating digestate and liquid digestate (LD), along with their low resource utilization rates, represent critical bottlenecks faced by the anaerobic digestion of organic matter. In this work, a novel method of waste functionalization was innovatively adopted, i.e., using food waste (FW) digestate and LD as the raw materials, and the LD-derived hydrochar (LD-HC) was prepared for the first time to improve the production of volatile fatty acids (VFAs) from anaerobic co-fermentation of sludge and FW. The experimental results showed that LD-HC had a large specific surface area and was rich in oxygen-containing functional groups. LD-HC significantly improved the co-fermentation efficiency with a maximum VFAs yield of 74.3 mg COD/gVS, which was about 1.3 times higher than that of the control. Mechanistic studies confirmed that LD-HC significantly promoted solubilization and hydrolysis of organic matter. Metagenomic analysis showed that LD-HC increased anaerobic co-digestion microbial diversity and enriched hydrolytic and acidogenic microorganisms such as Lentimicrobium, norank_f__Bacteroidetes_vadinHA17. Moreover, homologous genes of enzymes involved in glycolysis, pyruvate metabolism, ABC transporters, and quorum sensing pathways were enhanced in LD-HC. This study provides an effective strategy for enhancing the VFAs production in the anaerobic co-digestion system of FW and sludge, and offers a new pathway for resourceful reuse of digestate and LD.