Enhancing Humification and Shortening Composting Time of Food Waste Digestate with Mature Compost Addition: A Pilot-Scale Study

Direct agricultural use of digestate from food waste is hindered by its high phytotoxicity, and the addition of common auxiliary materials during composting increases the project cost. In this study, mature compost (MC) was used to replace part of mushroom residue as auxiliary materials for controlled experiments at a pilot-scale horizontal double-layer mechanical composting device. The results showed that the MC addition heated up more rapidly than the control group (NC), peaking at 72 °C on day 5. The moisture content was reduced from 47.17% to 25.36%, which was lower than the final value of 28.48% in the NC. The final humic acid (HA) content in the MC group (60.88 g/kg) was higher than that in the NC (44.82 g/kg). The seed germination index (GI) for both groups exceeded 70%, meeting the national standard. The MC group achieved a final GI of 119.37%, which was significantly higher than that of the NC (81.29%). The phylum Firmicutes became the dominant group in the MC group during the thermophilic phase, demonstrating strong thermotolerance and a high capacity for degrading recalcitrant organic compounds such as cellulose. At the genus level, Bacillus demonstrated a relatively high abundance during the thermophilic phase. These findings imply that the MC addition improves the composting property, enhances the degree of humification, and shortens the composting time, providing technical support for the improvement of aerobic composting of food waste digestate, thus contributing to more sustainable waste management by promoting a circular economy and reducing reliance on external inputs.