Impact of moisture regulation methods on the fish manure composting process and gas emissions

Industrial Recirculating Aquaculture Systems (IRAS) excel at both wastewater treatment and the conversion of wastewater into usable resources, thereby offering vital support for the sustainable advancement of aquaculture. In this study, we aimed to identify suitable moisture regulation methods for composting of fish manure from IRAS. Seven treatment groups were established to investigate the effects of moisture regulation strategies: no water replenishment (T1), tap water replenishment (T2), compost leachate replenishment (T3), hydrothermal liquid replenishment (T4), and their combinations with hydrochar (T5-T7). Results show that the T6 treatment (compost leachate + hydrochar) performed optimally. In addition to increasing microbial diversity, it prolonged the high-temperature phase of composting to 12 days, enhanced compost water stability, and resulted in a seed germination index of 87.72%. Additionally, T6 significantly reduced emissions of greenhouse gases (N 2 O and CH 4 ) and ammonia (NH 3 ). The incorporation of hydrochar effectively delayed the transition of capillary water to free water, and enhanced the overall stability of the reactor. In conclusion, the combination of compost leachate and hydrochar is an efficient and environmentally friendly method for regulating the moisture content of fish manure compost. Our study provides a theoretical basis and technical support for the resource utilization of fish manure.