Co-composting aquaculture sludge at full-scale: Two case studies evaluating process performance and compost quality

Two consecutive case studies assessed the feasibility of composting dewatered sludge (DS; fecal matter and uneaten feed) obtained from recirculating aquaculture systems. Utilizing a commercially available rotary drum composter (3.15 m 3 working volume; 14-d retention time), DS was combined with wood flakes (WF) bulking agent and co-composted with either spent mushroom compost (SMC) in case study 1 or fish mortalities (FM) in case study 2. The case studies characterized feedstocks, evaluated potential recipes, and assessed process performance and finished product quality. In case study 1, co-composting DS with SMC (DS-SMC) reached thermophilic temperatures (average hottest-zone temperature of 58.8 ± 0.6 °C), thereby accelerating feedstock breakdown. Compost was considered stable and mature post-drum, as indicated by a low CO 2 respiration rate of 0.2 ± 0.0 mg CO 2 -C/g OM/day, with a final C:N ratio of 19 ± 2 after a 6-month post-drum curing period. In case study 2, co-composting DS with FM (DS-FM) resulted in low hottest-zone temperatures (averaging 51.9 ± 1.0 °C), likely due to the heterogeneous nature of the combined feedstocks and increased drum rotations to minimize ammonia volatilization. High ammonium concentrations [571 ppm or 0.06%; 10% of total nitrogen (TN)] in the post-drum samples indicated incomplete decomposition. Six months post-drum, ammonia accounted for 1% of TN, but the C:N ratio remained elevated (28 ± 4). Additional analyses from both studies highlighted low concentrations of heavy metals and human pathogens. These two case studies suggest that co-composting with additional substrates may be an effective strategy for stabilizing aquacultural biosolids.