Valorization of fish waste using biochar and crude glycerin as additives in composting.

During composting, nitrogen loss primarily occurs in the form of ammonia, which negatively affects the quality of organic fertilizers, because nitrogen is a crucial macronutrient for plant growth. Additives are often employed to mitigate these losses, particularly when composted waste contains high nitrogen levels. This study aims to assess the effectiveness of biochar and crude glycerin as additives in the composting of fish waste in static windrows. Based on fresh weight, five treatments were evaluated: control (no additive), 5 and 10% biochar, and 5 and 10% crude glycerin, over three time periods (50, 70, and 90 days of composting). A 3:1 (mass/mass) ratio of fish waste to bulking agent was used, and the mixture was placed in nylon bags to enhance additive assessment. Thermophilic temperatures were achieved during the early stages of composting and after turning. There were no significant differences (P > 0.05) between the control and additive treatments in terms of the reduction in total solids, volatile solids, carbon, hemicellulose, cellulose, and lignin, with averages of 52.0%, 57.8%, 52.3%, 77.3%, 63.9%, and 60.7%, respectively. The additives accelerated fiber degradation (P < 0.05). The control treatment exhibited higher nitrogen loss (56.9%) than the biochar treatments (average of 50.6%), whereas the 5% glycerin treatment resulted in the lowest nitrogen loss (26.9%). No significant differences were observed in the macro- and micronutrient concentrations between the treatments (P > 0.05). Thus, biochar and crude glycerin are recommended as additives to reduce nitrogen loss without impairing the organic matter degradation.