Characteristics of aquaculture sludge produced from two different fish diets and its impact on biogas production

Assessing the feasibility of anaerobic digestion (AD) for treating recirculating aquaculture system (RAS) sludge requires understanding how fish diet changes affect waste characteristics and, subsequently, the AD process. In this study, sludge produced from RAS utilizing two fish diets (FA and FB) was anaerobically digested in batch reactors at 35 °C for 39 days. The two sludges differed in crude fat, chemical oxygen demand, total Kjeldahl nitrogen, total phosphorus, and volatile fatty acid concentrations. Normalized cumulative methane (CH4) production was significantly higher for FB (346 ± 5 mL CH4/g volatile solids [VS]) than FA (279 ± 4 mL CH4/g VS), which could be attributed to FB’s higher crude fat content (27.4 ± 4.7% compared to 12.2 ± 1.9% for FA on a dry matter [DM] basis). However, the CH4 production rate was higher for FA during the first 10 days of the study, with a maximum of 74.5 ± 1.8 mL CH4/day on day 8, most likely due to the higher initial total VFA concentrations in FA (3.0 ± 0.2 g/L) compared to FB (2.0 ± 0.2 g/L). Sludge FB also increased peak hydrogen sulfide concentration (2763 ± 82 ppm). Elevated total ammonia nitrogen concentrations (> 3 g/L) were observed in both FA and FB treatments post-digestion due to the high crude protein content in the two sludges (> 30% DM). The study showed that fish feeds with similar crude fat and protein content can result in different aquaculture waste characteristics, substantially impacting biogas production from AD.