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. • Aquaculture sludge from two different diets was subjected to anaerobic digestion. • Different diet ingredients impacted digestibility and final sludge characteristics. • The sludge containing more crude fat produced more biogas and methane. • Higher COD removal was observed in the sludge containing more crude fat. • High crude protein in the sludge increased ammonia concentration post-digestion.

Biofouling on aquaculture nettings poses several issues, especially increased drag on net panels or aquaculture structures to which the net is attached, potentially affecting the structural integrity of the infrastructure. Despite biofouling growth on nets not being uniformly distributed, only a few studies have explicitly investigated the influence of naturally fouled nets on drag. In this study, the effects of uniform and non-uniform (half- and quarter-concentrated) hard fouling distributions on the drag coefficient of knotless square net were numerically investigated using a Computational Fluid Dynamics (CFD) approach. Based on the experimental data obtained from the literature, a CFD model was developed, validated, and analyses were undertaken on the effects of fouling distribution on associated drag coefficients of netting panels. At low net solidities less than 0.6, nettings with half- and quarter-concentrated fouling had significantly lower drag coefficients, up to 40% and 35% respectively, than a netting with uniform fouling that had the same solidity. However, the difference in drag coefficient on nettings due to fouling distribution attenuated with a further increase in fouling (net solidity). The correlations of drag coefficient with net solidity for netting panels with half- and quarter-concentrated fouling were proposed, which can provide a conservative drag coefficient estimate for the evaluation of cage safety based on the fouling distribution. This study also highlights the need to take uniformity of fouling distribution on aquaculture nets into consideration when estimating drag force, especially at solidities less than 0.6, as it significantly differs from that of uniform fouling distribution. • At net solidities lower than 0.6, netting with nonuniform fouling show up to 40% lower drag coefficients than a uniformly fouled net with the same solidity. • Differences in drag coefficients between uniform and nonuniform fouling were associated with the alteration of flow patterns behind the net panel. • Correlations were proposed to predict drag coefficients of non-uniformly fouled nets with different net solidities.