Influence of biochar and zeolite on biochemical methane potential in saline aquaculture sludge

• Salinity decreased methane yields from freshwater to brackish and marine sludge. • Biochar boosted CH 4 yield by up to 11.3%, best under freshwater. • Zeolite enhanced methane yields in brackish and marine systems but not freshwater. • Targeted additive selection improved anaerobic digestion of aquaculture sludge. Aquaculture sludge from recirculating aquaculture systems (RAS) represents a growing waste stream with potential for biogas recovery; however, elevated salinity can inhibit anaerobic digestion (AD). This study evaluated the biochemical methane potential (BMP) of RAS sludge under freshwater (0%), brackish (1.2%), and marine (3.3%) conditions and assessed the effectiveness of biochar and zeolite. Batch BMP assays were conducted under mesophilic conditions at an inoculum-to-substrate ratio of 2:1, with additives applied at 0.8 g/g VS. Increasing salinity significantly reduced methane yields ( p < 0.05), from 533.6 ± 3.4 NmL CH 4 /g VS in freshwater to 478.1 ± 10.2 and 341.3 ± 0.6 NmL CH 4 /g VS in brackish and marine conditions, respectively. Biochar enhanced methane production by 5.9–11.3% across all salinities, while zeolite increased yields by 7.7% and 15.7% under brackish and marine conditions, respectively, but had no effect in freshwater. Methane production kinetics were well described by the modified Gompertz model (R 2 = 0.983–0.999). Overall, biochar was more effective at low salinity levels, whereas zeolite mitigated salinity-induced inhibition, indicating that targeted additive application can enhance methane recovery from saline aquaculture sludge and support sustainable RAS waste management.