Enhanced biofiltration coupled with ultrafiltration process in marine recirculating aquaculture system: Fast start-up of nitrification and long-term performance

In marine recirculating aquaculture systems (RAS), the prolonged initiation of biological nitrification and the proliferation of potential pathogens due to high salinity pose challenges. A combined biofiltration system (ceramic rings (CR) - granular activated carbon (GAC) - ultrafiltration (UF) process) was implemented in an actual marine RAS for selenotoca multifasciata. The research spans 200 days, focusing on ammonia nitrogen and dissolved organic matter (DOM) removal, membrane fouling, and microbial communities. Results showed that fast nitrification start-up achieved by the CR-GAC-UF process within 21 days under low ammonia concentrations with a daily ammonia removal efficiency of 91.7 %. UV254 and CODMn maintained at 0.05 ± 0.01 cm−1 and 1.34 ± 0.03 mg/L, in line with the stable removal of humic-like substances and soluble microbial products. UF membranes maintained low fouling with fluxes at 15 LHM and the protein/polysaccharide ratios in extracellular polymeric substances (EPS) were stable below 0.5. Specific dominant bacterial genera facilitate DOM removal such as Xanthomarina, Idiomarina and Hyphomicrobium were recognized. Candidatus_Nitrosopumilus, Nitrosomonas_sp. and Nitrospira sp. ENR4 were detected as the core nitrifiers in CR-GAC-UF process, which were closely related to the amoABC, hao and nxrA genes (p < 0.05). The multi-stage barrier of CR-GAC-UF process maintains low heterotrophic plate counts (3 ± 1 CFU/mL) and reduces potential pathogens in steps. Taken together, this study provides a theoretical basis for the wider application of marine RAS.