At the current stage of commercial land-based marine recirculating aquaculture systems (RAS) production, effluent discharge is inevitable. The effluent primarily comprises fish metabolites and a small portion of uneaten feed, accounting for approximately 70% of feed nutrients and 50% of feed carbon. Typically, due to its high salt content, this stream is released into the sea. However, environmental regulations are becoming increasingly stringent, necessitating treatment prior to discharge. In general, RAS systems consist of a solid separation filter and a nitrification biofilter. Therefore, in addition to sludge, the primary dissolved nitrogen form is nitrate. Denitrification is typically employed as the solution, with extensive work, mainly on small and pilot-scale systems. However, a commercial denitrification treatment for marine RAS is currently lacking. This study aimed to upscale and test a semi-commercial marine RAS effluent treatment facility for nutrient and sludge removal.
The facility was designed to treat the effluent of marine RAS receiving up to 500 kg fish feed per day at a maximum flow rate of 30 m3/h. It is based on the activated sludge concept with an intrinsic carbon source (fish sludge) for denitrification, and an anaerobic digestion pond for sludge stabilization (Fig. 1). The system comprises a 34 m3 mixed tank (operating under anoxic conditions) that receives RAS effluents, followed by a 32 m3 pressurized solid separator (Soliquator, Odis Filtering, Israel). Enhanced sedimentation is achieved through flocculation. The clear water passes through a pair of sand filters before being discharged into the sea. Most settled biomass is recirculated back to the mixed tank, while a small portion of excess biomass outflows to a 400-600 m3 anaerobic pond. The supernatants from the pond are pumped into a second 34 m3 mixed tank operating under aerobic conditions (aeration by diffusers) before retreatment in the anoxic tank. Water samples were collected weekly during the study and analyzed using standard methods.
Overall, the system operated for over a year, achieving nitrogen and phosphorus removal efficiency of 60-70%, primarily attributable to suspended solids removal. These results are relatively lower than previously published for pilot scale systems, and the system is currently undergoing optimization. The solid separation efficiency in the Soliquator increases with total suspended solids (TSS) concentrations, with flocculation polymer having a significant impact. Imbalance and high polymer addition resulted in sand filter clogging, necessitating media replacement. It was found that online continuous monitoring and control are critical factors in upscaling and require full attention. Economic analysis revealed that the initial investment in facility installation is the primary cost component, followed by energy consumption. Successful operation of commercial effluent treatment will enhance the feasibility and sustainability of the land-based mariculture industry, representing a significant milestone for its growth potential.