To date, marine land-based recirculating aquaculture system (RAS) still include effluent stream discharge. These effluents, originating from fish metabolism, feed residues, and sludge washouts, are rich in nitrogenous and organic compounds. Due to their salinity, such streams can not discharge to freshwater bodies or municipal wastewater treatment plants and are typically released into the sea. However, environmental regulations on nutrient discharge into marine environments are tightening globally, particularly regarding nitrogen and suspended solids. This creates an urgent need for effective, scalable, and regulatory-compliant treatment solutions. Denitrification, particularly heterotrophic, is the prevailing approach for nitrate (Nitrogen) removal from saline effluents, yet commercial-scale applications remain scarce.
System description
A full-scale effluent treatment system was established and operated to treat the discharge from a semi-commercial marine RAS (Fig 1) . Effluents consisted of solids filter backwashes, sludge removal by floor suction , and tank overflow. These were collected in a sump and pumped into a ~30 m³ denitrification reactor based on the activated sludge concept. The reactor was coupled with a pressurized settler (Soliquator SLS700) with optional flocculant dosing for enhanced separation. Polished water was filtered through twin sand filters before sea discharge. Excess sludge was diverted to an anaerobic stabilization pond (45 0–750 m³), followed by a ~30 m³ aerobic mixed reactor in a secondary tank and optional recirculation to the anoxic stage.
Operational results
The treatment facility operated for about a year under increasing feed loads and hydraulic conditions. As fish biomass increased and feeding rates gradually ramped up, effluent flow and pollutant loads intensified. Throughout the period, suspended solids separation in the settler improved with increasing reactor- biomass (MLSS) concentrations. Average nitrate removal efficiency reached ~69±7%, while overall total nitrogen (TN) removal averaged ~40% (~22 g N/m³/day), lower than the expected design load (300 g N/m³/day; based on pilot-scale previous studies). Factors limiting performance included mainly reactor geometry (low depth-to-diameter ratio) and retention time. In addition, low pH levels (6–7) may have impaired denitrification. Polymer screening showed optimal sedimentation at 4 mg/L with no observed inhibition of microbial activity.
Conclusions
In summary, the commercial-scale treatment system demonstrated stable operation and partial nutrient removal under real-world semi-commercial marine RAS conditions. However, significant optimization is required, particularly regarding reactor design, hydraulic control, and final polishing, to meet stringent discharge standards and fully support sustainable mariculture development.