RAS Effluent and Sludge Management

Where waste streams in a recirculating system come from, how much nitrogen and sludge leave a farm, how sludge is thickened and reused, and which discharge rules apply.

Where the waste streams come from

The FAO guide to recirculation aquaculture describes three outflows from a recirculating aquaculture system (RAS). Overflow water balances what goes in and out; it is the same water the fish live in and is not regarded as a pollutant unless the volume becomes excessive. The main waste water comes from the sludge outlet of the mechanical filter, where faeces and other organic matter are separated, and from cleaning and flushing the biofilters. SRAC Publication 452 adds that recirculating systems produce a considerable amount of highly malodorous sludge, which must be disposed of in an environmentally sound way, for example on agricultural land or by composting.

Nitrogen and phosphorus leave the fish by different routes. According to FAO, fish excrete most of their nitrogen in dissolved form through the gills, and only a smaller part with the faeces, while phosphorus leaves only with the faeces. A mechanical filter therefore captures much more phosphorus than nitrogen. Data from the Fisheries Research Station of Baden-Württemberg, reproduced by FAO, show the following removal efficiencies for a self-cleaning tank followed by a screen filter.

40 µm screen: suspended solids 60–91 %, total phosphorus 65–84 %, total nitrogen 25–32 % of the load removed.
90 µm screen: suspended solids 50–80 %, total phosphorus 45–75 %, total nitrogen 15–22 % of the load removed.

How much leaves the farm

FAO gives a theoretical example for a trout farm producing 500 tonnes per year, with a total water volume of 4,000 m3 of which 3,000 m3 is tank volume. A flow-through farm with a settling pond uses about 30 m3 of new water per kg of fish produced and discharges about 20 tonnes of nitrogen per year. A RAS with sludge treatment and a plant lagoon uses about 3 m3 per kg and discharges about 10 tonnes of nitrogen. A super-intensive RAS with sludge treatment and denitrification uses about 0.3 m3 per kg and discharges about 5 tonnes. FAO stresses that the reduction comes from the waste water treatment, not from recirculation itself; recirculation helps because it concentrates the waste into a much smaller volume that is easier to treat.

Measured sludge properties vary widely. Sludge from the sedimentation tank of one Korean freshwater rainbow trout farm, sieved before analysis, contained about 87.6 g/L total suspended solids, 36.7 g/L volatile suspended solids and 1.21 g/L total nitrogen (Chun et al., 2024, Frontiers in Microbiology). The same paper cites a salinity range of 0.2–44.0 psu for aquaculture sludge, which matters for any biological treatment.

Thickening and dewatering

FAO describes a common treatment chain: a buffer tank collects the waste water, sludge is separated from the discharge water, and the sludge goes to an accumulation facility for sedimentation or further mechanical dewatering. The options shown by FAO include sedimentation tanks, flocculation, belt filters and geotextile tubes. Mechanical dewatering makes sludge easier to handle and reduces its volume, which lowers disposal costs and fees.

Treating reject and overflow water

The water separated from the sludge is called reject water. FAO notes that it usually carries a high nitrogen concentration, while phosphorus can be almost completely removed in the sludge treatment. Reject water is most often discharged together with the overflow water. Remaining nitrogen and phosphorus can be reduced further in a plant lagoon, root zone or seepage system, or by denitrification, which converts nitrate to nitrogen gas. Reject water can also fertilise plants in aquaponics; for larger farms FAO suggests sending sludge to agricultural land and biogas, and reject water to aquaponics, which is simpler to adjust. Aquaponics is covered in its own guide.

Reuse routes for sludge

Fertiliser: Brod et al. (2023, Science of the Total Environment) tested dried sludge products and digestates from Norwegian smolt hatcheries. The dried products held 27–70 g nitrogen per kg dry matter, mostly as slowly available organic nitrogen, and gave lower grain yields than mineral fertiliser, while liquid digestate performed as well as mineral nitrogen. Cadmium and zinc were below EU limits for organic fertilisers in all but one product, and the organic pollutants analysed were detected in all of them.
Biogas: FAO, Nofima and University of Cambridge researchers writing for the Global Seafood Alliance list anaerobic digestion as a main route. Those researchers report that several studies suggest about 5 % of RAS energy demand could be covered by biogas from sludge.
Feed for invertebrates: Sele et al. (2024, Journal of Environmental Management) analysed 47 sludge samples from land-based Atlantic salmon (Salmo salar) farms in Norway. Sludge was rich in protein and fat, but arsenic exceeded EU maximum levels for complete animal feed in 43 % of samples and cadmium in 84 %. No veterinary medicines, salmonid viruses or bacteria were detected.

Limits of zero discharge

FAO notes that a farm can in some cases return no water at all to the environment, but such zero-discharge farms are costly to build, have significant running costs for waste treatment and need close daily attention. Some water exchange is always needed to prevent metals and phosphorus compounds from accumulating in the system. FAO concludes that the authorities and the farmer must agree on a discharge permit that protects the environment while keeping the farm viable.

Discharge regulation

In the United States, the EPA effluent guidelines for Concentrated Aquatic Animal Production (40 CFR Part 451), finalised on 23 August 2004, cover flow-through, recirculating and net-pen facilities that discharge wastewater directly and produce at least 100,000 pounds of fish, molluscs or crustaceans a year (about 45 tonnes, converted here). The requirements are written into NPDES permits; smaller facilities still need an NPDES permit to discharge but fall outside these guidelines. For the flow-through and recirculating subcategory, section 451.11 requires best management practices for solids control, safe storage of drugs, pesticides and feed with spill procedures, structural maintenance, record keeping and staff training. In the European Union, the Water Framework Directive (2000/60/EC) sets the framework for protecting all water bodies; discharge permits are agreed with the competent authority, as FAO describes.