A recirculating aquaculture system, or RAS, cleans and reuses most of its water instead of letting it flow through once. That saves water and gives the farmer control over conditions, but it also means the water treatment is the farm. If the loop goes wrong, the fish feel it quickly. This checklist covers the treatment steps a RAS project should think through. Our fish farming sector page explains how AFG systems fit.
1. Know your source water
Start with where make-up water comes from: sea, well or municipal network. Get a full analysis, including salinity, hardness, alkalinity, iron, manganese, nitrate, and for municipal water, chlorine or chloramine. Check how the source changes over the seasons. Coastal farms, for example around Umm Al Quwain, may see seasonal changes in turbidity and temperature at the intake.
2. Screen and filter the intake
Sand, silt and debris must not reach the tanks. Automatic backwash filters provide continuous screening without stopping the flow, followed by finer filtration where needed. For hatcheries and sensitive stages, ultrafiltration gives a physical barrier against fine solids and many microorganisms.
3. Remove chlorine from municipal water
Network water is disinfected, and chlorine and chloramine are toxic to fish. Activated carbon filtration or another dechlorination step is essential before municipal water enters the system. This step is easy to overlook when a farm moves from a well to the network.
4. Match salinity and chemistry to the species
If the source is too saline or carries dissolved contaminants that filtration cannot remove, reverse osmosis produces controlled make-up water. RO permeate is very low in minerals, so it is usually blended with raw water or remineralised to the values the species and life stage need.
5. Solids removal in the loop
Uneaten feed and faeces create solids that consume oxygen, feed bacteria and cloud the water. Mechanical filtration in the loop removes them before they break down. It is the foundation for everything that follows.
6. Biological filtration
Fish excrete ammonia, which must be converted by nitrifying bacteria in a biofilter. Biofilter design is usually part of the RAS supplier's scope, but it depends on the solids removal and water chemistry upstream, so the water treatment and the RAS design should be coordinated.
7. Disinfection
UV-C reactors reduce bacteria and viruses in intake and loop water without adding chemicals. Ozone also oxidises fine organics and improves clarity, but its dose has to be controlled so no residual ozone reaches the fish. Both work best on clear water, which is why solids removal comes first.
Sizing disinfection
UV is sized from the flow, the water's UV transmittance and the dose required for the target organisms. Ozone systems need contact time and a way to remove any residual before the water returns to the tanks.
8. Monitor continuously
Oxygen, temperature, pH, ammonia, nitrite and salinity are watched closely in a RAS. Treatment equipment should feed into the same monitoring, so that a blocked filter or failed UV lamp is noticed quickly.
9. Plan the effluent
Water that leaves the farm carries solids, nutrients and organic load. Treating it protects the surrounding water and can allow part of it to be reused. Physical, chemical and biological wastewater treatment can be sized for farm effluent.
10. Think about spares
UV lamps, filter media, cartridges and membranes all need replacing over time. Keep critical spares on site, because a RAS cannot wait long for a part.
11. Plan for power cuts and failures
A RAS depends on pumps, blowers and treatment equipment running continuously. Think about what happens during a power cut or an equipment failure: which units need standby, how long the fish can tolerate a stop and how alarms reach the staff at night. Water treatment should be part of that plan, not left out of it.
If you are planning a fish farm, hatchery or RAS, send the source, flows, species and system type through the quote form and AFG Dubai will reply with the next steps.