What is RAS?

(Recirculating Aquaculture Systems)

Recirculating Aquaculture Systems In General:

Recirculating Aquaculture Systems (“RAS”) is based on fish farming in a closed system (isolated hall with fish tanks and equipment),  in which water of the fish tanks is circulated through purification equipment back to the fish tanks. 

Daily amount of makeup water is 2-30 % from the system’s total water capacity. The amount of makeup water can also be indicated as liters per feed use (liters/kg).

Required amount of makeup water depends on fish species, fish densities in the tanks, quality of the makeup water and process technology.

Makeup water is needed for rinsing mechanical filters and for controlling nitrate concentrations (NO3), as well as for purging and bleeding the fish.

The RAS-Farm Concists of e.g. The Following Equipment And Systems:

  1. Fish tanks: e.g. round tanks, square tanks, raceway tanks, mixed cell raceway tanks
  2. Mechanical filtering for suspendid solids extraction (particles over 60 micrometers): e.g. drum filters 
  3. Water pumps: e.g. centrifugal pump, propeller pumps 
  4. Mechanical filtering for fine particle solids extraction (particles from 5-60 micrometers): e.g. particle filters (Fixed Bed Bioreactor, Bead filter, Granular filter) 
  5. Biofiltering (nitrification): ammonia (NH3/NH4) excreted by fish is oxidized trough nitrite (NO2) to nitrate (NO3): e.g. Moving Bed Bioreactor, Fixed Bed Bioreactor, Fluidized sand filter
  6. Degassing (carbon dioxide extraction): e.g. Trickling filter, Diffusers
  7. Oxygenation: e.g. pressured oxygen cone, column oxygenation, Low Head Oxygenation(LHO)
  8. Water disinfection: e.g. UV, ozonation 
  9. Adjustable automatic feeders
  10. Camera surveillance
  11. Online monitoring for water quality; pH, oxygen, temperature, solids etc.
  12. Alarm system
  13. Fish moving and handling systems: fish pumps, counters, and sorting machines

Example Of An RAS-System:


The chosen process technology and components depend on many factors,  such as temperature of farming water, solids tolerance of the fish, and on the farming densities.

 Largest problems are faced, if suspendid solids cumulate in the system, as this impacts negatively on water quality and condition of the fish. For this reason, makeup water use should always be slightly oversized (10-20%) so the system’s pipes and canals can be rinsed from time to time. 

 As the production model is market size fish farming, the water need for fish purging should be taken into account when estimating the amount of makeup water.

Usually the normal water use in slaughter fish production is ca. 500 – 1000 liters per one kilogram of feed, which is enough for fish purging, rinsing the mechanical filters and other usage, for example tank fill-ups.

Production method of RAS-farm is based on selling weekly growth (live/ gutted/ bled fish).

This keeps cash flow stable throughout the year.

Example Layout Of an RAS-Plant:

Example of an RAS-system:

The video below shows an example of a RAS-system at Laukaa fish farm (LUKE, Natural Resources Institute Finland).

If You Have Any Questions, Please Do Not Hesitate To Contact Us.