Water Recycling Aquaculture

What Are Water Recirculation Aquaculture Systems?

Recirculation aquaculture systems (RAS) represent an exciting, eco-friendly and unique way to farm fish. Instead of the traditional method of growing fish outdoors in open ponds, net cages or tanks, this system rears fish at high densities within a operator controllable rearing environment inside a building. Recirculation systems filter and clean the water for recycling back through the fish culture tanks. New water is added to the tanks only to make up for water loss due to splash out and evaporation and to replace that used to flush out fish waste materials to storage tanks.

RAS designs have several system process technologies all of which work together to ensure minimum loss of water, heat and of course fish stocks, while constantly cleaning and re-using the fish tank water. RAS farms can be designed to various levels of complexity, however all true water recirculation designs involve the use of equipment to remove and safely store wastes, clean and re-use water and maintain rearing conditions at or near optimum for the species of fish being raised.

In contrast, many older style tank farming systems that have been used to grow fish are termed "open" or "flow through" systems because the water makes only one pass through the tank and then is discarded. Although several improvements can be added to older designs to reduce water usage and create hy-bird types of operations, these are not exactly true water recirculation farms.

Benefits of Water Recirculation Aquaculture System Designs

Fish grown in any facility must be supplied with the correct conditions necessary to remain healthy and grow. Fish need a continuous supply of clean water at an appropriate temperature and a dissolved oxygen content that is optimum for growth. Water recirculation farms accomplish these tasks. A mechanical and biological system is necessary to purify the water and remove or detoxify harmful waste products and uneaten feed. The fish must be fed a nutritionally complete feed on a daily basis to encourage fast growth and high survival. With that in mind let's review the potential benefits of using Recirculating Aquaculture Systems.

- Low water Requirements

Because RAS farms recycle most of their water, they consume considerable less than other types of culture systems and are especially well suited to areas with limited water supplies. The required quantity of water needed to successfully grow fish varies with the species of fish selected, size of the farm system, and the unit process equipment design. As a general rule, a minimum water volume of 1-5 gallons is needed for every pound of fish reared and minimum new water flows of 5-20 gallons per minute are needed to grow 50,000 - 75,000 pounds of fish per year.

A properly designed and operated recirculating system requires a minimum daily input of water, just enough to clean the waste from the filters and to replace water lost to evaporation. This allows construction of fish farms in areas where ground water is limited and even opens the possibility of an operation being located in an urban area to use de-chlorinated municipal water. Such a production facility could be located close to the market. By comparison a recirculating system which produces the same number of pounds of fish as 1000 acres of ponds (about 4.8 million pounds of fish) would require only about 4000 gallons of fresh water each day or 1.5 million gallons per year, However, to fill 1000 acres of ponds averaging just 5 feet in depth, once only, would require 1.6 billion gallons of water. That would just to fill the ponds and does not include any water needed to maintain levels or freshen the ponds after the initial filling.

- Less Land Requirements

Since fish in a recirculation system are reared in tanks, with oxygen being supplied and their metabolic wastes removed by constantly recirculated water, fish can be be safely stocked and grown at high densities. Currently the goal which designers are striving to attain is 1 pound of fish per gallon of water. However, many people consider 0.5 to 0.75 pounds of fish per gallon of tank water as being acceptable. In pond aquaculture, the common maximum density is about 0.003 pounds of fish per gallon of water, Therefore, a water recirculating system can be located in areas where large amounts of level land (to build ponds) are not available. The low land requirement also permits the facility to be located in areas where the soil cannot hold water or, again, in urban areas, or housed in vacant farm or warehouse buildings converted to fish farming.

- Control of Water Temperature

The low water requirement of recirculation systems opens up the possibility of economically controlling temperature which, next to the lower water amounts need, controlling water temperature (and the resulting stock growth rates) is one the greatest benefits of these systems. Control of water temperature allows the aquaculturist to produce a fish species which could not normally be raised in a given geographic area. It also permits the water temperature to be maintained at the optimum level to maximize food conversion and provide optimum growth, Growth can also occur throughout the year, maximizing production and allowing rapid turnover of the product, In theory, marketing of the product is also enhanced, since fish can be supplied each week.

- Protection from the Elements and Potential Predators

By rearing the fish indoors, the farmer is no longer limited by weather conditions where a sudden cold spell can wipe out a year's production by killing the larval fish or disrupting the normal spawning of the brood fish. In addition pond and cage culturists can lose their crop to low oxygen during the summer or winter and have greater difficulty controlling predators. Having the fish indoors also permits harvest at times when heavy rain, snow or ice would stop the harvest of pond fish or cage; creating a definite market advantage to the indoor farmer. Indoor RAS farms are designed to prevent stock escapement providing mitigation of potential conflicts with regulations governing water bodies and fish escapements.

- Control of Water Quality

With recirculation systems, the aquaculturist has the opportunity to control water quality, to the benefit of the live fish stock to the final product and the environment. By maintaining dissolved oxygen at optimum levels, the fish have better food conversion and are less stressed, which translates into greater disease resistance, less wasted feed and faster stock growth. In addition, the fish are isolated from potential environmental contaminants such as off-flavor caused by algal blooms that can affect cages and from any potential pollution resulting from soil run-off or residual pesticides etc. into the water that the fish are held in. This results in a high quality product. In many cases where the fresh water is supplied by wells the likelihood of disease outbreaks are greatly reduced and medications are not required.


There are various designs for recirculation systems and most will work effectively if they accomplish oxygen and aeration, removal of particulate matter, biological filtration to remove waste ammonia and nitrite and buffering of water pH levels. These processes can be achieved by simple units that use combination filters in small farm systems or in the case of larger farms by several interconnected components and unit processes.

RAS systems also help make possible crop diversification with other alternative farming methods such as Aquaponics, which is the use of fish waste nutrients and water to grow other edible plant crops. Aquaponics is in fact becoming more common and in some cases the plant crop production has a greater value than the fish crop. Land based fish farms that use RAS technologies are indeed an alternative to pond and cage culture systems.

As with any technology or any other business the operators must have the knowledge and the inclination to manage the business and undertake the daily tasks required. In RAS farms the staff must care for and manage both the fish stocks and the systems. Knowledge is king and one should not attempt operating a farm without having the training and experience required. Water recirculation farms are also generally more costly to build and more complex to manage.

Persons looking at RAS designs should make every opportunity to visit existing farms, read available literature and be prepared to undertake training either in a classroom setting, on a farm, or preferably both. For those interested in water recirculation system design for Aquaculture, I recommend that you obtain a copy of Recirculating Aquaculture By M.B. Timmons and J.M. Ebeling. For additional reference books browse our Land and Sea Book Store

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