What Are Water Recirculation Aquaculture Systems?
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. Recirculating 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 repplace that used to flush out fish waste materials to storage tanks.
RAS designs have several system process techologies all of which work together to ensure minium 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 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 is just to fill the ponds and does not include any water added to maintain levels or freshen the ponds after the initial filling.
Less Land Requirements
Since fish in a recirculating 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 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 recirculating 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.