Recirculating Aquaculture Systems (RAS)

A recirculating aquaculture system is an enclosed system where the only water replacement is the water lost to evaporation and cleaning. These systems are being deployed in developed countries such as the United States where coastal land costs and labor costs are very high. The majority of tilapia grown in the US is from these types of systems.

There are several advantages in using recirculating aquaculture systems over traditional fish farming systems:

  • lower water requirements - a properly designed and operating RAS replaces less than 5% of the total water volume on a daily basis.
  • lower land requirements - in locations where land costs are very high, some sort of RAS should always be considered since they can produce a large volume of aquacultured product from a relatively small area. The amount of land required is less than 1/20th of the amount required for traditional pond farming.
  • reduced labor requirements - a typical 100 metric ton per year RAS can be run by as few as two people, which is at least a fivefold reduction in labor usage versus traditional fish farming methods.
  • increased control over water quality parameters - having control over water temperature allows a RAS producer to grow species which could not normally be raised in a given geographic area. This can provide a key market advantage. The traditional fish farmer essentially has no control over water temperature and must grow a species suited to the local environment or be a seasonal grower. Other important water quality parameters such as dissolved oxygen can theoretically be maintained at optimum levels in RAS, which leads to higher growth rates.
  • lower risk of negative impact from adverse weather conditions - the risk of crop loss from a natural disaster can be eliminated in a properly sited and constructed RAS. The traditional fish farmer is more vulnerable to natural disasters and the effects of natural weather patterns. By rearing fish indoors, the RAS producer is not limited by inclimate weather. An early season cold spell can spell disaster for a traditional fish farmer who has waited a week too long to harvest.
  • lower risk of creating adverse environmental impacts - if left untreated, the discharge plume from traditional fish and shrimp farms is essentially a source of pollution of local water bodies. Recirculating systems treat and reuse the water and there is zero discharge to the local environment. RAS should be selected when an environmentally friendly solution to the growing demand for seafood is required.
  • increased biosecurity - a properly designed and managed RAS has complete control over biosecurity concerns, whereas a traditional open system is open to attack.

RAS Design

RAS are typically run as intensive or super-intensive systems to compensate for the relatively high construction and operational costs. Their design and operation are varied. Circular tanks or raceways are the most common forms of tank design. Round tanks funnel wastes effectively, while raceways are excellent for manipulation of the stock and harvesting. Stocking densities as high as 0.75 pounds per gallon have been deployed in recirculating tilapia systems.

Super-intensive and intensive RAS have a high input demand for quality aquaculture feeds. The large inputs of feeds into the system are of major concern in the design phase of a RAS. All RAS must utilize processes to remove solid wastes, oxidize ammonia and nitrite-nitrogen and oxygenate the water. Feeding rate, feed composition, fish metabolic rate and quantity of wasted feed can all have a major detrimental impact on tank water quality and must be accounted for in the design and management of a RAS. Aquaculture feeds mainly consist of protein, carbohydrates, fat, ash and water. The portion of feed not utilized by the fish is excreted as an organic waste (fecal solids). These fecal solids, along with uneaten feed, are broken down by bacteria in the system. This process consumes oxygen and generates ammonia-nitrogen and must be dealt with in design and operation. To minimize their impact on water quality, waste solids need to be removed from the system as quickly as possible. Waste solids can be classified into four categories:

  • settleable
  • suspended
  • floating
  • dissolved

Settleable solids should be removed from the water in the tank as rapidly as possible. Settleable solids are those that will settle out of water within one hour under still conditions. Settleable solids can either be allowed to settle within round culture tanks, where they move towards the center drain, or they can be kept in suspension and then removed.

Suspended solids are those that will not settle out of the water column under still conditions within one hour. Fine suspended solids smaller than 30 microns can contribute more than 50% of the total suspended solids load in a RAS. Dissolved organic solids (proteins) can also contribute significantly to the total oxygen demand of RAS if left untreated. Dissolved solids and fine suspended solids can be removed using a process called foam fractionation or protein skimming. Foam fractionation introduces air bubbles at the bottom of a closed column. As the bubbles rise through the water column, fine suspended solid particles attach to the bubbles surface, creating a protein-rich foam at the top of the column. The foam buildup is then channeled out of the fractionation unit to a waste collection tank. A properly designed foam fractionation unit can reduce water turbidity and oxygen demand in the culture tank. Another option is to use bioenergetics to advantage in managed bacterial floc systems deploying active manipulation of the C:N ratio and the aerobic and anaerobic states of the RAS.

Perhaps the most important water quality variable in RAS is unionized ammonia nitrogen (NH3). To maintain a safe concentration of NH3 in the culture environment the rate of removal must equal the rate of production at the maximum sustainable capacity of the system. The efficiency with which the treatment system removes ammonia from the system, the ammonia production rate, and the desired concentration of ammonia nitrogen within the tank will determine the recirculating flow rate. While there are a number of different technologies available for removing ammonia-nitrogen from the water, RAS from AquaSol, Inc. utilize highly effective and cost efficient methods of biological filtration and flocculation. The goal here at AquaSol, Inc. is to employ system desigms that maximize the advantage of freely available natural biological processes.

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