Recirculating water systems are designed to minimize or reduce dependence on water exchange and flushing in fish culture units. These systems have practical applications in commercial aquaculture hatcheries, holding tanks, and aquaria systems, as well as small scale aquaculture projects. Water is typically recirculated when there is a specific need to minimize water replacement, to maintain water quality conditions which differ from the supply water, or to compensate for an insufficient water supply. There are innumerable designs for recirculating systems and most will work effectively if they accomplish:
- aeration,
- removal of particulate matter,
- biological filtration to remove waste ammonia and nitrite, and
- buffering of pH.
These processes can be achieved by a simple composite unit such as an aquarium filter, or in larger systems, by several interconnected components.
Recirculating Aquaculture Systems (RAS) offer significant advantages over open aquaculture systems. In the Netherlands the turbot industry is the first aquaculture industry that applies RAS on a large scale to produce market size marine fish. The use of RAS systems has been developed in the eighties in the research environment of Wageningen University and Fisheries Research. Because various technical installations are necessary dutch suppliers of filter- and pump systems were directly involved. Nowadays some Dutch companies are worldwide leaders in this business and are working together with the researchers to further optimize the systems. The targets are minimizing the energy and water needs of the almost closed systems.