Contact Person: Prof. Dr. Bela H. Buck
Land-based Recircualting Systems
Land-based intensive mariculture-systems are flow-throughs or recirculating systems. Flow-throughs are constructions, where the water inlet and outlet are on opposite ends, permitting aquatic organisms to be grown in streams of controlled velocity. In those systems, the water is used once only and then discharged into the receiving water with or without treatment. Recirculation systems are closed or partially closed systems employed in aquaculture production, where the effluent water from the system is treated and recirculated to enable its re-use.
Usually, a relatively small portion of the actual culture medium is exchanged per unit time. These systems are also called recirculating aquarium systems (RAS). It is assumed that use of recirculation systems in intensive commercial aquaculture will increase in the future. Moreover, these systems would be preferred for cultures of exotic species and genetically modified organisms (GMOs), because escapees can be prevented. Generally, recirculation systems are used for the culture of high value products such as crustaceans and other marine delicatessens.
The development of RAS started in the 1950s in Japan and was introduced experimentally in Europe in the 1970s. Commercial utilisation started in northern Europe, especially in the Netherlands, Denmark and Germany only as early as 1980s. Instead of being released, the wastewater is recycled after mechanical and biological filtration and oxygen injection. The combination of good water quality, appropriate temperature and oxygenation allows stocking densities of more than 50 kg/m2 without limiting the health or well being of the fish. Additionally, advantages of recirculation technologies are the minimisation of water consumption and pollution impacts on the environment, as well as the possibility to control the essential production parameters such as water temperature, oxygen concentration and the spread of diseases.
Careful choice of technology and management allows for optimum year round growth and food conversion. Systems can be adaptable for a wide range of species and the production cycle can be controlled to supply the markets on a year round basis. If seasonal prices are low, the system can be run with only half of the potential stocks until the market shows higher prices.
Major disadvantages of such systems are however the high-energy costs and initial capital costs for establishing the system. Also, the equipment used to move and treat the rearing water requires highly skilled workers, who require knowledge ranging from plumbing to computer systems. Such aquaculture systems require intensive, constant monitoring of the various parameters of water quality, as malfunction of the facility can prove quite costly if not noticed in time.
Primary functions of recirculation systems are
- to hold and culture fish, shellfish, crustaceans or plants under controlled conditions,
- to reduce water replacement when water supply is at a premium (??) and/or when energy costs for reheating or cooling are high and
- to reduce and manage effluents to minimize negative environmental effects.
The applications of recirculation vary widely, such as broodstock management, hatchery and nursery rearing, grow-out and quarantine holding.
Intensification stock densities can cause stress by disrupting the social structures of fishes - however, this varies from species to species. Therefore, fish may require pre-adaptation to the breeding conditions. RAS can also be designed to be highly species-specific. Species that are currently difficult to culture in other aquaculture systems can possibly perform better in recirculation systems. Strains that have been cultivated and adapted to recirculation systems seem to perform best.
The main water quality problems involve oxygen and ammonia levels. Elaborate treatment systems are required to reduce ammonia levels and to maintain oxygen levels in these systems. Ammonia is problematic as it is a natural by-product of metabolic processes in fish and acts toxic in high amounts. The high stocking and feeding rates used in intensive systems mean that waste production (uneaten food and feaces) can be very high and so waste disposal is an issue for this type of culture.
Recirculation systems usually are composed of tanks to support fish and a complex system for water treatment. The water in the tanks tends to be replaced quite rapidly (usually every few hours). This is necessary because of the high stocking rate and potentially toxic waste products such as ammonia. Water generally enters the tank at the top, thus creating a clockwise water movement around the tank and drains continuously through a centre bottom drain. The water is treated to remove wastes and add oxygen and is then re-used in the tanks. Efficient recirculation systems recycle 95% of the water, thus, a continuous supply of fresh "new" water is needed.
The recirculation system is build of some pre-treatment (bio-filtration, oxygenation, sterilisation). Water is pumped through the culture tanks where it has a constant aeration. From the culture tanks water passes through a hydrozyclon or other mechanical filters where larger particles, such as uneaten food, feaces and sediment, is removed. Afterwards the water passes through a biofilter and a protein skimmer. In the biofilter metabolic-by products, particularly ammonia, is removed. Those biofilters contain 'media' with a high surface area, such as silica sand or special plastic rings which provide a substrate for ammonia-scrubbing bacteria (Nitrosomonas, Nitrobacter).
There are two groups of bacteria in biofilters, those that digest organics and the second group that convert toxic ammonium into nitrate. The group that digest organic matter grows very much quicker than the ammonium reducing nitrifying bacteria. If the organic load on the biofilter is too high the heterotrophic bacteria that reduce the organics would simply displace the nitrifying bacteria from the filter. However, through the efficient removal of solids and the minimising dissolution of the particles by mechanical filtration, the organic load is minimised. This allows the nitrifying bacteria to develop and hence the performance of the biofilter is maintained. In addition to removal of organic compounds and ammonium from the recycled water, the trickling down flow in biofilters also reduces excess of carbon dioxide.
In the protein skimmer proteins are extracted from the water column by vigorously mixing the water to be treated with injected air or liquid oxygen. Some of the water may be disposed of but the main part is re-circulated back into the culture tanks; treated and clean water is added to the re-circulated stream.
Ozonisation within the protein skimmer cracks up any remaining organics that cannot be digested by the biofilters directly, and renders them into a stage in which they can then be extracted by the biofilters. In addition to this function, ozone cracks up pheromones that could otherwise adversely influence the development of fish.