V

Figure 8. Schematic representation of a typical closed water system design.

tance of aeration, ozone can be driven off or converted to molecular oxygen. Various commercial firms market ozone generators and can assist aquaculturists in selecting the proper equipment to meet system needs.

Ultraviolet (uv) light has also been used to sterilize the water in aquaculture systems. The effectiveness of uv decreases with the thickness of the water column being treated, so the water is usually flowed past uv lights as a thin film (alternatively, the water may flow through a tube a few centimeters in diameter that is surrounded by uv lights). Ultraviolet light systems require more routine maintenance than ozone systems. The bulbs emit lower and lower quantities of uv light with time and need to be changed periodically. In addition, organic material exposed to uv light will foul the surface of the transparent quartz or plastic tubes separating the bulbs from the water, thereby creating a film between the water and the uv source that reduces the effectiveness of the uv light.

Recirculating systems often feature other types of apparatus, such as foam strippers and supplemental aeration. Devices for denitrifying nitrate to nitrogen gas have been developed to the point that they may find a place in commercial culture systems in the near future.

The technology associated with recirculating systems is expensive. Redundancy in the system, that is, providing backups for all critical components, and automation are important considerations. When a pump fails, for example, the failure must be instantly communicated to the cultur-ist, who must have the ability to keep the system operating while the problem is being addressed. Loss of a critical component for even a few minutes can result in the loss of all animals within the system. Computerized water-quality monitoring systems that will sound alarms and dial emergency telephone numbers to report system failures are finding increased use among culturists who employ recirculating systems. The computer can also be used to put backup systems on line in the event of a primary system failure.

Recirculating systems can make aquaculture feasible in locations where conditions might otherwise not be conducive to successful operations. Tropical and subtropical animals such as tilapia and penaeid shrimp have, for example, been reared in recirculating systems in the northern tier of states in the United States. Recirculation systems can also be used to reduce transportation costs by making it possible to grow animals near markets. In areas where there are concerns about pollution from aquaculture system effluents or with the use of exotic species, recirculating systems provide an alternative approach to more extensive types of operations.

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