Recirculationand Water Re-use Systems
In water re-use or recirculation systems, the outlet water from the fish tanks is re-used instead of being released into a recipient water body (Fig. 10.1). The outlet water is cleaned and used again, which means that the amount of added new water can be reduced. Theoretically, all the outlet water can be re-used, as in aquaria where no new water is required except for that which is lost through evaporation. This is only theoretical, however, because in most cases the cost of removing all contaminants from the outlet water is very high, but this is of course dependent on the water quality requirements of the fish species being farmed. Usually, if maximum growth is required the water quality needs to be high.
A water re-use system includes the fish tank(s) or units for the aquaculture species, an adapted water treatment system and a pump to transport the water around the system. The pump and the water treatment system are the items that make the system distinct from traditional flow-through systems. The water treatment system, which is the heart of the re-use system, may include physical, chemical and biological processes to improve the water quality to acceptable levels.