How much water should be supplied?
The reason for adding water to a closed production unit is to give the fish or shellfish access to oxygen and remove waste products excreted by the fish. In this way, a water environment that creates optimal conditions for growth is established. The amount of water that must be supplied to a closed production unit depends on a number of variables, including species, fish density, growth stage and rate, water temperature, whether adding pure oxygen or not, and whether hydraulic self-cleaning occurs or manual cleaning is required.
Tables showing the supply of water necessary to satisfy the oxygen requirements of fish and shellfish at different water temperatures have been developed for many species. The amount of water that must be added to the production unit to cover the oxygen consumption of the fish can be calculated based on the amount of fish and water temperature.
Provided the quality of the water supplied is acceptable, one way to regulate the necessary supply of new water to a closed production unit can be to monitor the oxygen concentration in the outlet. This must be at an acceptable level for optimal growth of the actual species, and is normally around 7 mg/l. By using electronically con-trolled actuator valves for controlling the inlet flow,it is possible to have fully automatic flow control based on the oxygen concentration in the outlet water.
The amount of inlet water can be reduced by adding pure oxygen to the incoming water or inside the production unit. With high fish density and rapid growth, consideration must, however, be given to the concentrations of CO2, suspended solids (SS) and NH3 that might become excessive when adding oxygen. If this is the case, these sub-stances need to be removed; this requires water treatment as is carried out in a water re-use system.
The hydraulic forces in the water supplied to the tank may also be used to clean it; this is known as hydraulic self-cleaning. Extra requirements then apply to the amount of added water; This gives another method for calculating the necessary supply of new water. If this water supply does not fulfil the requirements for oxygen, pure oxygen gas must be added to make up the shortfall.