Oxygenation of water
To increase the amount of oxygen in the water above equilibrium and levels possible with traditional aerators, pure oxygen gas can be added. The addition of pure oxygen gas to the water is used in several cases. One is to increase fish production when there is not enough water. If the water has to be pumped to the farm, regardless of whether it is salt water or freshwater, it may be viable to add pure oxygen to reduce the necessary water flow and hence reduce the pumping costs. Normally it is more economical to add pure oxygen instead of pumping, but a calculation must be made in every single case. In systems with re-use of water it may also be worth adding pure oxygen to reduce the amount of new water and amount of water pumped through the system. When transporting fish pure oxygen is usually supplied.
It is important that the water is fully saturated with oxygen before starting to add pure oxygen gas. An aerator should therefore be installed before the point where pure oxygen gas is added; if there is no aerator, pure oxygen is used to saturate the water up to equilibrium before starting to supersaturate. This is, in most cases, unnecessarily ex-pensive, but calculations can be made to evaluate cost-effectiveness.
When the water is supersaturated with oxygen free gas bubbles are present; if this water were tohave a free surface to the atmosphere there would be gas exchange and the oxygen content would fall to 100% saturation. Therefore free surfaces should be avoided after supersaturating the water with oxygen; the best method is to have a short pipe run from where the oxygen is added to where it reaches the fish.
An oxygenation plant may be designed either to cover the whole oxygen requirement for the fish or as a top-up oxygenation facility. The design criteria for the oxygenation plant are determined by the situation where the biomass and water temperature are highest. A complete oxygenation system on a fish farm includes two parts: (1) the injection system that brings the gas into the water and (2) the source of oxygen gas.