Oxygenation Tech Info

Dissolved oxygen levels in aquaculture water are consumed through the respiration of fish and by the bacteriological degradation of organic matter. As fish densities increase, supplementation of the dissolved oxygen may be required to sustain a healthy environment for the fish. This process, called oxygenation, may be achieved through the use of specialized gas transfer equipment.

When a gas is in contact with water, the gas will continue to transfer in or out of the water at the water surface until equilibrium is reached. This point is the saturation concentration of the gas. The saturation concentration depends primarily on the temperature, pressure and the gas composition.

The rate at which gas transfer occurs is primarily dependent on the surface area, on the amount of contact time, and on the difference between the dissolved concentration and the saturation concentration of the gas. As flow rates in aquaculture facilities tend to be very high, either the oxygenation equipment must be very large or the contact time is very limited. To account for this limitation, oxygenation equipment is designed to maximize surface area, and to increase saturation concentration through modifying the gas composition and the system pressure.

Surface area is increased through one of two ways. Either the water is broken up into thin films or small streams or droplets, or the gas is broken up into small bubbles. The surface area is proportional to the size of the droplet or bubble; small bubbles have more surface area per unit volume than large bubbles.

Under normal conditions, oxygen is approximately 21% of the total gas in air. Increasing the concentration of oxygen in an atmosphere will proportionately increase the saturation concentration of dissolved oxygen in water. Oxygenation systems typically use an input of pure (or almost pure) oxygen which allows for greatly increased oxygen levels over systems that use an input of air (with only 21% oxygen). Discharge of water with the resulting high concentration of dissolved oxygen into culture tanks, which are open to the air, means that the water could be “supersaturated” with oxygen. If the fish (and associated bacteria) do not consume the excess oxygen, oxygen transfer from the water out to the air will occur in the culture tanks. Normally, however, rapid mixing with under-saturated water prevents the excess oxygen from coming out of solution.

At a given pressure, the water can only hold a certain total amount of gas. If the oxygenation device is kept at atmospheric pressure, the total amount of gas dissolved in the water does not change between the inlet and outlet. The oxygen rich environment causes the saturation concentration of oxygen to increase while the saturation concentrations of all other gases (such as nitrogen, argon, and carbon dioxide) decrease proportionally. As a result, as oxygen transfers into the water, these other gases transfer out resulting in no change in the total gas dissolved in the water. This is the principal used in the Low Head Oxygenator (LHO™). Within it, small streams of water rain through low pressure chambers filled with oxygen rich gas.

Some oxygenation technologies combine use of an oxygen rich environment with an increase in pressure. By increasing the pressure, the total amount of oxygen which can be dissolved in the gas is increased even further. This is the principal used in the oxygen cone, the oxygen saturator and the pressure packed column. The oxygen cone and oxygen saturator suspend small oxygen bubbles in the flowing water within the vessel, using the vessel geometry to prevent bubbles from carrying out of the vessel before the oxygen is absorbed. The pressure packed column breaks the water flow into thin films as it passes through a vessel filled with an oxygen rich gas.

Fine bubble diffusers may be used to deliver bubbles of oxygen directly to each culture tank. As the oxygen bubbles rise, they are absorbed. Although efficiency increases with tank depth, diffuser efficiency is low relative to other oxygenation equipment because any oxygen in bubbles that reach the water surface is lost to the air. However, diffusers can be incorporated into an effective emergency back up oxygenation system as they do not rely on water flow to deliver the oxygen to the fish.

PR Aqua offers a range of oxygenation equipment, both low head and pressurized, for aquaculture applications. For a more complete solution to your oxygenation needs, PR Aqua can also provide on-site oxygen generation equipment, and an integrated design for equipment installation and oxygen distribution.