Gas Balancing Tech Info

The main gases of importance to fish culture are oxygen (O2), nitrogen (N2), and carbon dioxide (CO2). Dissolved oxygen levels in hatchery waters, depleted through the respiration of the cultured fish and by the oxidation of organic matter in the water, must be replaced or supplemented to provide adequate oxygen to the fish. The same reactions that deplete oxygen tend to increase carbon dioxide concentrations which may accumulate to toxic levels in intensive culture systems. Nitrogen is primarily of concern in hatchery influent water and must be controlled to avoid the dangerous supersaturation condition. Other toxic gases, such as hydrogen sulfide (H2S), may also be present in influent water systems and will require removal.

Gas balancing is gas transfer where the objective is to have the concentrations of all dissolved gases approach a state of equilibrium with the gases in the atmosphere (i.e. dissolved gases approach their individual saturation concentrations). When air is in contact with water, gas transfer will continue until equilibrium is reached and all gases present are at their saturation concentrations. For gases that are supersaturated, some of the gas will transfer out of solution; for gases that are under-saturated, some of the gas will transfer into solution.

This process naturally occurs whenever water comes into contact with atmospheric gases but may be achieved more quickly and efficiently with gas transfer equipment. A common function of most gas transfer equipment is to break a flow of water into droplets or thin films in an ambient or altered atmosphere. This greatly increases the exposed surface area of water and accelerates the transfer of gas molecules into and out of the water. Alternately, small bubbles are passed through the water to provide the gas to liquid interface. Small bubbles have larger surface areas per volume than larger bubbles.

A special case requiring gas balancing occurs when the combined concentration of all gases, expressed as the total gas pressure (TGP), exceeds the saturation level of gases in the water. This condition, known as supersaturation, is unstable and dangerous in fish rearing water as it may cause a condition called Gas Bubble Disease. As water passes over the gills, the dissolved gases from the water are diffused into the blood of the fish. Gas Bubble Disease results when supersaturated gases come out of solution, forming bubbles (emboli) in the blood and tissue of the fish. The bubbles restrict circulation, blocking oxygen supply to the tissues and commonly resulting in death (Colt, 2001).

Supersaturation with nitrogen is the most common cause of Gas Bubble Disease. Nitrogen comes out of solution very easily due to its relatively low solubility in water. Nitrogen supersaturation is a common issue in hatchery influent water which is extracted from depth (such as wells or deep lake intakes). Supersaturation may also occur when cold, nitrogen-saturated water is heated, decreasing the solubility of the gases and forcing the gases out of solution. For this reason, gas balancing should always occur after heating the water.

PR Aqua designs and manufactures products and custom designs solutions to address many gas balancing challenges. The selection of a technology will depend on the specific gas targeted and the application. Aeration columns may be used to increase oxygen levels and/or for degassing of low levels of gases such as nitrogen. Vacuum degassers may be used to treat high TGP due to nitrogen supersaturation. Forced-air aeration columns are effective at simultaneously reducing super-saturated gases (such as CO2 and/or H2S) and increasing under-saturated gases (such as O2) to approaching their natural atmospheric balances. A special application of forced-air aeration is for CO2 stripping in intensive Recirculating Aquaculture Systems (RAS) where the removal of CO2 and the addition of O2 are required to restore a healthy gas balance.

Whether it’s a hatchery influent or recirculation/reuse treatment application, PR Aqua can help you to find a solution tailored to your gas transfer needs.