Recirculation Aquaculture: Oxygenation cont'd

18. Oxygenation Continued

Use of Pure Oxygen

Photos Courtesy of Aquatic Ecosystems, Inc.

Oxygen Injection

Pure oxygen can simply be injected into pipes leading to the tank. If these lines are under pressure from a pump, the increased hydrostatic pressure will increase oxygen diffusion. Reasonable transfer efficiencies can be achieved with oxygen injection, though some oxygen is still lost to the atmosphere.

Aeration Cone (more properly Oxygenation Cone)

Sometimes called a Speece cone or a down-flow bubble contactor, this oxygenation technique cleverly uses the change in water velocity that occurs in different diameter pipes to ensure complete diffusion of pure oxygen bubbles. Water flows down through a cone, as the cone widens, the water slows. At the same time, oxygen bubbles are injected into the bottom of the cone. The bubbles rise against the counter flowing water until the point that the velocity of the down flowing water equals the speed of the bubbles rise. At this point, the bubbles hover in the water flow as they slowly diffuse away. There is no off-gas and thus no loss of expensive oxygen.

Theoretically it would be possible to adjust a down flow of water in a cylinder to equal the velocity of rising bubbles and achieve the hover necessary for complete absorption, but practically it would be impossible to maintain the exactly correct flow rate for very long. Flow would either increase or decrease just enough to let the bubbles rise through the column and stall the pump or be blown down and out into the fish tank. The trick of the cone is to automatically generate a range of velocities which, even with small fluctuations in flow, will allow the bubbles to hover somewhere in the cone. Fiberglass cones are marketed for this technique, but it is also possible to buy (or make) a series of three pipes of increasing diameter adapted together. These are sometimes called oxygen saturators. This arrangement approximates a cone and works in the same way. Since this apparatus only offers three velocities, what happens is that the bubbles hang at the union between two sections, but the effect is the same.

Carbon Dioxide Stripping

As mentioned previously, the use of oxygenation almost demands that carbon dioxide stripping be employed. Even in oxygen systems that generate an off gas, the high rate of diffusion of pure oxygen (because of a gas pressure of one atmosphere or more) precludes the adequate movement of carbon dioxide out of the system because of insufficient gas/water interface. The most common way to increase gas/water contact and increase diffusion of carbon dioxide out is to employ a stripping column (degassing tower). A carbon dioxide stripping column is a counter flow packed column. A similar arrangement was described in the last chapter as an oxygenation method. The difference is that a carbon dioxide stripper is open to the atmosphere.

Assignment 18