Degassing and CO2

Degassing is the removal of dissolved gases from the culture water by contacting it with air. In a recirculating system the target is almost always carbon dioxide, though the same process also relieves nitrogen supersaturation and brings the water toward equilibrium with the atmosphere generally.

Unlike aeration, whose purpose is to add oxygen, degassing works in the other direction: it exploits the very low carbon dioxide content of air to drive CO2 out of the water.

• Dissolved CO2 forms carbonic acid, consuming alkalinity and depressing pH.
• Falling pH slows nitrification, so a CO2 problem becomes an ammonia problem.
• Pure oxygen systems make it worse: they raise oxygen without providing the gas exchange that would strip CO2, so intensive systems on pure oxygen accumulate it fastest.
• The symptoms — poor appetite, reduced growth, respiratory distress — look like an oxygen problem and are frequently misdiagnosed as one.

Stripping depends on contact between water and a large volume of air with a low carbon dioxide content. Two variables govern performance: the surface area created between water and air, and the ratio of air volume to water volume passing through the contactor. A degasser with insufficient ventilation saturates its own air stream with CO2 and stops working, however well distributed the water is.

Degassing belongs after biofiltration and before oxygenation. After biofiltration, because nitrification itself produces carbon dioxide and stripping upstream of it would miss that contribution. Before oxygenation, because a degasser brings water toward air equilibrium and would strip away any supersaturated oxygen added beforehand.

• Air-to-water ratio is the primary design variable. Too little air and the exhaust saturates with CO2, ending transfer regardless of contact area.
• Column height and packing set the contact area and therefore how close to equilibrium the water gets.
• Removal is driven by the concentration difference, so a degasser removes a great deal from heavily loaded water and progressively less as the concentration falls.
• Exhaust must leave the building. Recirculating CO2-laden air back through the degasser defeats it and creates a workplace hazard.
• Alkalinity and CO2 are coupled: stripping CO2 raises pH, and dosing alkalinity changes how the carbonate system responds to the CO2 present.

• Size for CO2 production at peak feeding, including the biofilter's contribution, not for fish respiration alone.
• Provide adequate and adjustable airflow, and duct the exhaust outside the building.
• Place after biofiltration and before oxygenation.
• Provide even water distribution over the packing, and access to inspect and clean it, because fouled packing channels.
• Account for the degasser's head requirement in the hydraulic design; it is often the highest point in the loop.
• Provide the means to measure pH, temperature and alkalinity so that dissolved CO2 can be tracked at all.
• Consider building ventilation as part of the same design, since the CO2 removed from the water ends up in the air.

• Track pH and alkalinity together as the routine indicator of the carbonate system, and calculate dissolved CO2 from them.
• Verify that the degasser fan or blower is actually moving air, rather than assuming it from run hours.
• Inspect packing for biofouling and for uneven wetting, which are the two common causes of gradual performance loss.
• Suspect CO2 whenever fish show respiratory signs and dissolved oxygen measures adequately.
• Watch building air quality where degasser exhaust is poorly ducted, for the sake of the staff as well as the process.

• Clean packing and distribution trays on a schedule; biofilm growth is continuous and its effect is gradual.
• Verify and record airflow rather than inferring it from equipment status.
• Service fans, blowers and their filters, and inspect exhaust ducting for obstruction.
• Calibrate the pH and temperature instruments the CO2 calculation depends on.
• Inspect for air entrainment on pump suctions and at inlets, which is the usual source of nitrogen supersaturation.

• Diagnosing every respiratory symptom as an oxygen problem and never calculating dissolved CO2.
• Placing the degasser before the biofilter and missing the CO2 the biofilter produces.
• Oxygenating before degassing and stripping out the oxygen just paid for.
• Providing insufficient airflow, so the degasser saturates its own exhaust.
• Exhausting CO2-laden air into the building and drawing it back in.
• Adding alkalinity to hold pH without addressing the CO2 that is depressing it.

Total dissolved gas pressure is the parameter that matters for gas bubble disease, and it is not the same as dissolved oxygen. Nitrogen supersaturation arises from entrained air on pump suctions, from pressurised leaks and from certain water sources, and it is relieved by the same degassing process — which is why a degasser has a safety function beyond CO2 control.

The carbonate system links CO2, pH and alkalinity into one interacting equilibrium. Managing pH by alkalinity dosing alone, without adequate stripping, treats a symptom and increases chemical consumption. The two levers are used together, and understanding which is doing the work at any time requires measuring all three variables.

Carbon dioxide is produced by fish and by the biofilter, and accumulates unless stripped. Degassing sits after biofiltration and before oxygenation, depends on air-to-water ratio as much as on contact area, and must exhaust outside. When respiratory signs appear in water with adequate oxygen, dissolved CO2 is the first thing to calculate.