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GAS TRANSFER IN AQUACULTURE

 Fiche conseil élaborée par J. PETIT (INRA) et M. MOUTOUNET - Sté SEDIA / Sté AQUINOVE  


AERATION Gas transfert in aquaculture             






 


Aeration – oxygenation degassing of carbon dioxide, nitrogen and ammonia


Gaseous exchange (and this concerns not just oxygen but also ammonia, nitrogen and carbon dioxide) is of great importance in aquaculture and is, in general, the main limiting factor in intensive breeding. The machines or devices used by fish farmers, whatever their main function, are involved in the equilibrium of all gasses present. The type of the device depends on the concerns of the farmer, regarding one gas or another and the optimization of the equipment by the designer in view of the desired result. 
In reality, an 'oxygenator' can eliminate carbon dioxide, a 'degassing tower' can aerate water and an 'aeration turbine' can help to eliminate ammonia. Although those notions were not very important when the sole aim was to supply oxygen to the fish, the same is not true for tanks fed with oxygenated water above natural concentrations (beyond saturation) to maintain a large stock with limited water flow. 


Carbon dioxide and ammonia produced by the stock will interfere with the oxygen when exchange is being carried out by the oxygenator. If these interferences are not taken into consideration, they can for example, affect the functioning of the oxygenator (due to carbon dioxide accumulation) or even affect the health of the fish (by ammonia accumulation).


As a result, all apparatus for mixing the water or introducing gases should be operated with care to optimise the technical and economic results (and thus lower production costs). To be able to determine the best operation, the fish farmer must recognise the different effects that the equipment can have on gas transfer which affects the lives of the fish: such as oxygen, but also, in intensive breeding conditions, carbon dioxide and ammonia, and in the case of pressurised or heated water, nitrogen. The following discussion is intended to stimulate thinking on this subject.












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aqualog@aqualog.fr                Tel. : 0033 (0)4 94 10 26 26




 







 


 


Gas transfer


The principles of dissolution (passage of gas to a dissolved state) and degassing (passage of dissolved gas to a gaseous state). The rules that govern gas transfer at the surface of the water, of a bubble or droplet, are the same whatever the direction of passage (towards water or towards a gaseous state). Two simple basic rules should be remembered:

a) The direction and extent of the transfer are determined by the difference in the pressure of a given gas (oxygen, carbon dioxide) on either side of the 'liquid film' which separates the gas from the water. (1) Pressures are converted to concentrations with the help of coefficients as a function of temperature (Bunsen coefficients). Consequently, if there is no difference in concentration at the interface, there is no exchange.
An aerator which produces a strong concentration of oxygen in its immediate proximity will therefore transfer only a tiny amount of oxygen.


b) To maintain a difference in gas concentration between the two components (bubbles and water) requires a constant renewal of the liquid phase (water flow) and gas phase (gas flow). Thus, so that there will be a difference in oxygen content allowing oxygen diffusion, it is necessary to install a flow of water in the zone influenced by the aerator (where there is bubbling). The oxygenation capacity will only reach its full potential in these conditions. 


Note:
- The bubbling acts as a barrier for the water, which has a tendency to pass 'around' the oxygen zone if the apparatus allows it. This is not a problem in a purification station when the oxygen consumers (bacteria) come into contact with the bubbling (not the case in aquaculture).
 


To contact AQUALOG


aqualog@aqualog.fr                Tel. : 0033 (0)4 94 10 26 26




 





Performance parameters in the transfer from gas to liquid


Whatever the fate of the oxygen (oxygenation, degassing) its use can be determined with a knowledge of a machines performance standards, established if possible by certified bodies (for example, CEMAGREF - agricultural engineering) which provide a trial report to the manufacturers. Standard performances (in France) are from the mixing of deoxygenated fresh water, in the presence of air, at a temperature of 10°C,

They are characterised by;
-the standard oxygen capacity ( S.C.O.) per hour, expressed in kg of dissolved oxygen /hour.
-the gross specific supply (G.S.S. expressed in kg of oxygen transferred per kWh)
This data allows the calculation of the performance under all forms of application.
The application conditions are defined as:
-the water and gas temperatures
-the concentrations of gas to be transferred into the liquid and gaseous phases.
-the concentration of dissolved salts and suspended material.







Particularities of the degassing of carbon dioxide and ammonia


These two constituents in a gaseous state follow the laws of exchange described above. However, in practice, it must be remembered that these constituents also form a part of the dissolved salts:


H2O + HCO3 <->  1 HCO3- + H
H2O + CN3  <->  1 NH4+ + OH



The extraction of a gas in a machine may have no apparent effect if the 'chemical reservoir' is considerable. The gas extracted is replaced by gas created from that in the ‘chemical reserve’. The 'stripping' of ammonia and carbon dioxide must take into account the chemical equilibriums at different pH. In some cases, the chemical composition of the water is such that there is no economically acceptable solution to eliminate the excess of dissolved gases.


 


To contact AQUALOG


aqualog@aqualog.fr                Tel. : 0033 (0)4 94 10 26 26




 





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