Gas saturation and its implications
By
Dr.Howard T Dryden
Published in
Hatchery International, Issue 5 Ovum 4 Sept/Oct 2003
Much is said of the requirement for adequate dissolved oxygen in aquaculture, its
importance is implicit to the successful culture of fish and aquatic animals of all
species. However oxygen represents only 21% of the gas constituents of air, the principal
remaining gas being nitrogen, the importance of which is often overlooked. This is a
surprising statement. Equally surprising is that nitrogen, or rather the excess
concentration of nitrogen in fish culture water, may be directly or indirectly related to
some 20% of fish mortalities in aquaculture.
The above is a rather bold claim, however at Dryden Aqua we have been measuring nitrogen
gas concentrations on fish farms for nearly 20 years and frequently observe sub-lethal
conditions. Because of the nature of the symptoms such as recurring myxobacterial gill
infections in mild cases, the underling cause of the problem is oft en missed. Nitrogen
supersaturation can occur as a result of both man-made and natural events. For example air
and water mixed together at pressures greater than atmospheric, e.g. in pumps drawing air
in from the suction side, venturi injection systems, spillways, monks or pressurized
oxygenation systems using oxygen generators, will cause varying levels of gas saturation.
Supersaturation will also occur when water is heated in a boiler, or even by the sun when
a length of black pipe is used above ground. Ground water and borehole water is nearly
always supersaturated. Even when an atmospheric depression passes over a fish farm, the
drop in air pressure of 1 - 2% will result in an equal increase in the total gas pressure
in the water, which will diminish only slowly as equilibrium is re-established.
Fish sense high gas pressures, and like a diver, will go deeper in the tank to compress
the gases and thereby prevent nitrogen bubble formation in their blood. When water is at
102% gas saturation, for example, the compensation depth, or depth at which bubbles will
not form in the blood of the fish, is 200mm (8 inches). As a guide, for every 1% increase
in gas pressure, the fish have to swim 100mm (4 inches) deeper in the water to
equilibrate. If the total gas pressure increases to 110% and the depth of the tank is only
500mm (20 inches), the fish can not escape and the consequences will be 100% mortality in
about 30 minutes. If the gas pressure increases to only 103% the fish will survive but
will be subjected to sublethal stress.
At low sublethal concentrations, whenever the fish swim up into the danger zone, there
will be some bubble formation in the surface capillary vessels of the fish. The organs
most at risk are the gills where any damage will result in both bacterial and fungal
infections that keep recurring. The distal ends of the fins, the eyes, swim bladder and
intestine can also be affected. For example, fish can become very exited by sudden changes
in light intensity, they may swim with the head down and may die after feeding because of
their inability to digest food.
A more insidious problem can occur where fish are exposed to elevated gas pressures while
in shallow hatchery tanks. Exposure of young fish, even for a short period could
potentially damage the developing capillary vessels and nervous system. The effects may go
unnoticed for many weeks or months since an animal damaged at an early stage in
development may not exhibit symptoms until later. A classic example of this is exposure of
salmon fry in a hatchery, which results in mortalities during smoltification or on
transfer to sea cages. Exposure to low level gas supersaturation can also predispose the
fish, or act as a precursor to other infections such as anaemia and PKD. The importance of
nitrogen gas supersaturation should not be ignored. One fish farm we worked on described
it as being like exposure to radiation; once the fish have been exposed they will never be
the same.
One simple way to check for supersaturation is to fill a clean glass bottle with the fish
culture water, but leave the bottle in the water with the top off, and with the neck
sticking out. After 30 minutes gently lift the bottle out of the water and inspect for gas
bubbles adhering to the inside. If bubbles are present you definitely have gas
supersaturation, if the oxygen levels are at 100% or less then the gas will be nitrogen
and the fish will be suffering from gas bubble trauma. Even if no bubbles are observed
does not mean that the water is safe since low saturation levels will not be detected by
this technique. To be certain, an accurate total gas meter should be used to check the
water.
The function of a degasser is to restore the equilibrium partial pressures of dissolved
gases in the water to be the same or lower than the partial pressure of the same gases in
the atmosphere. In order to achieve this, the water is trickled down though a media-packed
column. The water flowrate for a 1 metre diameter vacuum degasser column is 100 m3/hr
which translates to a 10% water hold up within the packed bed. For example if the bed
volume is 2000 litres, no more than 200 litres of water will be falling through the column
at any one time.
Water vapour exerts a gas pressure on the water equivalent to approximately 2% of the
total gas pressure. Passive open degassing columns will not compensate for water vapour
pressure, so their best performance will be 102% gas saturated water. Passive degassing
columns are never 100% effective so there will always be a slightly elevated residual
nitrogen partial pressure in the water. In active degassing columns in which air is blown
up through the column, the act of blowing the air through the bed means that there will be
a slightly elevated positive pressure within the column. Such an arrangement will cause
gas supersaturation that could lead to chronic sub-lethal problems among the fish, and
should be avoided.
In the Dryden Aqua columns air is pulled (rather than pumped) through the column creating
a slight negative internal pressure, which compensates for the water vapour pressure. A
large volume of air is drawn counter-current against the flow of water. If water flowrate
is 100 m3/hr, then 100 m3/hr of air will be pulled through the degasser. This volume of
air is not required to effect degassing of the water. However water which has a high
nitrogen pressure (e.g. ground water) will normally have high nitrogen content, high
carbon dioxide and volatile organics such as hydrocarbons, and low levels of oxygen. By
pulling a large volume of air against the flow of water, the water is scrubbed of all
volatiles, increasing the dissolved oxygen level and providing for a very stable partial
vacuum in the column. Typically the total gas pressure on the discharge from the column
can be held at 97 ± 0.5%.
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