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Degassing & Air stripping

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Water often contains high partial pressure of nitrogen, especially borehole water or water drawn from depth in lakes or marine systems abstracting  from Fjords.  High partial pressure of  dissolved nitrogen gas is extremely dangerous for fish and will cause  chronic sub-lethal problems, to acute toxicity depending upon the partial pressure of nitrogen gas in solution.

In addition to nitrogen gas, borehole water or water abstracted from depth may have a low redox potential, high heavy metal concentrations, especially iron and manganese. There may also be VOC`s ( volatile organic carbon) such as methane or  hydrogen sulphide. The zeta potential also tends to be low which makes is very difficult to remove heavy metals.

Passage of water through a Dryden Aqua partial vacuum degassing / stripping column can have a profound impact to the quality of the water from the following typical sources;

Water type & source of elevated dissolved nitrogen
  • Bore Hole Spring Water
  • deep water abstraction from Fjords or freshwater lakes
  • Any anoxic or low redox or low zeta potential water
  • Long pipe runs High water pressure systems
  • Pumped systems Hydro Power Generating stations
  • Ice formation Intake pipe drawing air
  • Heated water systems Faulty pipe/pump seals
  • cavitating pumps venturi injectors and some aeration systems

Borehole water

All Bore hole water should be degassed prior to drinking water treatment of aquaculture applications.  The benefits of the Dryden Aqua partial vacuum degassing system are as follows;

  • reduce total gas pressure to less than 100%
  • reduce  nitrogen partial pressure to less than 100%
  • reduce and restore carbon dioxide partial pressure
  • stabilise the pH of the water (usually increase pH)
  • blow off volatile organic gas such as methane
  • blow off and oxidise hydrogen sulphide
  • raise the oxygen level close to 100% saturation
  • oxidise heavy metals such as iron and manganese
  • raise the oxidation potential REDOX of the water and reduce fungi & pathogenic bacterial risk

The following paragraphs are concerned primarily with gas supersaturation of water with nitrogen and the impact on fish. For drinking water click here for a typical schematic suitable for drinking water.


Dangerous levels
Eggs can tolerate a relatively high total gas pressure (110% to 115%), however as the eggs hatch they become very sensitive to elevated gas pressure. Fish moralities can be expected at gas (nitrogen) pressures above 105%. However at lower levels ranging from 102% to 105% moralities might also be expected.  The low gas pressures may not cause moralities directly, however they will stress the fish and predispose them to infection and possibly long terms problems.

Gas pressure and gas bubble trauma
Salmonids and most aquatic animals will tolerate up to 102% gas supersaturation with nitrogen. However long term damage may be inflicted on the organisms. For example in the case of juvenile salmon, exposure to low level gas pressure may damage the gill structure, bone formation, blood capillaries and the nervous system. These conditions will render the fish susceptible to a high rate of infection in the hatchery.

Symptoms
Recent evidence has shown that levels up to 102% can cause an increased incidence of myxobacteria gill infections, the fish also become sensitive to light and mass moralities may be experienced shortly after feeding the fish. Mass moralities may also be experienced during periods of low atmospheric air pressure or if water level in the tanks are reduced.

There is also some evidence which suggests that exposure of salmon to elevated gas pressures for a short period of time after egg hatch may reduce the ability of the smolt to osmoregulate when transferred to the sea. It may therefore be the case that exposure to elevated gas pressures at an early stage in the Atlantic Salmon life cycle could be a possible reason for 'Failed Smolt Syndrome, or it could predispose the fish to increased incidence of disease or other as yet un-recognized problems.

Carbon Dioxide
Gas supersaturation can also occur with carbon dioxide in certain water types, such as spring or bore hole water in which the geology of the surrounding area is predominantly of carbonates.

High levels of carbon dioxide can also occur in recirculation systems due to the respiration of the fish and bacterial filters. The haemoglobin in the blood of fish has a high affinity for carbon dioxide, and at levels of 25mg/l free carbon dioxide the ability of the fish blood to absorb oxygen is reduced by 50%.

The carbon dioxide will also tend to acidify the water, thus pH buffering is essential in recirculation systems. Maintaining stable pH or slightly alkaline conditions will greatly assist in reducing the concentration of carbonic acid. However, assisted degassing by ventilated bacterial nitrification or vacuum degassing may also be required in systems with greater than 95% water reuse

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Last modified: November 17, 2009