Gas oversaturation

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Gas oversaturation in the water plays an important role in fish farming and aquaristics . It means that from the various gases that make up the air, water quantity, more resolved is when the respective pressure-dependent diffusion equilibrium corresponds with the air. In this case the fish get sick with the so-called gas bubble disease .

Gas oversaturation in the blood or in the tissue fluid of humans is the cause of diving illness . It is caused by pressure relief when surfacing too quickly.

Development of gas oversaturation in fish farming

Gas oversaturation in the water occurs in fish farms as in aquariums in three different ways:

  • by solution of air or other gases under elevated pressure. One possibility is the ventilation of the water via air outlets, since in the depths of the water due to the hydrostatic pressure the correspondingly higher pressure also prevails in the air bubbles flowing out. Frequently, however, air is also drawn into the water at the beginning of pipelines through which the water is distributed in the fish farm and then increasingly dissolves under the back pressure and the hydrostatic pressure in the pipe. The water often arrives at the fish tank oversaturated.
  • by heating water, which was in solution equilibrium with the air at a lower temperature . The gas solubility decreases as the temperature rises, so that the dissolved gas now forms supersaturation after being heated. The typical example of this is aerated spring water in fish farming that heats up in the fish pond . In the aquarium hobby, cold tap water is often aerated and subsequently heated when changing the water. As a rule of thumb, a 3 ° C increase in saturation increases the saturation by 5%.
  • through photosynthesis of aquatic plants and floating algae , which produce oxygen during the day and thereby consume the much more soluble carbon dioxide .

Over-saturation caused by photosynthesis is usually reduced again at night by the oxygen consumption of breathing. The resulting carbon dioxide hardly contributes to “overall supersaturation” because of its much better solubility.

The warming of water is also linked to the daily cycle in many cases. However, pressure-related oversaturation is usually caused by the technical water flow in fish farms and is therefore permanent. As a result, they represent a particular danger for the fish because the gas bubble disease has enough time to develop.

Hydropower plants also usually discharge water that is oversaturated with gas into the river, which makes it uninhabitable for fish and other aquatic animals for a certain period of time.

Total oversaturation as a danger for the fish

Total supersaturation is a condition in which the sum of the partial pressures of all dissolved gases (including the water vapor pressure) is greater than the mechanical pressure at the location of a bubble (air pressure plus hydrostatic pressure).

Such total oversaturation is responsible for the development of the gas bubble disease, not the oversaturation of individual gases. It can be determined directly with the Saturometer . Only when a bubble shows a positive balance in the diffusion of the molecules of all types of gas does it have growth. Only then can it arise at all.

Once a bubble has formed, the molecules of all gases diffuse in both directions at its boundary with the surrounding water. Therefore, contrary to a common misconception, especially in fisheries, there cannot be any bubbles which selectively consist of just one gas that is supersaturated by itself. This is an important difference, for example, from the selective crystallization of a particular supersaturated salt from a mixed salt solution.

Elimination of gas oversaturation

If the path of formation described is observed, it is often possible to avoid gas oversaturation from the outset. So you won't saturate the still cold spring water with air and then let it heat up in the fish pond. This results in almost exactly 5% total supersaturation for every 3 ° C of warming. The aeration should therefore only take place in the warmed water as close as possible to the fish. In the case of piped water in a fish farm you should also carefully avoid that air is drawn into the pipes.

Where it cannot be avoided, gas oversaturation can be removed from the water by long-term contact with (overpressure-free) air, e.g. B. by trickling. In fish farming, this is achieved using surface aerators (water mushroom, paddle wheel) or by trickling over fillers or perforated sheets, which are often stacked in cascades.

literature

  • Colt, JE (1983): The computation and reporting of dissolved gas levels. Water Research vol. 17 No. 8 pp. 841-849.
  • Mortimer, CH (1981): The oxygen content of air-saturated fresh waters over ranges of temperature and atmospheric pressure of limnological interest. In: Mitt. Internat. Society. Limnol. No. 22 Stuttgart, ISBN 3-510-52022-X
  • Weiss, RF (1970): The solubility of nitrogen, oxygen and argon in water and seawater. Deep-Sea Res. 17, 721-735
  • Weitkamp, ​​DE, Katz, M. (1980): A review of dissolved gas supersaturation literature. Trans. Am. Fish. Soc. 109, 659-702

Web links

  • Saturometer , measuring device for determining gas oversaturation in fish ponds and aquariums
  • Government report on gas saturation in the Columbia river system, USA, in connection with power plants