As a self-cleaning is generally refers to the ability of an article or system to be able to free itself independently and in particular without external intervention of pollution or of specified substance inputs.
A distinction is primarily made between two meanings:
- Self-cleaning of surfaces, for example via the lotus effect or photocatalytic self-cleaning , but also technical processes such as pyrolysis, for example in ovens . See also: self-cleaning glass and self-cleaning concrete .
- Self-cleaning of biological or ecological systems. This concerns both living beings and their ability to break down or excrete pollutants, as well as the ability of complex ecosystems to deal independently with the input of environmentally unrelated or environmentally harmful substances.
An example of the self-cleaning of animals is their excretion of end products of the metabolism as well as of unusable or harmful substances that were ingested with food.
The best-known use of the term refers to bodies of water whose biological degradation process of organic compounds by plant, animal and bacterial organisms ( saprobionts ) with simultaneous consumption of oxygen is called self-cleaning.
Self-cleaning of waters
The self-cleaning power of bodies of water depends on the oxygen content of the water and thus on factors such as the soil and the material transported, the quality of the structure and the water temperature.
Anaerobic putrefaction processes occur when there is a lack of oxygen . In oxygen-rich water, on the other hand, a limited amount of organic matter, e.g. B. from waste water by bacteria , fungi and smaller animals (see. Destruents ) are decomposed. Therefore, the self-cleaning potential in flowing water is higher than in stagnant water, because the entry of atmospheric oxygen via the moving surface is greater there. The water temperature also plays a role, since when the water is heated (e.g. by drained cooling water ) the oxygen solubility decreases, but the degradation processes accelerate. The floating algae phytoplankton , the vegetative algae and the higher aquatic and bank plants also serve as natural sources of oxygen .
In connection with self-cleaning, there is the water quality classification of flowing waters according to the saprobic index system . Here, by determining indicator organisms, a measure of the load of degradable organic material is obtained and classified according to a standardized method.
The self-cleaning power is also harnessed in the so-called biological clarification stage in sewage treatment plants .
Defined a little further, self-cleaning includes the following aspects:
- Microbial: microbiological conversion of organic and inorganic substances
- Chemical: dissolution and precipitation processes , sorption and desorption , conversion of organic and inorganic substances
- Physical: dilution , sedimentation , filtration and outgassing
Benefits of self-cleaning
The term self-cleaning refers to the ability of water bodies to reduce organic water pollution or pollution.
Through the processes described above, such pollution is "reduced" and thus converted into a less complex - no longer necessarily organic - compound ( organic compounds are converted into mineral salts , for example ). Some processes actually remove substance - nutrients - from the water and thus automatically prevent eutrophication and thereby also the overturning of the lake, as long as the anthropogenic pollution is not too great:
- Long-term conversion of the pollutants into fertilizer and then into living biomass (e.g. producers such as phytoplankton , consumers such as fish and ducks)
- Outgassing of nitrogen , CO 2 or NH 3 ( ammonia )
The self-cleaning of the water often fails if the pollution is too high for the following reasons:
Some of the phosphates and nitrates remain in the water as fertilizer and in any case contribute to eutrophication . Part of it is initially sedimented (see also: phosphate case ), but in the event of lack of oxygen to the sediment it will return back solved (see. Upset ). Filtration and sorption also only have a temporary relief.
Even the conversion into living biomass mentioned as an advantage does not represent any real relief. The biomass takes part in the material cycle of the water. She leads z. B. again to increased detritus formation (e.g. dead plants, excrement), which in turn is mineralized by the destructors to form fertilizer salts. These nutrients, and thus the water pollution, are retained in the water.
The self-cleaning power refers to organic inputs as they occur in nature , e.g. B. Leaf fall , dead wood , entries from swamps , moors . The anthropogenic inputs from industry and agriculture far exceeded the self-cleaning power of water bodies in the past. The self-cleaning power of the water bodies was also reduced by the technical waterway control (straightening and fortification). The water bodies have no self-cleaning power whatsoever against many anthropogenic pollutants such as heavy metals, many pesticides and industrial waste, for which there are no metabolic pathways in the aquatic organisms.
Increasing or restoring the natural self-cleaning power of watercourses is one of the goals of the renaturation of rivers.
- Ulrich Stottmeister, Erika Weißbrodt, Jörg Tittel: Natural self-cleaning. From the contaminated site to the lake. In: Biology in Our Time. 32, 5, 2002, , pp. 276-285.