Assimilative capacity

As a self- cleaning is generally defined as the ability of an object or system to be able to free themselves independently and in particular without external intervention of pollution or of specified material inputs.

A distinction is primarily between two meanings:

• Self-cleaning of surfaces, for example about the lotus effect or photo- catalytic self-cleaning, as well as technical methods such as pyrolysis, for example at ovens.
• Self- purification of biological or ecological systems. It includes both organisms and their ability to break down pollutants or excrete, and the ability of complex ecosystems, independently of environmentally foreign to the entry or environmentally damaging substances to cope.

An example of the self-cleaning of animals is their excretion of end products of metabolism as well as useless or harmful substances that were ingested with food.

The most common use of the term refers to water whose biodegradation process of organic compounds by plant, animal and bacterial organisms ( Saprobionten ) is called with simultaneous consumption of oxygen as a self-cleaning.

Self- purification of water bodies

The self-purification capacity of water bodies is dependent on the oxygen content of the water and thus factors such as the soil and the transported material, the structural quality and the water temperature.

Under oxygen deficiency leads to anaerobic decay processes. In the oxygen-rich water, however, a limited amount of organic matter will decompose eg from waste water by bacteria, fungi, and small animals ( cf. decomposers ). Therefore, the self- cleaning potential in flowing waters is higher than in stagnant water, because the entry of atmospheric oxygen on the moving surface is greater there. A further factor is the water temperature, as during heating of the water body ( such as through secondary cooling water) oxygen solubility decreases, but the degradation processes accelerate. As a natural source of oxygen also serve the suspended algae phytoplankton which nursery algae and higher aquatic and waterside plants.

In connection with the self-cleaning the water quality classification of rivers under the system of saprobic index. This is obtained by the determination of a measure of the indicator organisms load of degradable organic material, and classified according to a standardized procedure.

The self-cleaning capacity is made available in the so-called biological clarification stage in sewage treatment plants.

A little further defines the self-cleaning includes the following aspects:

• Microbially: microbiological conversion of organic and inorganic substances
• Chemical: dissolution and precipitation processes, sorption and desorption, transformation of organic and inorganic substances
• Physical: dilution, sedimentation, filtration and degassing

Benefits of self-cleaning

The term self-cleaning ability of water is meant to be able to decompose organic water pollution or stress.

Through the processes described above are such loads " degraded " and thus in a less complex - not necessarily organic - Convicted connection ( from organic compounds, for example mineral salts). Some operations actually remove substance - Nutrients - from the water and thus prevent automatically eutrophication and thus also the overturning of the lake, as long as the anthropogenic pollution is not too large:

• Long-term transformation of pollutants into fertilizer and then in living biomass (eg producers such as phytoplankton, consumers such as fish and ducks)
• Outgassing of nitrogen, CO2 or NH3 ( ammonia)

Critical review

The self-cleaning of the water fails due to excessive pollution often for the following reasons:

A subset of phosphates and nitrates remains effective in fertilizing waters and takes in every case to eutrophication. A part thereof is initially sedimented ( see also: phosphate case ), but upon the occurrence of a lack of oxygen to the sediment it is returned dissolved (see tipping ). Also, filtration and sorption act only temporarily relief.

Also mentioned as an advantage in living biomass conversion provides no real relief represents the biomass takes part in the material circulation of the water. It leads, for example, again for increased detritus (eg dead plants, feces ), which in turn is mineralized by decomposers to fertilizer salts. These nutrients, and thus the water pollution remain, so get the water.

The self-cleaning capacity refers to organic items as they occur in nature, such as fallen leaves, dead wood, entries from swamps, bogs. The anthropogenic inputs from industry and agriculture exceeded the self-purification capacity of water bodies in the past, by far. The self-purification capacity of water bodies was also reduced by the technical Gewässerverbau ( straightening and fixing). No self-cleaning capacity have the waters to many anthropogenic pollutants such as heavy metals, pesticides and many industrial wastes for which there is no metabolic pathways in aquatic organisms.

Increasing or restoring natural self-purification capacity of water is one of the objectives in the renaturation of watercourses.