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Rainwater is water from precipitation in liquid form, the rain . As a product of rain, rainwater is an essential part of the earth's water cycle .

Rainwater on a street

Rainwater runoff

Rainwater pumping station when pumping out a full rainwater sewer

The infiltration rate of water (and other liquids, in the case of snow during melting ) in the respective subsoil is identified and measured with the infiltration rate. Their size depends on the already existing saturation of the respective soil as well as on its chemical, physical, biological and mechanical properties. In agriculture, the method of tillage and the respective planting have a considerable influence on the porosity of the subsoil ( soil culture ). Working with large and therefore heavy machines promotes soil compaction . The colonization by living things also plays a certain role: Numerous earthworms loosen the crumb to a considerable extent. In vineyards, the creation of transverse grooves promotes the retention capacity of the more or less inclined locations . The lower the absorption capacity of a soil, the faster surface runoff will occur in the event of heavier precipitation ( heavy rain ).

Water that does not seep away collects in depressions. If they overflow, the water continues to flow above ground to the next lower point. Depending on the temperature, solar radiation and wind speed, some of the water evaporates .

In the case of paved or sealed surfaces, the proportion of seepage is small (under certain circumstances, the moisture evaporates when there is little rainfall). Most of the water flows more or less completely above ground. When draining roads outside of built-up areas, the rainwater is often directed over the shoulder to the side of a road ditch or a drainage basin . If the conditions of the terrain or the pollution of the rainwater will not let the water is about gutters and gullies into a storm sewer run.

With the so-called separation system , the rainwater is directed into a rainwater canal via rain gutters and street gullies, while the dirty water is diverted separately from it into wastewater canals. In heavily polluted areas, because of the high degree of pollution (high-traffic streets - DTV value), rain or surface water is also discharged into a sewer, contrary to normal drainage.

In the so-called mixed system , the rainwater is also collected via rain gutters and street gullies, but drained together with the dirty water ( mixed ) in a mixed water channel. Since the peak flows in heavy rain not in the wastewater treatment plant to be treated, at suitable locations combined sewer overflows built where forwarded mixed water on the order of two or three times wastewater effluent to the treatment plant and the remaining diluted water into a water body relieved is. This relief is usually combined with rainwater treatment.


Due to the increasing surface sealing , heavy rains repeatedly lead to overloading of the sewage systems and floods , as the falling water runs off immediately and cannot seep into the ground. Rain retention systems are installed in the sewer system or between the sewer system and water in order to achieve a flow equalization to the receiving water and a dampening of the discharge peaks.

Pollution of rainwater

Rainwater picks up pollutants of various origins in the atmosphere and during runoff on paved surfaces and in the rainwater sewer or mixed water sewer:

In mixed sewer systems, higher concentrations of particulate suspended matter , organic substances as well as nitrogen and phosphorus compounds occur rather than in separate sewer systems. In some cases, there are high concentrations of filterable substances there, which can significantly exceed the values ​​of mixed sewer systems.

The concentrations are very different depending on the catchment area , the duration of rain, the amount of rain and other factors. In terms of magnitude, however, the pollution of rainwater from the rainwater sewer system and from unloaded mixed water are the same.

Rainwater treatment

In order to minimize ecological damage to the bodies of water into which rainwater is discharged, measures for its purification ( rainwater treatment ) are provided and rainwater clarifiers are built in rainwater sewers . Rain overflow basins are set up in mixed sewer systems .

Use of rainwater

Collected rainwater for garden irrigation

The use of rainwater as process water to drinking water to save, are gaining popularity. The rainwater is diverted from collecting areas, for example via rainwater collectors , rainwater filters, etc. and stored in underground or above-ground rain storage tanks, e.g. B. collected in cisterns , IBC containers or rain barrels. At least 60% of the rainwater can be collected with special rain collectors. Such devices can be installed in standard zinc downpipes with a diameter of approx. 100 millimeters. From there, the rainwater is transported to the individual tapping points by pumps . A 4-person household can e.g. B. save approx. 70,000 liters of drinking water per year by using rainwater. The actual amount depends on the location (rain discharge) and the size of the usable roof area. In Germany, rainwater can be used for flushing toilets , washing machines and watering gardens. There are also numerous possible uses in industry and commerce.

A cistern can save costs when it comes to fresh water. If economic reasons are part of the purchase of a rainwater cistern, a cost-benefit analysis must be carried out. It must be taken into account that fees may have to be paid for the wastewater and costs for the installation of separate pipes for the gray water may arise. Without taking direct sustainable effects into account, rainwater systems and cisterns in Central Europe often only cover costs over very long periods of time.

More and more municipalities are now starting to view the wastewater from rainwater harvesting systems as "positively influenced wastewater" that would have been discharged into the sewer one way or another. Since in such cases there is often no wastewater fee to be paid for the rainwater, a sensibly dimensioned rainwater utilization system can definitely pay off in the course of its long operation (cistern almost unlimited, filters / pipes 50 years and more, pumps over 20 years). This is especially true if there are grants from the community and the cistern is taken into account when building a house. In many places today this is also part of the building regulations. In addition, the prices for fresh water and waste water have been rising sharply in some municipalities for years. Rainwater use in the commercial sector is almost always economical, as the rain yield is determined by the size of the roof area and there is usually a large, constant water requirement.

The increasing spread of rainwater cisterns can reduce the risk of flooding in some sealed, canalised areas, because the sudden, extreme increase can be sufficiently delayed or even held back in the event of brief heavy rain (e.g. in the case of almost empty cisterns after long periods of dryness). The risk of flooding from typical rivers can of course hardly be reduced when it rains continuously .

The use of rainwater in rainy areas such as the DACH countries does not bring any advantages for remote dry areas, as the water balance in these countries does not change as a result. However, the use of rainwater is still to be classified as an environmental protection measure, since water consumption and groundwater abstraction in the immediate vicinity, as well as the infrastructure effort (treatment, transport) for the provision of drinking water, are reduced. Many large cities obtain their very high water requirements z. Sometimes from distant mountains (Frankfurt / M. E.g. from Vogelsberg, which is over 50 km away).

The collection and use of rainwater as drinking water can enable shipwrecked people to survive (see corresponding self- experiments by Alain Bombard and Hannes Lindemann ).

As irrigation water for plants, rainwater is more advantageous than tap water, for example, due to its low lime content (low water hardness ) . However, components of rainwater can be harmful to some plants. This includes, for example, zinc absorbed by the drainage channels.

Sewage fee

Rain drainage basin of an industrial company

Rainwater or surface water that runs from sealed surfaces (roofs, parking lots, streets) into the public sewer system has not been recorded as wastewater to date (only as a surcharge to the fee for the wastewater ). Its removal was co-financed by the municipal wastewater charges levied. These in turn were calculated according to the fresh water consumption (probability scale). The sealed area is only taken into account as a benchmark if a split sewage fee is charged for water and sewage.

In the meantime, the rulings on the necessary calculation of the water and wastewater charges according to a “split” charge are increasing nationwide.

A sewage fee for rainwater only has to be paid if the sealed surfaces are actually connected to the public sewer system. In the federal states there are different options for leaving rainwater on the property. In some cases, all of the rainwater that accumulates on the property can be used or drained away. Then no rainwater fee has to be paid.

See also


  • fbr (Ed.): Energetic use of rainwater . (Vol. 16) Fachvereinigung Betriebs- und Regenwassernutzen eV, Darmstadt 2013, ISBN 978-3-9811727-5-1 .
  • fbr (Ed.): Rainwater use in public and social institutions . (Vol. 14) Fachvereinigung Betriebs- und Regenwassernutzen eV, Darmstadt 2011, ISBN 978-3-9811727-3-7 .
  • fbr (Ed.): Rainwater Management - Synergies with Rainwater Use . (Vol. 13) Fachvereinigung Betriebs- und Regenwassernutzen eV, Darmstadt 2009, ISBN 978-3-9811727-2-0 .
  • Patrick Herzer: Influences of near-natural rainwater management on urban planning - spatial, economic and ecological aspects. Fraunhofer-IRB-Verl., Stuttgart 2004, ISBN 3-8167-6440-1 .
  • Wolfgang F. Geiger (et al.): New ways for rainwater - manual for retention and seepage of rainwater in building areas. Oldenbourg, Munich 2001, ISBN 3-486-26459-1 .
  • Keith J. Beven: Rainfall-runoff modeling - the primer. Wiley, Chichester 2006, ISBN 978-0-471-98553-2 .
  • Karl-Heinz Böse: Rainwater for the garden and home. Ökobuch, Staufen near Freiburg im Breisgau 2009, ISBN 978-3-936896-42-8 .

Web links

Wiktionary: Rainwater  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Lucas L. Alonso, Pablo M. Demetrio, M. Agustina Etchegoyen, Damián J. Marino: Glyphosate and atrazine in rainfall and soils in agroproductive areas of the pampas region in Argentina. In: Science of The Total Environment. 645, 2018, p. 89, doi: 10.1016 / j.scitotenv.2018.07.134 .
  2. ^ Rainwater harvesting 2012 . 2012. Accessed in 2012.  ( Page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Toter Link / aramo.de  
  3. VGH Mannheim, judgment of February 11, 2010 - 2 S 2938/08.