How was rainwater polluted

  Rainwater is the collected water from precipitation (meteor water) or rain.

Rainwater runoff

Precipitation (in the case of snow only after thawing) takes different paths depending on the surface on which the precipitation falls:

Water seeps away at a rate (infiltration rate) that depends on the amount of water already in the soil and the permeability of the soil. Water that has not seeped away collects in depressions. When these overflow, the water flows off above ground to a body of water. Depending on the temperature, solar radiation and wind speed, some of the water evaporates.
In natural unpaved areas, the proportion of seepage is high, and water only flows off above ground in heavy rain.

In paved areas, the amount of seepage is small and most of the water flows off above ground. When draining roads outside built-up areas, rainwater is often used over the shoulder led directly laterally into a road ditch or an infiltration trough. If the terrain or the pollution of the rainwater do not allow this, the water is channeled through rain gutters and street gullies. With the separation system in built-up areas, rainwater is channeled through rain gutters and street gullies into a rainwater sewer, while the dirty water is drained away separately in a sewer. In heavily polluted areas, the rainwater is directed into the dirt sewer contrary to normal drainage due to high levels of pollution (high-traffic streets - DTV value).

With the mixed system, the rainwater is also collected via rain gutters and street gullies, but together with the dirty water (mixed) derived in a mixed water sewer. Since the peak runoff cannot be treated in the sewage treatment plant in the event of heavy rain, mixed water reliefs are built at suitable locations, where mixed water in the order of 2 or 3 times the wastewater discharge is forwarded to the sewage treatment plant and the remaining diluted water is transferred to a body of water relieved becomes. Usually this relief is combined with rainwater treatment.

Sewage fee

Rainwater that runs from sealed surfaces into the public sewer system usually has to be paid for as wastewater. If a split fee is charged for waste water and rainwater, the fee is calculated according to the sealed area, otherwise as a surcharge to the fee for the waste water.

The wastewater fee 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. Sometimes it is the case that all of the rainwater that accumulates on the property can be used or drained away. Then no rainwater fee has to be paid.


Due to the increasing surface sealing, heavy rainfalls 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:

  • atmospheric pollutants and pollutants
  • substances absorbed at the earth's surface
  • Wastewater constituents of the dry weather runoff
  • Resuspended substances from sewer deposits
  • Eroded sewer skin

In mixed sewer systems, higher concentrations of particulate suspended matter, organic substances as well as nitrogen and phosphorus compounds tend to occur than in separate sewer systems. In some cases, high concentrations of filterable substances occur there, which can significantly exceed the values ​​of mixed sewer systems.
The concentrations are very different depending on the catchment area, the duration of the rain, the amount of rain and other factors. On the order of magnitude, however, the pollution of rainwater from the rainwater sewer system and from unloaded mixed water are equally large.

Rainwater treatment

To minimize ecological damage to bodies of water into which rainwater is discharged, measures to purify it (Rainwater treatment) planned and built in rainwater sewers. Rain overflow basins are set up in mixed sewer systems.

Use of rainwater

The use of rainwater as process or industrial water to save drinking water is enjoying increasing popularity. The rainwater is diverted from collecting areas and collected in underground or above-ground rain storage tanks, e.g. in cisterns. From there, the rainwater is transported to the individual tapping points by pumps. For example, a 4-person household can save up to 70,000 liters of drinking water per year by using rainwater. In Germany, rainwater can be used for flushing toilets, washing machines and watering gardens. There are also numerous possible uses in industry and commerce.

However, before purchasing a rainwater cistern, the economic viability should be checked, since a cistern can save costs in terms of fresh water, but the investments have to be offset from an economic point of view. There is almost always no profitability. This is all the more true if fees for the wastewater are properly paid and the costs for a double installation level for the gray water are taken into account.

Owners of rainwater harvesting systems do not need to pay the sewage fee only if they can prove that the water is only used for the garden.
Even a widespread use of rainwater cisterns can only slightly reduce the flood runoff in the bodies of water, since the total volume available is far too small.

The use of rainwater in rainy areas, such as the DACH countries, does not bring any advantages for arid areas, as the water balance in these countries does not change as a result. The use of rainwater can therefore only be classified per se as an environmental protection measure in the rarest of cases, as there are no benefits for the environment.

For shipwrecked people, the collection and use of rainwater as drinking water can enable survival (see corresponding self-experiments by Alain Bombard and Hannes Lindemann).

See also

Category: water