Can surface water drain a polluted sewer

Bavarian State Office for
environment

Evaporation, seepage and runoff - these are the possibilities that precipitation has that falls on the earth.
Almost two thirds of the rainwater evaporates on natural, unpaved, overgrown areas such as a meadow. Around a quarter seeps into the subsoil, is cleaned in the process and contributes to the formation of new groundwater. Only a small part of the rain that has fallen flows away on the surface.

As the development progresses, more and more areas are being paved or sealed, so that significantly less rainwater can evaporate or seep into the ground. By far the largest part flows off on the paved surface.

Paths of precipitation on natural subsoil
Paths of precipitation on solid ground

In this context, the long-standing practice of simply discharging rainwater has led to ever larger amounts of water in the sewage system and sewage treatment plants with correspondingly rising costs. From today's point of view, a "natural" approach to rainwater should therefore be aimed for in order to impair the natural balance of the water cycle as little as possible.

The goals of near-natural rainwater management are:

  • Promote evaporation
  • Increase in infiltration
  • Reduction of surface runoff
The essential elements of near-natural rainwater management

This supports the formation of new groundwater and makes an important contribution to avoiding floods and sewer overloads. This aspect is of particular importance given the expected increase in heavy rain events due to climate change. It goes without saying that reliable drainage of settlement areas must be guaranteed within the framework of the applicable assessment approaches. Rare and extraordinary heavy rain, however, cannot be managed with natural rainwater management and public drainage systems alone.

In order to cope with such events without damage, further measures such as the identification of danger areas, sustainable land-use planning and land use, personal precautions (e.g. backflow protection on buildings, insurance) and comprehensive public risk communication are necessary.

Detrimental effects, for example through the entry of pollutants into the groundwater through the seepage of heavily polluted rainwater, must also be prevented. Dealing with rainwater should therefore by no means be taken "lightly".

There are many options for dealing with rainwater in a natural way:

  • (decentralized) restraint
  • Infiltration
  • above-ground collecting
  • delayed discharge

The use of clean rainwater can also make sense in the private as well as in the commercial or industrial sector.

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