estimated water pollution during rainfall

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Overflows from the combined system or even from separate systems, excess water that occurs during a rainfall episode will discharge significant quantities of pollution into the natural environment and, when a water treatment plant deals normally with dry weather water, this excess water can become a major contributor of pollution to the natural environment. This can be seen from tables 32 and 33 that provide examples from the Paris (France) region.

If we wish to improve this situation, we can:

town covering impermeabilisation combined separate systemsSecured image
Table 32. Theoretical case of a 10,000 inhabitant town covering a 170 hectare, 30 % impermeabilisation, site: mass discharged in t · year–1 by the plant and by the combined or separate systems
catchment basins Paris wastewater treatment plantSecured image
Table 33. Average for 10 catchment basins in the Paris (France) region showing the annual input per impermeabilised hectare by comparing it with the discharge from the same hectare (100 inhabitants) through a wastewater treatment plant

It should be noted that run-off water mainly contains suspended solids (leached from impermeabilised soils) and it can be said that approximately 85% of COD and BOD, 70% of nitrogen (NK), 90% of hydrocarbons and over 95% of metals (Pb, Zn, Cu) will be present as particulates.

Similarly, run-off water usually contains more metals, especially Pb, Zn, Cd cause by rusting roofs, downpipes, tyre wear, hydrocarbon combustion residues… These residues also contribute to microbial pollution (leaching of animal manure in particular) and, during the first hours of a rain storm, separate systems reveal concentrations that are close to those found in wastewater in dry weather for instance. 108 total coliforms·100 mL–1.

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