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Retention and treatment of overflows from combined sewer systems and of storm water from separate sewer systems

There are two types of sewer system to dispose of storm water and waste water: separate and combined. In the separate system, storm water and waste water are collected and disposed of via separate sewer networks. In the combined system, waste water and, in the case of storm or rain events, storm water are collected in one sewer network. In both systems, the most important hydraulic design element is flood protection – that is, the sewers should be designed to avoid floods in built areas. However, the sewer systems cannot be designed to accommodate any magnitude of heavy rain event.

For both the combined and separate sewer system, it is best environmental management practice to implement environmentally friendly and more sustainable water drainage to reduce the flow, and to locally infiltrate storm water. This also means disconnecting already developed impervious areas from the sewer.

For the combined sewer system, it is best practice to:

  • minimise overflows using local infiltration, including disconnecting already developed areas (impervious pavements), retaining combined waste water (mixture of waste water and storm water) in dedicated tanks, and increasing the capacity of waste water treatment plants for combined sewer flow (considering the waste water treatment plant and storm water management, especially overflows, as a whole, in an integrated way) to limit overflows to events of unusually heavy rainfall
  • treat the overflow of the retention tanks using fine screens (4–6 mm) and sediment tanks and, depending on the quality of the receiving water, by soil retention filters or other techniques with a similar removal efficiency of suspended solids, COD, heavy metals and organic pollutants.

For the separate sewer system, it is best practice to treat the storm water depending on its level of pollution and, as indicated above, to directly discharge storm water with no or low pollution. For this purpose, the areas of a municipality must be systematically categorised according to their level of pollution.

Concerning the disposal of storm water as run-off from impervious areas, all areas must be systematically assessed with respect to their pollution potential and classified into three categories: low, moderate and high contamination.

As no individual technique fits all applications, each public body must assess the specific circumstances and choose the most suitable technique.

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