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Contaminated and rainwater is collected in a sewer system. When the weather is dry, all the waste water goes to the sewage plant, when it rains, some of the waste water goes to the sewage plant, but some goes directly into the receiving water course.

The advantage compared to the separate system is the lower cost for construction, maintenance and cleaning of the sewers (only one sewer network). Runoff from polluted areas is also included (agriculture, tanker accidents, etc.).
During dry weather however, only very little water flows into the sewer, which often leads to deposits. Even relatively little rairecipient n then leads to a sudden increase in runoff and to the deposits being washed away. The annual quantity of domestic waste water is of approximately the same order of magnitude as the quantity of rainwater diverted from the same drainage area. In order to limit the cross sectional areas of the sewer network for technical and economic reasons, rainwater relief structures (rainwater overflow basins, rainwater overflows, sewer storage spaces or rainwater retaining basins) are provided. In the event of heavy rain there is therefore introduction of mixed water into the receiving water course.

Qualified combined system:

If it is economical in terms of water and technically feasible for some of the rainwater to seep away or be diverted separately, the collection and diversion of all the rainwater with the contaminated water can be dispensed with. This results in smaller pipe diameters and in consequence also smaller relief structures.

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