The dirty water and the rainwater from the roof and the street are collected in a common canal. From there it is directed to the sewage treatment plant.

The discharge in rainy weather (with rainwater) is approx. 100 times greater than in dry weather (without rainwater). The rainwater is therefore decisive for the size of the canal. However, large fluctuations in the discharge can lead to problems in the sewage plant.

That is why mixed water overflow structures are arranged. These structures channel part of the runoff into the river during rainy weather. However, the runoff is heavily diluted by the rainwater and only a few pollutants enter the river. Therefore, the overflow has only a minor effect on the river. In addition, less wastewater is discharged to the wastewater treatment plant, which means that the wastewater can be better purified in the wastewater treatment plant. In dry weather, the water level in the canal is very low.

• Lower investment and maintenance costs than a separation system (only one channel)
• Lower space requirements (particularly suitable for narrow roads)

• Dry weather deposits due to low flow velocity (channel cross-section too large for dry weather runoff)
• If there is too much precipitation untreated wastewater is discharged into the river
• Worse cleaning effect of the wastewater treatment plant due to strongly different inflows (dry or rainy weather)