Combined wastewater systems convey wastewater and storm water together in the same pipeline network. In order to alleviate the treatment facility from a high hydraulic load during storm events, at selected locations along the network are placed weir chambers, which allow for the discharge of the diluted wastewater into receiving water bodies during storm events. The level of dilution is prescribed by regulations. These are the classic and oldest types of wastewater systems. The illustration is shown on Figure 2.

Separated wastewater systems convey wastewater and storm water in completely separated networks. The wastewater network leads to the treatment plant and the treated water is discharged into a natural water body. The storm water is collected by a separate network of pipelines or open channels and discharged into the receiving environment at numerous suitable locations without treatment. In selected cases only, there is provision of storm water treatment facilities, which may also serve to balance high floods.

Note that, in some cases of population centres is possible to have the two systems applied. The older parts of the area could be served by a combined system, but more recent urban developments or retrofitted areas can have a separate system.

Figure 2 The combined wastewater system