Classification based on hydraulic principles

This classification refers mainly to the sewerage or the conveying structure of the whole system and does not affect the treatment plant. Based on the hydraulic principle of flow in the pipeline sub-system, we can mention:

Gravity flow systems convey the fluid in pipelines or open channels, which are designed based on the hydraulic principle of free-surface flow. This principle requires that the pipe is laid off with a prescribed slope, downwards the direction of flow, in order to allow for the wastewater to flow due to gravity forces. The vast majority of wastewater networks fall in this category.

Pressurized flow systems convey the fluid in pipes under pressure, and are designed on the hydraulic principles of pipe flowing full. Such types of networks might be designed in combination with gravity systems at specific topographic conditions (e.g. flat terrain). They are applied at relatively short lengths or in small population centres and require pumping stations for their functioning.

Vacuum flow systems convey the fluid in pipes under vacuum, which is created by vacuum stations. As shown on Figure 4, the wastewater from the household flows by gravity to an interface chamber, (pit), which has a special valve, ensuring that the vacuum in the following pipeline is maintained and allowing the suction of the fluid in the chamber. The vacuum in the pipeline system is maintained by the vacuum station, where the fluid is discharged into a tank and transported further by gravity or by pressure. The system is relatively complex and contains a number of mechanical/electrical elements, which requires a high level of operation and maintenance. Because the length of the pipelines linked to one vacuum station is limited to several kilometres, when applied to larger population centres, a large number of vacuum stations is required. For this reason, its application is recommended for specific geological conditions and small population centres.