Vacuum Sewers

Vacuum sewerage systems use a central vacuum source to convey sewage from individual households to a central collection station (UNEP 2002). It is a mechanised system of wastewater transport. Unlike gravity flow (e.g. conventional sewers, separate sewers or simplified sewers), vacuum sewers use differential air pressure (negative pressure) to move the sewage. A central source of power to operate vacuum pumps is required to maintain vacuum (negative pressure) on the collection system. The system requires a normally closed vacuum/gravity interface valve at each entry point to seal the lines so that vacuum can be maintained. These valves, located in valve pits, open when a predetermined amount of sewage accumulates in collecting sumps. The resulting differential pressure between atmosphere and vacuum becomes the driving force that propels the sewage towards the vacuum station (PDH ENGINEER 2007).

A traditional gravity line carries wastewater down to the collection chamber (it is recommended to combine it with vacuum- or low-flush toilets). As soon as the level reaches a defined height, the vacuum interface valve opens and the negative pressure sucks the wastewater into the vacuum sewer main. At the end of the pipe system, there is a collection tank. When the tank fills to a predetermined level, sewage pumps transfer the contents to a treatment plant via a conventional or separate sewer system. It is important to understand that the collection system is hold on permanent level of vacuum.

The wastewater from the houses is held back in collection chambers, with pneumatic regulating valves close to the houses. When a given volume of wastewater is collected in the chamber sump, a pneumatic controller is activated by hydrostatic pressure. The controller opens an interface valve for an adjustable time period. The wastewater (10 to 50 L) and a certain amount of air (20 to 60 L) are evacuated through the open valve into the vacuum sewer line. The pressure gradient between the vacuum station and atmospheric pressure at the collection pits is responsible for the movement of sewage to the vacuum tank (GTZ 2005).

All the vacuum sewers are connected to the vacuum collection vessel installed at the central vacuum station, where vacuum pumps create the required negative pressure (approximate -0.6 bar). The vacuum vessel can be placed inside or buried outside the vacuum station. Transfer pumps convey the wastewater from the vessel to the wastewater treatment plant or to an existing sewer. The capacity and dimensions of the vacuum station are dictated by the particular requirements of the sewer system. Operation of the vacuum and transfer pumps is controlled by a software (ROEDIGER 2007).

In contrast to gravity sewer pipes, it is easier and cheaper to build the vacuum sewer piping. No manholes or sewer pumping stations are necessary, just inspection points for pressure testing. This avoids settling of sludge and no manholes are needed to be cleaned out (see also dignity or human powered emptying). The pipes of a vacuum sewer system have a smaller diameter (80 to 250 mm) and the trenches are narrow and shallow (1.0 to 1.2 m). That is also an advantage if there is a high groundwater table. Unexpected obstacle can easily be bypassed by a modified and more flexible pipe design. If a pipe is damaged, the risk of sewage infiltration is very low, because of the negative pressure in the sewer line. Expert design is required, but the construction and installation work can be done by local contractors and pipe suppliers. No heavy machinery is necessary and there is no danger of a collapse of deep trenches (ROEDIGER 2007).

As it is a high-tech system, it is costly. But in comparison with a conventional sewer system, it is cheaper. Piping costs are lower, because the dimensions are smaller. Therefore, less material is required. Also the installation can be cheaper because piping is independent from the topography. Furthermore, no heavy machinery is necessary for excavate deep trenches, thus the work can be done by local workers, which creates employment. Finally, big amounts of flushing water can be saved which is economical and ecological reasonable. However, the constant energy requirement for the permanent vacuum generation can increase the costs.

The risk of clogging is low and there is almost no cleaning/emptying work to do. From time to time pressure in the vacuum sewer system should be tested. The system needs instructed workers for maintenance and operation works. More complex and/or technical problems are in the responsibility of the manufacturer.

As long as it is maintained properly, a vacuum sewer system guarantees a high level of comfort and hygiene. There is a very low risk of contamination due to leakage. It is a closed system, thus there is almost no contact between wastewater and operators. However, a treatment system at the end of the pipe needs to be in place.