System Description

The pressure sewerage system consists of a sump or collecting tank, usually made of prefabricated concrete or GRP, a pump and the pipes that form the branch network. In a pressure system, small pumps, usually submersible ones with grinding devices, deliver the wastewater from individual houses or from a group of houses into a pressurized pipe network that carries the wastewater to the disposal point.

The sumps are normally 1.5 to 2 m deep and 1 m diameter. They are completely standardised to obtain the lowest possible production costs and to simplify the construction. The buffer volume (volume between in- and out-level of the sump) is small enough to prevent sedimentation and the development of unwanted odors.

Grinder pumps commonly used for pressure sewer systems have capacities of 2 to 4 l/s, operating at heads between 2 and 3 bar and have electrical motors of 2 to 3 kW. The pumps are usually equipped with grinding or cutting devices to reduce the size of particles in order to prevent clogging of the small diameter pipes.

A pressure sewer is a small diameter pipeline that is not deeply buried and follows the profile of the ground. Typical diameters are 50 to 120 mm, according to the size of the community to be served. Polyvinylchloride (PVC) or High Density Polyethylene (HDPE) are the most common piping materials. Burial depths are usually below the frost line. Small pipe diameters and the possibility to follow the ground surface without the need for deep trenches are the main advantages of pressure sewer systems. This type of system can terminate at an existing gravity system or can lead directly to a wastewater treatment plant.

System design
Pipelines are usually laid to follow the terrain. Odor release and/or water hammer might occur, and these possibilities must be taken into consideration at the planning stage.
A minimum velocity of 0.7 m/s is required in order to prevent sedimentation and settling out of solids. Velocities below 0.7 m/s may be acceptable for certain operation conditions provided that the velocity exceeds 0.7 m/s at least once a day. As gas may form within the system, wastewater should not be retained within the pipe system for more than 8 hours; sometimes local regulations and conditions requires even shorter times.
If a malfunction occurs at the collecting chamber, a high-liquid-level alarm should be activated. If the available storage volume of the chamber is not large enough, it must still be protected from overflow.