The Vacuum Mains (either PE or PVC) are designed to be as shallow in the ground as possible and are laid at a 1/500 grade. The contractor builds steps in the pipework to keep the mains from getting too deep. The more steps that are installed, the more hydraulic losses are created, lessening the distance that the mains can go. It is important to have collection pits evenly distributed around the catchments and along the vacuum mains to ensure fast movement of sewage to the VPS.
The first stage is connecting the gravity lines from the house to the Flovac collection pit. usually between 4 and 6 houses are connected to a collection pit, but this depends on a few things: the ground conditions, the size of the house and the amount of flow likely to enter the sewer at this point. All collection pits are usually located outside of the property boundary so no easements or entering of a private property is necessary.
From the Collection Pit to the vacuum mains takes careful design to ensure that the flow is heading correctly towards the vacuum pump station. It is also important to ensure that the wastewater is being injected into the vacuum mains at the right places within the system so as to avoid water logging.
Connecting pipework from a collection pit to the vacuum main
Vacuum Mains are laid in a shallow trench
It is important to lay the vacuum mains at a grade sloping downward towards the vacuum pump station. Due to the negative pressure within the vacuum mains it is possible to lay the pipework at a 1:500 grade. This suggests that the system is actually a hybrid gravity system, which is true, so as to minimize energy use.
Vacuum Mains are laid at a depth of between 1-2 meters (3-6 feet) and do not require manholes. Steps in the mains are required to maintain a shallow depth.
The flexibility and shallowness of all of the pipework is very useful when dealing with high water table, hard rock or acid sulphate soils. Contractors can lay pipe at a much faster pace which minimizes disruptions to homeowners and delays in the project.
There is also an environmental benefit in that there is little to no impact on the water table level. Vacuum systems are considered to be the safest technology to use when dealing with environmentally sensitive areas. Vacuum Mains cannot leak which is very important when systems are close to waterways.
Pressure pipe is used so that it can handle the negative pressure in the pipes. Either PVC with rubber ring joints or HDPE pipe with electrofusion couplings is acceptable.
Systems are now being installed with the vacuum main being installed in the same trench as the water main as well as the force main. This is because the vacuum main has a negative pressure. If a vacuum main breaks, it sucks in. This leak is quickly discovered as the vacuum pumps would run longer to keep up the depleted vacuum.
