Vacuum Sewer Systems (cont)
The resulting pressure differential between the positive pressure of atmosphere air and the negative pressure in the vacuum main becomes the driving force that propels the sewage towards the vacuum station. The pressure differential that exists at the normal operating vacuum levels provides the energy to propel the sewage at velocities of 15-18 ft/s. When the sewage enter the vacuum main it travels as far as its initial energy allows, until frictional forces cause it to come to rest. As other valves in the piping network open, additional slugs of sewage and air enter the system. Each subsequent energy input continues to move the sewage toward the vacuum station. Many view the vacuum pipeline as a “vacuum-assisted gravity sewer”. Like gravity sewers, vacuum sewers are installed with a positive slope toward the vacuum station. When vacuum mains start to become deep, a “lift” is used to return the main to a more acceptable depth. It is at these lifts that vacuum “assists” the sewage on its travel toward the vacuum station. The lifts are part of the sawtooth configuration of the vacuum mains, which is a key feature of a vacuum system. The saw-tooth profile is used to keep an open passageway on the top of the piping network, thereby preventing the pipe from becoming sealed. By doing this, air flows above the liquid, and the vacuum that is created at the vacuum station can be transferred to every valve pit. This ensures that the maximum pressure differential, and hence, maximum energy, can be obtained at each valve pit.
Eventually the sewage reaches the vacuum station. The vacuum station has 3 major components: the collection tank, the vacuum pumps and the sewage pumps. The vacuum pumps and the vacuum mains are connected to the top part of the collection tank. This part of the tank is kept open so that the vacuum that is created by the vacuum pumps can be transferred to the vacuum mains and ultimately to the valve pits.
The vacuum pumps do not run continually, but rather in cycles. They run for a short period in order to establish the high level vacuum. When this level is achieved, they turn off. As valves throughout the system open and admit atmospheric air, vacuum levels gradually drop. When the vacuum levels become low, the vacuum pumps come on again and run to reestablish the high vacuum levels. Sewage from the vacuum mains enters the collection tank and accumulates in the bottom part of the tank. When enough accumulates, the sewage pumps come on and pump the sewage out of the collection tank through a force main to the ultimate point of disposal.
Experience has shown that for vacuum systems to be cost effective, a minimum of 75 to 100 customers (houses or equivalents) per vacuum station is generally required. The average number of customers per station in systems presently in operation is about 200 to 500, but that average is increasing every year. Vacuum systems are to a degree limited by topography. The most successful applications have been in relatively dense developments with moderate terrain changes. The vacuum produced by a vacuum station is generally capable of lifting sewage 15- 20 ft, depending on the operating vacuum level of the system. This amount of lift is often sufficient to allow the designer to avoid all or many of the lift stations that would be required in a conventional gravity system.
The consulting engineer usually drives the community’s choice of collection system type during the planning stages of a wastewater facilities project. This choice is normally based on the result of a cost-effectiveness analysis. While gravity may appear to be less costly in situations where the terrain is favorable for gravity flow, many small factors considered collectively may result in a vacuum system being the proper choice.
Below are the general conditions that are conducive to the selection of vacuum sewers.
• Unstable soil
• Flat terrain
• Rolling land with many small elevation changes
• High water table
• Restricted construction condition
• New urban development in rural areas
• Existing urban development where built-out conditions exist
• Sensitive eco-system