Wednesday, July 8, 2009

Sewer System Technology: Vacuum Sewer vs Pressure Sewer

At first thought, a vacuum sewer system sounds like an easy, convenient alternative to a gravity sewer system. Vacuum sewers require shallow trenches, small mains, and can be used in flat or rocky areas. No individual pump stations are required at each home. But they aren’t ideal for every situation or every location, and require regular maintenance.

How a Vacuum Sewer System Works
Vacuum systems use differential air pressure to move wastewater. The system consists of one or more vacuum stations, collection system piping, valve vaults and vacuum interface valves. The sewage is transported in small diameter pipes placed at variable grade and deflection underground. The wastewater is drawn to the station and pumped to a wastewater treatment plant. Vacuum interface valves regulate the entry of wastewater and air from the valve vault into the collection system piping.

Vacuum System Disadvantages
There are a number of disadvantages to vacuum systems. First, while the mains are small and require shallow burial (just below the frost line), they are limited to approximately 20 feet of head. The sewer lines must have a specific profile of pockets or running traps, so installation requires the same attention to grade as a gravity sewer main.

The biggest disadvantage is system size. The central vacuum stations require a large capital investment, so a system for less than 50 homes is not economically feasible.

Pressure Sewer Systems
Pressure sewer systems use pump installed at each home to grind wastewater and move it to the sewer system. Pressure sewer mains are also small-diameter pipes that are buried just below the frost line, but follow the contour of the land – therefore, they don’t require the pockets or running traps that vacuum systems need. Head is limited by the capabilities of the grinder pump installed at the home (185 ft for E/One pumps), allowing for uphill pumping.

Small systems are also economically feasible. A few homes – or even one home – can connect to the sewer system at a reasonable cost. E/One’s grinder pump does not require preventive maintenance and boasts an average mean time between service calls of 8 to 10 years. Visit the Case Studies section of E/One’s web site to read more about communities that have used E/One Sewer systems.

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