Monday, April 13, 2009

Sewer Systems: Pressure Sewer vs Gravity Sewer


When most people think of sewer systems, they think only of gravity sewers. Gravity sewers have been around for centuries, with origins in the Roman aqueducts. While often considered the "simple sewer solution" when septic tanks cannot be used, gravity sewers are not always feasible. What other choice is there? Pressure sewer systems.

Gravity sewer systems use large mains (up to 24" in diameter) that must be accurately placed and bedded along a continuous downward grade. Large, costly lift stations are often involved as well. All of the sewer mains must be in "straight lines," adding to the complexity of the system design. and, because the mains are all "below grade" of structures, large, deep trenches will be required. If the land has a high water table or a lot of bedrock, gravity sewers may be cost prohibitive or impossible to install at all.

Pressure sewer systems offer many more options. The small mains (usually 2" to 4" in diameter) are buried just below the frost line, eliminating the need for large, deep trenches. The mains are flexible, so they can often be installed via directional drilling. Grinder pumps transport waste from the homes to the pressure sewer mains. Because the wastewater is pressurized, it can be transported horizontally up to two miles, or 185 feet vertically.

Case Study: Great Sky, Canton, Georgia
The Great Sky development is located in the foothills of the Appalachian Mountains. Steep hills and bedrock made gravity sewers incredibly expensive -- deep trenches and 20 lift stations were estimated. Septic systems were not allowed.

The solution? A pressure sewer system with E/One grinder pumps installed at each home, allowing wastewateer to be pumped uphill. Shallow trenching was used to install the force mains. Only three lift stations were required.


Read the entire article, as published in Land Development Today magazine, on E/One's web site.

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