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New Developments With Slow Rate Of Buildout

In a typical new development, all the lots are platted, roads built, and some community facilities put up initially in order to begin selling lots. This represents a large "up front" investment at the start of the project. Since only a few houses are actually built and occupied each year, resulting in a proportionally small revenue stream, the "up front" cost of gravity sewers is often prohibitive.

On the other hand, if pressure sewers and grinder pumps are chosen, all of the small diameter shallow buried pressure piping system can be installed initially at very low cost per foot. The grinder pump station, which comprises the majority of capital cost, needs to be purchased and installed only as each house is built. This is especially critical in providing an affordable and effective sewer system, initially to the first few houses -- often scattered throughout a large tract far from their nearest neighbor.

Layout of a typical centrifugal grinding pump system.

Projects With Large Lots And Consequent High Cost Per Dwelling Unit

The cost difference between gravity and pressure sanitary sewers is a function of the pipe size, depth, and the necessity to deeply bury gravity sewers to ensure downhill flow.

Pressure sewer piping, besides being smaller and shallower, need not be laid on a downhill grade, but can follow the contour of the land at a constant shallow depth dictated by the local frost penetration depth or, in very mild climates, by the need for protection from mechanical damage.

Since costs are assessed to serviced properties on a dollars per front foot basis, the cost advantage for pressure sewers increases rapidly as lots become larger.

Difficult Terrain Conditions

In steep terrain, especially on uphill runs, gravity sewers very quickly become too deep and costly to be feasible. The only answer is to put at least one pumping station on each significant uphill reach. Gravity can usually be used on the downhill sections, but the capacity of pump stations become successively larger as the piping progresses toward the ultimate discharge point.

Pressure sewers can be designed to work successfully and economically in either situation. A useful analogy to water system hydraulics can be drawn which shows that appropriate attention must be paid to the need for air and vacuum release valves at significant high points in the profile, as well as at the beginning of long downhill runs discharging to atmosphere.

Design flow, in any given downstream section of line, as a function of number of pumps contributing, has been determined empirically; and is available in tabular or graphic form. Here are these criteria as suggested by several competent authorities.

The ability to construct sewers that follow the contour of the land not only makes development affordable, but has also preserved natural rolling topography and trees.