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Grinder Pumps in Pressure Sewers

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Grinder Pumps in Pressure Sewers Forging the Future Grinder Pumps in Pressure Sewers Charles G. Stolberg, Submersible Wastewater Pump Association An explanation of why the application of grinder pumps and pressure sewer systems provides a cost-effective, long life answer to allow more sites, existing and new, access to a public sewer system. ince the early 1970s, pressure sewer systems have been an effective method to move residential wastewater through small diameter pipes to collection facilities S 1 where other methods are less economical or less feasible. The keys to understanding the differences between con- ventional gravity sewer systems and pressure sewer systems are the piping network and the reduction of solids size in the wastewater. Pressure sewer systems use grinder type pumps to reduce the solids present to particles, which can easily be moved through small diameter pipes. This discussion describes the use of pressure sewer system technology to solve challenging sewage disposal requirements where other methods may be less economically feasible or environmentally unacceptable. Pressure sewers can be used where gravity systems just won’t work because of uphill topography, surface rock, high water tables, waterfront locations, very fl at land, extreme Aerial view of Levittown, Pennsylvania. Following WWII, subdivi- cold weather, stream crossings, restricted access, and con- sions such as this used septic tanks and soil absorption systems, straints on blasting. where sewage is digested in a large tank and then leached into Because the piping systems are sealed, pressure sewers the surrounding soil, instead of public sewers and water supply. offer freedom from infi ltration/infl ow. This provides a two- fold benefi t. The wastewater treatment plant can be built diately following the Second World War. This boom, epito- with smaller capacity since it is not necessary to allow for mized by William Levitt at his Levittowns on Long Island storm peaks, and the treatment process will not be upset by and in southeastern Pennsylvania, provided a fi rst home to storm related fl ows which can literally swamp out plants with millions of homecoming GIs. infi ltration/infl ow plagued gravity collection systems. Many of these subdivisions, in all parts of America, were Generally speaking, these systems are installed outdoors carved out of raw land adjacent to existing cities and towns. below grade. Indoor systems are also available. A typical Most provided only streets, electricity, and telephones. Such system includes a pump, basin, controls and piping and valv- niceties as curbs and gutters, streetlights, fi re protection, and ing. Let’s consider some well-known applications of submers- especially public sewers and water supply were notable by ible grinder pump pressure sewer technology. their absence. Into this breach was thrown an old technology, devel- Failing Septic Tanks In Existing oped early in the 20th century by agricultural engineers for use on the farm: namely, septic tanks and soil absorption Subdivisions systems where sewage is digested in a large tank and then The biggest building boom in American history began imme- leached into the surrounding soil. 42 JULY 2007 www.pump-zone.com PUMPS & SYSTEMS Historical Timeline of Pressure Sewers successful and widely used methods is pres- sure sewer systems powered by grinder pumps. • ASCE-sponsored project (mid 1960s) inspired by Professor Gordon M. Fair Most public health offi cials, developers, con- of the Harvard School of Public Health (Fair 1968) sulting engineers, contractors, and public • Development (1963-66) at General Electric of world’s fi rst prototype grinder works personnel have had at least some pump. Field test of fi rst pressure sewer system (1969-70) by Environment experience with pressure sewer systems since One Corp. in cooperation with NYDEC and the US EPA (Carcich 1972) their development in the 1960s. • Introduction of fi rst commercial grinder pump at WEF (then WPCF) Hundreds of thousands of homes that Annual Conference in Boston, 1969 once suffered from marshy, odorous chil- • Other EPA-sponsored demonstration projects (early 1970s) in Pennsylvania dren’s play areas, lake water quality degrada- (Mekosh 1973), Oregon (Eblen 1978) and Indiana (Sanson 1973) tion, and even hepatitis and E-coli epidem- • Earliest projects approved by state regulatory agencies (1970-72) in Indiana, ics caused by septic tank failures are today New York, Ohio, Texas, Virginia the proud owners of successful grinder • Federal Construction grant eligible (c. 1970s) as an Alternative Technology pump pressure sewer systems. These are in • Adopted (early 1980s) into “Ten States Standards,” as well as most indi- everyday use in neighborhoods all over the vidual state guidelines. world. Out in the country, land was abundant. As long as the well was prudently located up hill and on the other side of the New Developments With Slow Rate Of house, there was no problem. A little odor or a soft spot in the Buildout middle of a fi eld, far from habitation, was no cause for concern. In a typical new development, all the lots are platted, roads Limited use of such septic systems had also been made in some built, and some community facilities put up initially in order to city neighborhoods with large lots, prime soil conditions and begin selling lots. This represents a large “up front” investment careful operation (limited loading and frequent pump outs). at the start of the project. Since only a few houses are actually In retrospect, septic tanks (when brought to town) were built and occupied each year, resulting in a proportionally small a very poor choice on these small “postage stamp sized” lots. revenue stream, the “up front” cost of gravity sewers is often Nonetheless, in the rush to provide criti- prohibitive. cally needed homes, millions of septic tanks were built. By the late 1950s the U.S. Public Health Service (USPHS) was conducting serious studies in the fi eld and in labs at the Taft Sanitary Engineering Center on behalf of the FHA. UPSHS reported that over 24 million septic systems were in ser- vice and that they were failing, on average, in 11 years – far less than the 20-year or 30-year term of the typical VA or FHA mortgage loan. These same USPHS studies showed that over 50 percent of the available build- ing land in the U.S. was unsuitable for septic tank systems. Despite these facts, and because of continuing tremendous pressure from the public and the home building and real estate industries, several million more septic tanks were installed. The sad fact that failing septic tanks are a ubiquitous feature of so many American suburbs today makes it clear that they have been misapplied and not properly main- tained or inspected. Fortunately there are several proven solutions to this need for affordable, dependable, safe sewers. One of the most Layout of a typical progressing cavity grinding pump system. PUMPS & SYSTEMS www.pump-zone.com JULY 2007 43 Forging the Future 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 afford- able and effective sewer system, initially to the fi rst few houses — often scattered throughout a large tract far from their nearest neighbor. 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 down- hill fl ow. 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 dic- tated by the local frost penetration depth or, in very mild climates, by the need for protection from mechanical damage. Layout of a typical centrifugal grinding pump system. 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. Diffi cult 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 signifi cant 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 economi- cally 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 signifi cant high points in the profi le, as well as at the beginning of long downhill runs discharging to atmosphere. The ability to construct sewers that follow the contour of the land not only makes development affordable, but has also preserved natural Design fl ow, in any given downstream section of line, as rolling topography and trees. a function of number of pumps contributing, has been determined empirically and is available in tabular or Rocky Soil Conditions graphic form. Here are these criteria as suggested by Rock can be one of the most costly and diffi cult factors in construction. several competent authorities. Gravity sewers have wide excavated trenches and go deeper with each 44 JULY 2007 www.pump-zone.com PUMPS & SYSTEMS lower trenches makes it feasible in places like solid rock where gravity systems are liter- ally impossible. High Groundwater Levels Locations with high groundwater, whether seasonal or year round, present other chal- lenges in both construction and operation of gravity sewers.
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