A description of the use of pressure sewer system technology to solve challenging sewage disposal requirements where other methods may be less economically feasible or environmentally unacceptable
Since the early 1970s, pressure sewer systems have been an effective method to move residential wastewater through small diameter pipes to collection facilities where other methods are less economical or less feasible.1The keys to understanding the differences between conventional 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 flat land, extreme cold weather, stream crossings, restricted access, and constraints on blasting.Because the piping systems are sealed, pressure sewers offer freedom from infiltration/inflow. This provides a twofold benefit. The wastewater treatment plant can be built with smaller capacity since it is not necessary to allow for storm peaks, and the treatment process will not be upset by storm related flows which can literally swamp out plants with infiltration/inflow plagued gravity collection systems.
Generally speaking, these systems are installed outdoors below grade. Indoor systems are also available. A typical system includes a pump, basin, controls and piping and valving. Let's consider some well-known applications of submersible grinder pump pressure sewer technology.
Failing Septic Tanks In Existing Subdivisions
The biggest building boom in American history began immediately following the Second World War. This boom, epitomized by William Levitt at his Levittowns on Long Island and in southeastern Pennsylvania, provided a first home to millions of homecoming GIs.
Many of these subdivisions, in all parts of America, were carved out of raw land adjacent to existing cities and towns. Most provided only streets, electricity, and telephones. Such niceties as curbs and gutters, streetlights, fire protection, and especially public sewers and water supply were notable by their absence.
Into this breach was thrown an old technology, developed early in the 20th century by agricultural engineers for use on the farm; namely, septic tanks and soil absorption systems where sewage is digested in a large tank and then leached into the surrounding soil.
Out in the country, land was abundant. As long as the well was prudently located up hill and on the other side of the house, there was no problem. A little odor or a soft spot in the middle of a field, far from habitation, was no cause for concern. Limited use of such septic systems had also been made in some city neighborhoods with large lots, prime soil conditions and careful operation (limited loading and frequent pump outs).
In retrospect, septic tanks (when brought to town) were a very poor choice on these small "postage stamp sized" lots. Nonetheless, in the rush to provide critically 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 field and in labs at the Taft Sanitary Engineering Center on behalf of the FHA. UPSHS reported that over 24 million septic systems were in service 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 building 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 maintained or inspected.
Fortunately there are several proven solutions to this need for affordable, dependable, safe sewers. One of the most successful and widely used methods is pressure sewer systems powered by grinder pumps. Most public health officials, developers, consulting engineers, contractors, and public works personnel have had at least some experience with pressure sewer systems since their development in the 1960s.
Figure 1. Layout of a typical progressing cavity grinding pump system
Hundreds of thousands of homes that once suffered from marshy, odorous children's play areas, lake water quality degradation, and even hepatitis and E-coli epidemics caused by septic tank failures are today the proud owners of successful grinder pump pressure sewer systems. These are in everyday use in neighborhoods all over the world.
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.
Figure 2. 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.
Figure 3. 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.
Rocky Soil Conditions
Rock can be one of the most costly and difficult factors in construction. Gravity sewers have wide excavated trenches and go deeper with each foot of length. This means the price per foot is significantly higher than installations in normal soil.
Installation in areas of solid rock can make system costs economically unfeasible. In cases such as these, contractors recommend alternatives such as pressure sewer systems. The fact that this alternative requires dramatically narrower and shallower trenches makes it feasible in places like solid rock where gravity systems are literally impossible.
High Groundwater Levels
Locations with high groundwater, whether seasonal or year round, present other challenges in both construction and operation of gravity sewers.
During construction, the work site must be dewatered by generous use of pumps and well points distributed along the proposed trench route, and powered 24 hours a day. Such dewatered soil can be very unstable and potentially dangerous to work in. Therefore, continuous shoring and bracing are usually required.
Even if these obstacles are overcome by expenditure of much money, care and effort, there remains the necessity to successfully operate the completed gravity sewer for the next 40 or 50 years. Consider that once the dewatering pumps are shut down and the ground water returns, the sewer must operate in what is tantamount to a submerged condition - this without causing infiltration and or inflow, both notorious enemies of overall water quality goals.
Lakeside Or Oceanfront Properties
One of the most desirable properties, sought out by millions of people around the world, is "a place beside the water." It doesn't really matter whether it's a pond, creek, lake or reservoir, riverfront, an estuary or an ocean. People desire to live near water.
The topographical features, which create these precious water bodies, are dominated by the fact that the land almost always slopes down toward the shore. With failing septic systems, the untreated wastewater can potentially pollute the body of water.
Since these systems must be down slope from the houses, they cause the disturbance, degradation and sometimes destruction of the most important feature of waterfront properties; namely, the "front yards" facing the shore. In some cases, land is so precious and the demand so great that tiny cabins are crowded against each other and literally pressed down as close as they dare to the water. It is very expensive, environmentally damaging, and seldom entirely satisfactory to put gravity sewers in such waterfront locations.
The pressure sewer has proven to be an environmentally friendly, cost-effective solution in these waterfront applications.
Lots On The Wrong Side: Sewer Must Go Under A Stream Or Highway
Sometimes, property is developed in a strip along one side of a highway, road or stream. Often there are highly desirable, perhaps isolated, building lots on the "wrong side of the street." Until pressure sewers came along, these choice lots were listed as "unbuildable" and might be ignored for decades with a casual "that's too bad."
Figure 4. Layout of a typical 230-volt grinder pump control panel circuit.
Pressure sewers bored under the stream or highway using a trenchless technology, or carried overhead on a bridge crossing, make such difficult sites easily accessible to whatever sewers already serve the strip community.
Houses Near But Not Directly Serviceable By An Existing Gravity Sewer
It is always desirable, and sometimes absolutely mandatory, that public sewers be deep enough to serve fixtures at, or just under, the basement floor level.
It often happens that when a gravity sewer is designed to serve a certain area, the basements of houses at the ends of the served streets end up just level with the sewer. If such streets are later extended "further out into the country," the new houses will be too low to have basement sewer connections. Or the distance to the gravity sewer may require a pump system.
The answer is to put grinder pumps in or next to these basements and create a pressure sewer line that can pump into the nearest gravity pipe or pumping station with available capacity for the additional flow.
Indoor Installations
As strange as it sounds to us today, when flush toilets were first invented, they were installed in the outhouse. Now, more than a century later, the flush toilet and other convenient water using fixtures are firmly ensconced as not only necessities, but also beautiful adjuncts to the modern American home.
However, we still seem to have a residual mental block from those days which whispers, "anything to do with sewage goes out in the yard." Some manufacturers have systems specifically designed to be installed indoors in basement areas.
Elimination Of Infiltration/Inflow
Pressure sewers are constructed of pressure pipe and leak tested to the same American Water Works Association (AWWA) standards used for potable water supply; thus, they are, for all practical purposes, watertight. This eliminates most infiltration problems so characteristic of old gravity sewers.
New collection systems consisting entirely of pressure lines fed by grinder pumps have been shown to be entirely free of extraneous water flows.
Conclusions
- It has taken three decades for pressure sewer systems to begin to take their proper place within the public health engineering field. Today, there are hundreds of thousands of grinder pumps in routine daily operation in systems ranging in size from a single pump to thousands of pumps.
- Extremely low operating and maintenance (O&M) costs have been documented. Data is now available from many successful systems - some in operation for more than 20 years. By taking advantage of the experience which these systems offer, a new system can be planned which will have good performance, high reliability, and reasonable O&M costs.
- Pressure sewer systems using grinder pumps are particularly useful in new construction of subdivisions and second home communities, and in existing communities with failing septic tanks. The problems of failing septic tanks, unsatisfactory soil conditions, and an increased emphasis on environmental issues can be economically solved with pressure sewers. Pressure sewers are compatible with other collection system techniques. The advantages of each technology can be blended into site-specific designs using grinder pumps, gravity, large submersible pumping stations, and force mains. There is virtually no limit to the type discharge point to which a pressure sewer using grinder pumps can be connected.
References
1 Adapted from "The Secret Life of Pressure Sewers" by R. Paul Farrell, Consulting Engineer Niskayuna, NY, USA. Presented at the Small Drinking Water and Wastewater Systems Conference, January 12-15, 2000 Phoenix, AZ
Pressure Sewer System Technology Bibliography
- Almquist, Carl, Chief Operator, Town of Groton, CT, personal communication, December 1991
- Bendixen, T.W., and Weibel, S. R., "Study on Septic Tanks and Septic Tank Disposal Systems", NTIS Report Number PB-216 760, 24p, 1951
- Carcich, ltalo G.; Farrell, R. Paul and Hetling, Leo, "A Pressure Sewer System Demonstration", EPA R2-72-091, 218p, November 1972
- Coulter, J.B., "Sewage Disposal Systems Applicable to Subdivisions", NTIS Report number PB-217 475, 12p, 1957
- Eblen, J.B. and Clark, L.K., "Pressure and Vacuum Sewer Demonstration Project - Bend, Oregon", EPA 600/2-78-166, 1978
- Environment One Corporation, "GP Report", Summer 1988
- Fair, Gordon M., "Converted Sewer System" - US Patent 3,366,339, filed Nov. 26, 1965, issued June 30, 1968, assigned by the inventor to the public
- Gray, Donald D., "TN Community's Grinder Pumps Provide Positive O&M Statistics", Small Flows Clearing House, 5(4), October 1991
- Gray, Glenn C., ‘Environmental Constraints Challenge Designers of Shoreline Community Near Kansas City", Professional Engineer 45(6) pp42-44, 1975
- Head, A. L. et al, "Low Pressure Sewer System Replaces Septic System in Lake Community', http://towtrc.tamu.edu/sewer.htmi, 8p, 1998
- Ierley, Merrit, "The Bathroom an Epic", American Heritage magazine, 50.3, May 1999, 76(1)
- Mayhew, Chuck and Fitzwater, Richard, "Grinder Pump Sewer System Saves Beach Property", Water Engineering and Management, 4p, September 1999
- Mekosh, G. and Ramos, D., ‘Pressure Sewer Demonstration at the Borough of Phoenixville, PA", EPA-R2-73-270 (NTIS PB-224456/4), 71 p, 1973
- Milnes, Thomas R. and Smith, Nacky, "Community Action at Quaker Lake", WPCA of Pennsylvania Magazine, 6p, November/December 1978
- Palmer Lynn H., "Preserving the Antietam Battlefield at Affordable Cost" in WEF Specialty Conference on Collection System Operation and Maintenance, Tucson, AZ, pp377-383, 1993
- Sanson, R.L., "Design Procedure for a Rural Pressure Sewer System", Public Works, (I 04)1 0, pp86-87, 1973
- Wetsel, David A., "The O&M History of the Low Pressure Sewer Systems for the Towns of Parish, West
- Monroe, and Cleveland, New York", at NYWEA Environmental Technical Conference, Saratoga Springs, NY, 11 p, June 12-14, 1995
Historical Timeline of Pressure Sewers
- ASCE-sponsored project (mid 1960s) inspired by Professor Gordon M. Fair of the Harvard School of Public Health (Fair 1968)
- Development (1963-66) at General Electric of world's first prototype grinder pump. Field test of first pressure sewer system (1969-70) by Environment One Corp.. in cooperation with NYDEC and the US EPA (Carcich 1972)
- Introduction of first commercial grinder pump at WEF (then WPCF) Annual Conference in Boston, 1969
- Other EPA-sponsored demonstration projects (early 1970s) in Pennsylvania (Mekosh 1973), Oregon (Eblen 1978) and Indiana (Sanson 1973)
- Earliest projects approved by state regulatory agencies (1970-72) in Indiana, New York, Ohio, Texas, Virginia
- Federal Construction grant eligible (c. 1970s) as an Alternative Technology
- Adopted (early 1980s) into "Ten States Standards," as well as most individual state guidelines.