Pumps & Systems , July 2007
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. 1
The 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 sept ic 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
