Pressure Sewer Systems

A pressure sewer system is a holding tank that detains the wastewater flow from a home or group of homes until a set point is reached and a pump in the tank grinds and then pumps the wastewater under pressure to a treatment facility or further into the collection system.

The information below was taken from the Wastewater Technology Fact Sheet provided by the United States Environmental Protection Agency (EPA). For the full article, visit

http://water.epa.gov/scitech/wastetech/upload/2002_10_15_mtb_presewer.pdf.

Pressure sewer systems are most cost effective where housing density is low or in hilly areas, and where the pressure system outfall must be at the same or a higher elevation than most or all of the service area. They can also be effective where flat terrain is combined with high ground water or bedrock, making deep cuts and/or multiple lift stations excessively expensive. They can be cost effective even in densely populated areas where difficult construction or right of way conditions exist, or where the terrain will not accommodate gravity sewers. Basically pressure systems can be utilized into almost any wastewater situation.

Because wastewater is pumped under pressure, gravity flow is not necessary and the strict alignment and slope restrictions for conventional gravity sewers can be relaxed. Network layout does not depend on ground contours: pipes can be laid in any location and extensions can be made in the street right-of-way at a relatively small cost without damage to existing structures.

There are advantages for a utility in installing a pressure system. Lower cost of materials, smaller pipe and less excavation cost along with low-cost clean-outs and valve assemblies rather than manholes. Infiltration should be reduced, but inflow may still be a problem. A 10-gallon per minute sump pump that runs for 24 hours will feed 14,400 gallons per day to a system. Another advantage is that the homeowner pays the electric bill for the pumping unit.

There are also disadvantages to this type of system. The operation and maintenance cost for a pressure system is often higher than a conventional gravity system due to the high number of pumps in use. Annual preventive maintenance calls are usually scheduled for grinder pump components of pressure sewers. The number of pumps that can share the same downstream force main is limited and power outages can result in overflows if standby generators are not available. Odors and corrosion are potential problems because the wastewater in the collection sewers is usually septic. Educating the public is necessary so they will know how to deal with emergencies and how to avoid problems with blockages.

Since pressure systems do not have the large excess capacity typical of conventional gravity sewers, they must be designed with a balanced approach, keeping future growth and internal hydraulic performance in mind.

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