solution finder

27 April 2018

Pressurised Sewers

Author/Compiled by
Beat Stauffer (seecon international gmbh)
Dorothee Spuhler (seecon international gmbh)
Executive Summary

Pressurised sewers or pressure sewers differ from conventional gravity collection systems, because they use pumps instead of gravity to transport wastewater. The primary effluent is delivered to the collection tank by traditional gravity drainage methods where it is grind before being transported into the pressurised system by pumps. As for central network, no gravity is required; the system can be built with only shallow trenches and relatively small-diameter pipes. It is an effective solution where conventional systems are impractical such as in rocky, hilly or densely populated areas or areas with a high groundwater table.

Advantages
Effective wastewater transportation at minimum depth, minimising the excavation for the piping system
Gravity free; independent from land topography
Less costs compared to a conventional gravity sewer (pipe size and depth requirements are reduced and many small pumps are cheaper than some large-capacity lift stations
Requires little water only for transporting the excreta
Disadvantages
Needs expert design
Needs a permanent energy source for the grinder pumps
High capital costs
A proper recycling of nutrients and energy becomes difficult, because all kinds of wastes (faeces, urine, greywater, industrial waste) are mixed
Unsuitability for self-help, requires skilled engineers & operators
It is still a flushing system which transports wastewater away. If there is no treatment plant and an unprofessional discharge it can contaminate the environment
Leakages may lead to contaminations
In Out

Blackwater, Greywater, Brownwater, Urine or Yellowwater, Non-biodegradable Wastewater, Energy

Blackwater, Greywater, Brownwater, Urine or Yellowwater, Non-biodegradable Wastewater

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Introduction
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Sewers are a means of collecting wastewater from multiple sources and delivering the wastewater to an existing collection sewer, and/or to semi-centralised secondary treatment system such as constructed wetlands (free-surface, horizontal or vertical), or waste stabilisation ponds. Conventional sewers transport wastewater and sludge by gravity and therefore require a slope and pumping station. This can lead to high construction cost in hilly or flat terrain. In some cases, a conventional system is almost impossible to construct, e.g. where the groundwater table is high or the topography is extremely flat.

 

A pressurised sewer system is independent from land topography and does not need deep excavation work. Source: ITT (n.y.)

A pressurised sewer system is independent from land topography and does not need deep excavation work. Source: ITT (n.y.)

Pressurised sewers (and also vacuum sewers) are not dependent on gravity to move wastewater; thus there is less concern about the local topography. Moreover the system can be built with shallow trenches only and relatively small-diameter pipes. Thus pressurised sewers are an effective solution where conventional systems are impractical, e.g. in very hilly or flat areas, densely populated areas or where the groundwater table is high. However, pressurised sewers require a lot of pumps relying on electricity supply and make the system more vulnerable to failure. Thus if the topography, climate, country do/does not require pressurised sewers other systems such as simplified and condominal sewers, solids free sewers or separate sewers or conventional sewers may be preferable.


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Pressure sewer unit including grinder and pump
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     Schematic design of a pressurised sewer system. Source: WERF (2010)

Schematic design of a pressurised sewer system. Source: WERF (2010)     

A typical arrangement for a network of pressurised sewers is for each connection (or small cluster of connections) to have a tank that receives wastewater. When the tank fills to a set point, a pump within the basin injects the wastewater further into the sewer. This transfer of wastewater pressurises the sewer. As various pumps along the length of the sewer inject sewage into the line, the wastewater is progressively moved to the treatment facility (WERF 2010). 


 

         A prefabricated pressure sewer unit made out of plastic for outside placement. Source: SHOALHAVEN WATER (n.y.)

A prefabricated pressure sewer unit made out of plastic for outside placement. Source: SHOALHAVEN WATER (n.y.)         

The storage tank at the entrance of the system is the heart of the system. It is generally a prefabricated plastic pit that provides wastewater storage, grinding (this allows small diameter piping after the pressure sewer unit) and pumping all in one in a single self-contained unit (see picture below). This all-in-one tank is called a pressure system unit. The wastewater flows first by gravity downhill into the pressure system unit from where it is grinded and then pumped into the sewer, generating the pressure. The unit is installed in a property in consultation with the owner. A small diameter discharge pipe goes from the unit to the pressure sewer pipe into the street. A small box (boundary kit) is installed just inside the property on the discharge pipe. A non-return valve (to prevent backflow from the pressure sewer) and isolation valve is housed in this kit. The unit is wired to the household power supply and controlled by a small panel located near the unit (control panel), either on a wall, fence or pole (SHOALHAVEN n.y.).


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Septic tank effluent pump
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             A septic tank effluent pump. Source: EPA (2002)

A septic tank effluent pump. Source: U.S. EPA (2002)             

An alternative to the all in one pressure sewer unit described above is to first install a septic tank situated outside in the ground or in the basement of the building to settle the solids. The septic tank effluent then flows in an underground holding tank containing a pump and control devices from where it is then pumped into the pressurised sewer system and transferred for treatment. Retrofitting existing septic tanks in areas served by septic tank/leach field systems would seem to present an opportunity for cost savings, but a large number (often a majority) must be replaced or expanded over the life of the system because of insufficient capacity, deterioration of concrete tanks, or leaks. The pump in this system does not have to be as powerful as grinder pump, since it just pumps liquid (U.S. EPA 2002).


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Costs considerations
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(Adapted from WERF 2010)

The cost of a pressure sewer system can be divided into two major components: The on-lot costs and the collection network costs. On-lot costs include the pump, basin, controls, building sewer, lateral piping, electrical service, and installation. The collection network includes all the piping in the utility easements that directs the sewage to the treatment facility. Depending on the style of pump and basin selected by the managing utility, on-lot costs are estimated to be US$ 4,800 to US$ 7,200 (prices are estimated for the United States of America, 2009 dollars) for an existing single-family home. Typical solids-handling pumps will use less than 1 kW-hr of power per day and the electrical cost would be about 50 dollars per year depending upon local electrical rates (prices are estimated for the United States of America, 2009 dollars). Using many low power-consuming pumps reduces installation cost as compared to a conventional gravity system (e.g. conventional sewers) that may require one or more large-capacity lift stations (sewer pumping stations) as conventional large scale systems barely can be operated by gravity only. A pressurised system also allows more flexibility in choosing locations for and routes to treatment facilities.

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Operation and maintenance
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Regular service is important for all system components to ensure best long term performance to protect public health and the environment. This also protects the investment. Frequency of operation and maintenance is dependent upon wastewater volume, relative risk to public health and the environment as well as the complexity of components used (Adapted from WERF 2010). Electricity needs to be available all the time, the pumps should be checked regularly and the pipe connections should be controlled for leakages.

 

Three examples of damages on pressurised piping system. Therefore it is very important to inspect and maintain the sewer system. This avoids high repair cost, environmental damages and public health hazard. Source: WSAA (2003) 


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At a glance
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Working principle

A pit containing a grinder and pump or a settling unit (septic tank) connected to a holding tank with a pump are installed close to the user interface (single household to an apartment building). When a certain level of effluent has been collected, it is pumped into the sewer, generating the pressure for transportation.

Capacity/adequacy

Can be used in densely populated areas and where the groundwater table is high. It is independent from the topography (hilly or flat area) and can pass any obstacles without any problems.

Performance

High performance

Costs

High capital costs, but still lower than gravity sewer system

Self-help compatibility

Very low

O&M

Electricity required. The components and especially the pumping and piping must be inspected and maintained regularly to avoid and/or repair any damages.

Reliability

If well maintained it is reliable

Main strength

Shallow trenches, independent from topography and it requires a small amount of flushing water. Very high level of comfort and hygiene.

Main weakness

It is costly and it needs a permanent energy source for the grinder pumps. It needs expert design and depends on a centralised system.

Applicability

Basically, pressurised sewer systems have the same advantages as vacuum sewers. Therefore they are used in similar surroundings;

  • Hilly or flat topography: gravity systems demand installation at great depths to maintain adequate flow or requires pumping and lift stations
  • Rock layers, running sand or a high groundwater table make deep excavation difficult
  • Densely populated areas
  • High groundwater table
Library References

Pressure Sewer System.

The Schoalhaven City Council describes the system of pressurised sewering and its experiences.

SCHOALHAVEN WATER (n.y): Pressure Sewer System., Shoalhaven: Shoalhaven City Council URL [Accessed: 06.06.2011]

Pressure Sewer

This wastewater technology fact sheet is a detailed description of the pressurised sewer system. This factsheet describes the pressurised sewer system as an alternative option to conventional sewers. It describes the system as well as advantages/disadvantages, performance, costs and applicability.

U.S. EPA (2002): Pressure Sewer, Pennsylvania: United States Environmental Protection Agency (US EPA) URL [Accessed: 11.07.2012]

Pressure Sewer System

This factsheet gives an overview about pressure sewer systems. It is illustrated with graphics and photos.

WERF (2010): Pressure Sewer System, Alexandria, Virginia: Water Environment Research Federation URL [Accessed: 11.07.2012]
Further Readings

Pressure Sewer

This wastewater technology fact sheet is a detailed description of the pressurised sewer system. This factsheet describes the pressurised sewer system as an alternative option to conventional sewers. It describes the system as well as advantages/disadvantages, performance, costs and applicability.

U.S. EPA (2002): Pressure Sewer, Pennsylvania: United States Environmental Protection Agency (US EPA) URL [Accessed: 11.07.2012]

Design Criteria and Specifications for Pressure Sewers, Force Mains, Grinder Pumps and Appurtenances

These specifications give the minimum requirements for installation of pressure sewer lines, force mains and appurtenances in the City of Tullahoma, Tennessee.

TUB (2008): Design Criteria and Specifications for Pressure Sewers, Force Mains, Grinder Pumps and Appurtenances, Tullahoma: Tullahoma Utility Board (TUB) URL [Accessed: 19.09.2011]

Pressure Sewer System

This factsheet gives an overview about pressure sewer systems. It is illustrated with graphics and photos.

WERF (2010): Pressure Sewer System, Alexandria, Virginia: Water Environment Research Federation URL [Accessed: 11.07.2012]
Case Studies

Pressure Sewer System.

The Schoalhaven City Council describes the system of pressurised sewering and its experiences.

SCHOALHAVEN WATER (n.y): Pressure Sewer System., Shoalhaven: Shoalhaven City Council URL [Accessed: 06.06.2011]
Training Material
Awareness Raising Material

Rural Water Supply and Sanitation Challenges in Latin America for the Next Decade

Based on market research, this new WSP technical paper analyzes the main features of the fecal sludge collection businesses in each city, including the marketing mix, potential demand, supply capacity, and legal frameworks. In addition, the paper spotlights major challenges and opportunities in fecal sludge management, describing the current and potential market for fecal sludge removal, collection, and disposal in peri-urban areas—which typically struggle with high population density, limited land planning, high citizen insecurity, and low coverage of basic services.

PEARCE-OROZ, G. (2011): Rural Water Supply and Sanitation Challenges in Latin America for the Next Decade, Lima: Water and Sanitation Program (WSP) URL [Accessed: 14.05.2012]

Alternative Versions to