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Schematic of a vacuum sewer system
Schematic of a vacuum sewer system
A vacuum toilet at Lau Fau Shan Roundabout in Hong Kong
A vacuum toilet at Lau Fau Shan Roundabout in Hong Kong
A portable vacuum sewer system on the underside of a train carriage, serving the vacuum toilet and sink inside the train
A portable vacuum sewer system on the underside of a train carriage, serving the vacuum toilet and sink inside the train

A vacuum sewer or pneumatic sewer system is a method of transporting sewage from its source to a sewage treatment plant. It maintains a partial vacuum, with an air pressure below atmospheric pressure inside the pipe network and vacuum station collection vessel. Valves open and reseal automatically when the system is used, so differential pressure can be maintained without expending much energy pumping. A single central vacuum station can collect the wastewater of several thousand individual homes, depending on terrain and the local situation.[1][better source needed]

Vacuum sewers were first installed in Europe in 1882. Dutch engineer Charles Liernur first applied negative pressure drainage to sewers in the second half of the 19th century.[2][non-primary source needed] Technical implementations of vacuum sewerage systems began in 1959 in Sweden.[citation needed]

Historically, vacuum sewers have been a niche product, used only in trains, airplanes, and flat areas with sandy soils and high ground water tables. Gravity sewers were used for most applications, because although vacuum sewers were cheaper to install, they were more expensive to maintain. In the 20th century, vacuum sewer technology has improved significantly: fault-locating sensors have reduced operation and maintenance costs, and some operators now consider that vacuum sewers can be cheaper to run than conventional gravity sewers.[3]

Basic elements

A cutaway of a vacuum valve
A similar vacuum valve opening and closing

The main components of a vacuum sewer system are a collection chambers and vacuum valve parts, sewers, a central vacuum station and monitoring and control components.

Some vacuum systems have vacuum toilets are connected directly to a vacuum line, which requires less water for flushing (less than a quarter of a liter per flush).[4] Others use standard gravity drainage for the first phase of collection; sewage flows by means of gravity from each house, as in a standard system. It discharges into a collection sump that might collect sewage from 2-6 houses and is located in a public area.

Vacuum technology is based on differential air pressure. Rotary vane vacuum pumps generate an operation pressure of -0.4 to -0.6 bar at the vacuum station, which is also the only element of the vacuum sewerage system that must be supplied with electricity.

Interface valves are installed inside the collection chambers. They work pneumatically. After a certain fill level inside this sump is reached, the interface valve opens. The impulse to open the valve is transferred by a pneumatically mechanical controlled controller unit. No electricity is needed to open or close the valve. The energy is provided by the vacuum itself.

A vacuum station for a student dorm in Norway, providing suction for all the vacuum toilets in the dorm
A vacuum station for a student dorm in Norway, providing suction for all the vacuum toilets in the dorm

While the valve is open, the resulting differential pressure between atmosphere and vacuum becomes the driving force and transports the wastewater and air towards the vacuum station. Besides these collection chambers, no other manholes, neither for changes in direction, nor for inspection or connection of branch lines, are necessary. High flow velocities keep the system free of any blockages or sedimentation.

Large systems with numerous collection chambers benefit from the provision of a monitoring system for remote monitoring of the vacuum valves and sump pits. Such systems allow much faster troubleshooting and easier preventive maintenance of collection chambers and valves. However, monitoring systems are optional systems and not required for operation of vacuum sewer systems.

Vacuum station in Hooper, Utah.
Vacuum station in Hooper, Utah.

Vacuum sewer systems are considered to be free of ex- and infiltration which allows their use even in water protection areas. For this reason, vacuum sewer lines may even be laid in the same trench as potable water lines (depending on local guidelines).

In order to ensure reliable transport, the vacuum sewer line is laid in a saw-tooth (length-) profile.[5] The whole vacuum sewers are filled with air at a pressure of -0.4 to -0.6 bar. The most important aspect for a reliable operation is the air-to-liquid ratio. When a system is well designed, the sewers contain only very small amounts of sewage. The air-to-liquid ratio is usually maintained by collecting liquid/air simultaneously or controller units that adjust their opening times according to the pressure in the system.

Sewers can be laid in flat terrain, and parts may flow uphill (within limits). A saw-tooth profile keeps sewer lines shallow;[5] in frost-free climates, trench depth can be about 1.0 – 1.2 m. By contrast, gravity sewers need a monotonically falling slope of at least 0.5 - 1.0%, which can mean that expensive trenching and pumping stations are needed.

Once the wastewater arrives in the vacuum collection tank at the vacuum station, it is pumped to the discharge point, which could be either a gravity sewer or the treatment station. As the dwell time of the wastewater inside the system is very short and the wastewater is continuously mixed with air, the sewage is kept fresh and any fouling inside the system is avoided (less H2S).

Advantages

Minimal disruption for vacuum sewers
Minimal disruption for vacuum sewers

Disadvantages

Sign indicating a buried vacuum sewer in Germany.
Sign indicating a buried vacuum sewer in Germany.

Applications

Vacuum sewer systems may be the preferred system in the case of particular circumstances, such as:

Transport

Vacuum toilet on a train
Vacuum toilet on a train

Trains, aircraft, buses, and many ships with plumbing generally have vacuum systems with vacuum toilets. The lower water usage (less than a quarter of a liter per flush)[4] saves weight, and avoids water slopping out of the toilet bowl in motion.[9] Aircraft toilets may flush with blue disinfectant solution rather than water.[4] A portable collection chamber is used; if it is filled from an intermediate vacuum chamber, it need not be kept under vacuum.[10][better source needed]

Dry areas

Lack of water in many countries and drastic water savings measures have led to difficulties with aging gravity networks with solids blocking in the pipes. Vacuum systems save water.

Boggy, rocky, or permafrost terrain

Flat terrain, unfavorable soil (rocky or swampy ground), or a high groundwater table (which requires dewatering trenches) can make gravity sewerage systems much more expensive. Vacuum sewers are small in diameter and leak inwards, and in frost-free areas, they can be laid close to the surface in small trenches.

Water protection areas, environmental use

Vacuum sewers can pass through water protection areas and areas with sensitive high ground water tables, because there is no danger of spoiling groundwater resources (vacuum sewers have a high leak tightness due to their material; and if they leak, they leak inwards). Vacuum systems are used in many environmentally sensitive areas such as the Couran Cove Eco Resort close to the Barrier Reef in Australia. They've also been used to replace septic tanks to reduce nitrogen levels in ground/surface water.[11]

Vacuum systems have also been applied to collect toxic wastewater from the environment.

Seasonally sub-freezing climates

If the temperatures in an area dip below freezing in winter, the vacuum line is buried below the frost line, in ground that stays unfrozen year-round (as are conventional gravity sewers). Valves, collection pits, intake vents, and control systems need to be designed to keep functioning despite cold, snow and ice. Temperature-monitoring sensors are also standard, so problems can be noticed early.

In the case of Plum Island (Massachusetts), the island was prone to freezing temperature and excessive snowfall that initially made it difficult to locate a potential problem. Changes to their Pit setup and monitoring at the valve pit has helped with maintenance.

Many Nordic Countries utilize vacuum sewers, it is helpful to have some type of marker or monitoring to locate valves when they are buried under the snow for extended periods.

Low or seasonal population density

With lower population densities, the costs for the collection chambers and vacuum stations are less important than the costs of installing pipe and, for gravity sewers, pumping stations, etc.. Pneumatic pipes are generally smaller than gravity-drained hydraulic ones[citation needed] In frost-free climates, the pipes for a vacuum system can also be buried more shallowly than a gravity system.

High specific conduit lengths, where the required pipe length is longer than ~4 metres per inhabitant[citation needed], will tend to make a vacuum system cheaper.

In seasonal settlements (recreation areas, camping sites etc.) with conventional gravity sewer systems, sedimentation problems can easily occur as automatic flushing by daily waste water does not take place. High flow velocities within vacuum sewers prevent such sedimentation problems. The Formula 1 race tracks in Shanghai and Abu Dhabi are using a vacuum sewer system for that reason.[citation needed]

Historic sites

A sideroad in Flavigny-sur-Ozerain where it would be difficult to install a conventional gravity sewer.
A sideroad in Flavigny-sur-Ozerain where it would be difficult to install a conventional gravity sewer.

Historic sites may have old buildings, narrow streets, and steep terrain. Tourism may also cause strong seasonal fluctuations in population density. Vacuum sewer systems may be selected for their fast (avoiding conflicts with traffic and tourism), cost-effective and flexible installation. Examples include Flavigny-sur-Ozerain, France, and Khasab and Al Seeb in Oman.

Treatment

Vacuum sewer systems can be set up so that they collect concentrated blackwater (toilet wastewater) only, with the greywater from sinks and baths being collected separately (it is much easier to treat greywater than blackwater). The biosolids from a vacuum system need not be diluted with flushing water.

Sewage systems usually thermophillically compost biosolids which have been separated and dewatered from a standard gravity sewer.[12][13] This process is simpler if the biosolids are never watered.

Composting at high temperatures kills pathogens and seeds.[13][14] Biosolids compost is required to be composted at high temperatures.[15]

Sewage can also be treated in an anaerobic process with the production of biogas. This design has the potential to increase sustainability of water infrastructures.[16]

Examples

Arctic

Arid regions

High water table areas

Areas with seasonal freezing

Ruling technical guidelines and norms

See also

References

  1. ^ a b "Vacuum Sewerage System - Palm Jumeirah, Dubai" (PDF), sswm.info, Corodex Electromechanic
  2. ^ Liernur, C.T., "Pneumatic Sewerage system" (PDF), sewerhistory.org
  3. ^ See (Gibbs 2016): "'When we did the pilot project study in 1993, we calculated that the present value estimate comparison for vacuum sewers and gravity sewers would intersect in 20 years,' he notes. 'At that point, we predicted that vacuum sewers would be more expensive due to maintenance and upkeep. This was based largely on data known at that time. It has been 23 years now, and we’re still not close to seeing the O&M costs that were originally estimated,' Beach continues. 'We have not crossed the present worth value line, and it will likely be another 10 years before these lines intersect.'"
  4. ^ a b c Cromwell, Bob. "Aircraft Toilets". toilet-guru.com. Toilets of the World.
  5. ^ a b See (Gibbs 2015): "'One of the interesting things I learned about vacuum sewers is that they are gravity assisted,' Garri notes. 'The collection lines have a sawtooth profile. Vacuum pressure in the lines assists gravity to help move sewage slugs along to the treatment plant. This type of innovative design allows for the vacuum sewer mains to be installed at a much shallower depth than gravity sewers.'"
  6. ^ See (Gibbs 2016): "Leaks in a vacuum main or lateral likewise are rare... 'That’s one of the things we appreciate about vacuum sewers; if there is a leak in the line, vacuum pressure sucks in whatever is around the hole rather than sewage pouring out into the environment,' notes Read. 'We don’t have to do a line repair standing in sewage.'"
  7. ^ See (Gibbs 2016): "Read said his team can detect the location of small leaks by looking at the diagnostic data at the vacuum station to determine the specific area, then listening for the sucking sound of air rushing in... 'We can usually locate a leak in less than 30 minutes. If you can’t find it in 30 minutes, it must be a very small hole'... Vacuum sewer line repair usually is quick and easy, because collection lines are buried only 3 to 4 feet deep. A small backhoe typically is all that’s required to excavate a leaky pipe."
  8. ^ See (Gibbs 2015): "He notes that vacuum stations have emergency generators that kick in when power is lost"
  9. ^ "How does the toilet in a commercial airliner work?". howstuffworks.com. HowStuffWorks. 1 April 2000.
  10. ^ "EVAC Bus Vacuum Toilet". evac-train.com. Evac GmbH.
  11. ^ See (Gibbs 2015): "An important step in the Kings Bay restoration effort was a two-phase vacuum-sewer installation. The project’s goal was to rid the area of nearly 600 aging septic tanks that were contaminating groundwater and contributing to nitrogen buildup in the bay."
  12. ^ http://infohouse.p2ric.org/ref/12/11513.pdf
  13. ^ a b Turovskiy, Izrail S.; Westbrook, Jeffrey D. (1 October 2002). "Recent Advancements in Wastewater Sludge Composting". www.wwdmag.com. Water & Wastes Digest.
  14. ^ http://www.blockislandwater.org/Biosolids-WaterOpsdata/Simmons-Composting%20Biosolids-July2010.pdf[dead link][dead link]
  15. ^ "BioSolids Composting". www.cdaid.org. City of Coeur d'Alene Idaho.
  16. ^ a b "KREIS - Kopplung von regenerativer Energiegewinnung mit innovativer Stadtentwässerung". kreis-jenfeld.de. Archived from the original on 29 December 2013. Retrieved 10 April 2016. (in English, despite German title)
  17. ^ "Alaska Native Tribal Health Consortium Division of Environmental Health and Engineering Final Report" (PDF), energy.gov, US Department of Energy, September 2014
  18. ^ Herbst, Heinrich (November 2009), Ökonomische Bewertung von Abwasserinfrastruktursystemen (PDF) (in German)
  19. ^ See (Gibbs 2016): "The city of Ocean Shores, Wash., offers a valuable test case for vacuum sewer reliability and endurance. The city’s vacuum system is one of the largest in the world, and it’s also mature; most of its components are more than 20 years old and must function in a challenging operational environment. What makes Ocean Shores even more valuable as a sewer system comparison study is that it also has gravity sewer and grinder pump systems to maintain."
  20. ^ "PCS: Vacuum Sewer Construction". scgov.net. Sarasota County. 13 October 2007. Archived from the original on 13 October 2007.
  21. ^ "Booms and Blooms", wwdmag.com, Water and Wastes Digest
  22. ^ "Vacuum Sewer System Fact Sheet" (PDF). ci.carnation.wa.us. City of Carnation. 23 May 2004. Archived from the original (PDF) on 23 May 2004.
  23. ^ a b "Frozen Vacuum Sewers and the Lessons from Europe". flovac.com. Flovac Vacuum Sewerage Systems. 29 May 2015.

Works Cited