Vacuum sewer

Vacuum sewers were first installed in Europe in 1882 but until the last 30 years it had been relegated to a niche market. The first who has applied the negative pressure drainage (so called vacuum sewerage) was the Dutch engineer Liernur in the second half of the 19th century. It was only used on ships, trains and airplanes for a long time. Technical implementations of vacuum sewerage systems were started after 1959 in Sweden by Joel Liljendahl and afterwards brought on the market by Electrolux where QUA-VAC took over the vacuum sewer technology and Electrolux concentrated on the vacuum toilets for ships only. Nowadays several system suppliers offer a wide range of products for many applications.

This section covers land based vacuum systems but the technology is also used on aircraft, ships, trains as well as in buildings. Supermarkets, prisons, marina's and many commercial buildings are using vacuum systems as well as vacuum toilets which can reduce the amount of water flushed away to less than 1 litre per flush. The NASA Space Shuttle uses vacuum toilet technolgy to reduce water requirements.

A vacuum sewer system uses the differential pressure between atmospheric pressure and a partial vacuum maintained in the piping network and vacuum station collection vessel. This differential pressure allows a central vacuum station to collect the wastewater of several thousand individual homes, depending on terrain and the local situation. Vacuum sewers take advantage of available natural slope in the terrain and are most economical in flat sandy soils with high ground water.

Basic elements

# Collection chambers and vacuum valve units
# Vacuum sewer lines
# Central vacuum station

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 that are installed inside the collection chambers work pneumatically. Any sewage flows by means of gravity into each house’s collection sump. After a certain fill level inside this sump is reached, the interface valve will open. The impulse to open the valve is usually transferred by a pneumatically (pneumatic pressure created by fill level) controlled controller unit. No electricity is needed to open or close the valve. The according energy is provided by the vacuum itself. While the valve is open, the resulting differential pressure between atmosphere and vacuum becomes the driving force and transports the wastewater towards the vacuum station. Beside these collection chambers, no other manholes, neither for changes in direction, nor for inspection or connection of branch lines, are necessary. High flow rates keep the system free of any blockages or sedimentation.

Vacuum sewer system are considered to be free of ex- and infiltration which allows the usage 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). The system supplier should certify his product to be used in that way. To achieve the condition of an infiltration-free system and therefore allowing to reduce the waste water amounts that need to be treated, water tight (PE material or similar) collection chambers should be used. Concrete collection chambers are very likely to be infiltrated by ground water (wall) and/or surface water (covers) which will lead consequently to higher energy costs as this water needs to be transported, too. Valve and collection sump (waste water) should be physically separated (different chambers) in order to protect service personal against direct contact with waste water and to ensure longer life cycles (waste water is considered to be corrosive).

In order to ensure reliable transport, the vacuum sewer line is laid in a saw-tooth (length-) profile, which will be referred to more precisely afterwards. 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 – they should never be totally filled with sewage since the hydraulic friction loss would be too high! The air-to-liquid ratio is usually maintained by "intelligent" controller units or valves that adjust their opening times according to the pressure in the system.

Considering that the vacuum idea relies on external energy for the transport of fluids, sewers can be laid in flat terrain and up to certain limits may also be counter-sloped. The saw-tooth profile keeps sewer lines shallow, lifts minimise trench depth (approx. 1.0 – 1.2 m). In this depth, expensive trenching, as it is the case for gravity sewers with the necessity to install continuously falling slopes of at least 0.5 - 1.0%, is avoided. Lifting stations are not required.

Once arrived in the vacuum storage tank at the vacuum station, the wastewater is pumped to the discharge point, which could be a gravity sewer or the treatment station directly. As the dwell time of the watewater 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

*closed, pneumatically controlled system with a central vacuum station. Electrical energy is only needed at this central station
*no sedimentation due to self-cleansing high velocities
*spooling and maintenance of the sewer lines is not necessary
*manholes are not required
*Usually only a single vacuum pump station is required rather than multiple stations found in gravity and low pressure networks. This frees up land , reduces energy costs and reduces operational costs.
*investment costs can be reduced up to 50 % due to simple trenching at shallow depths, close to surface
*flexibility of piping, obstacles (as open channels) can be over- or underpassed
*reduced installation time
*small diameter sewer pipes of HDPE, PVC materials; savings of material costs
*aeration of sewage, less development of H₂S, with its dangers for workers, inhabitants, as well as corrosion of the pipes may be avoided; sewage is kept fresh
*no odours along the closed vacuum sewers
*no infiltration, less hydraulic load at treatment station and discharge sewers
*absolutely no leakages (vacuum avoids exfiltration)
*sewers may be laid in the same trench with other mains, also with potable water or storm-water, as well as in water protection areas
* Lower cost to maintain in the long term due to shallow trenching and easy identification of problems

Limitations

*vacuum systems are not capable of transporting sewage over very long distances
*vacuum sewerage systems are only capable for the collection of wastewater within a separated system (not for the collection of storm-water)
*the lines can only reach up to 3-4 km laid in flat area (restrictions of the system due to headlosses (3-4.5 m) (friction and static))
*systems should be designed with help of an experienced manufacturer (concepts are usually free of charge)
*external energy is required at a central point for collecting sewage
*odours close to the vacuum station can occur, biofiltre eventually necessary

Application Fields

Vacuum sewer systems becomes more and more the preferred system in the case of particular circumstances:

*Especially difficult situations as ribbon, peripheral settlements on flat terrain with high specific canal lengths of longer than 4 metres per inhabitant are predestined for the application of vacuum sewerage systems. In the case of sparse population density the influence of the costs for the collection chambers and vacuum stations are less important in comparison to the costs of long and deep sewers on gravity.

*Missing incline of the ground, unfavourable soil (rocky or swampy grounds) and high groundwater table (with the necessity of dewatering trenches) lead to enormous investment costs in regards to gravity sewerage systems. On the contrary vacuum sewers that are small in diameter can be laid close to the surface in small trenches.

*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; moreover the vacuum itself does not allow exfiltration). Vacuum systems has also been applied to collect toxic wastewater. Vacuum systems are seen as a priotity in many environmentally sensitive areas such as the Couran Cove Eco Resort close to the Barrier Reef in Australia.

*In seasonal settlements (recreation areas, camping sites etc.) with conventional gravity sewer systems, sedimentation problems can easily occur as automatic spooling from the daily waste water does not take place. High flow velocities within vacuum sewers prevent such sedimentation problems. The Formula 1 race track in Shanghai uses a vacuum sewer system for that reason.

*Even in old narrow and historical villages, the use of vacuum sewer systems becomes more and more important due to a fast (traffic, tourism), cost-effective and flexible installation. Good examples and references can be found in France, such as the village of Flavigny and in Oman at the township of Khasab.

*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. Neither the lack of water nor solids affect resp. occur in vacuum sewer systems. That's why this technology becomes interesting for such kind of applications. As PE or PVC pipes are used, no solids from aging pipes will enter the system. All other solid are kept out at the collection chambers. vacuum sewer systems don't have any manholes to dump big solids into the system.

The county of Sarasota, Florida [ [http://sewers.scgov.net/Tech_Vacuum.aspx PCS: Vacuum Sewer Construction ] ] and the city of Carnation, Washington [http://www.ci.carnation.wa.us/sewer/factsheet-vacsystem.pdf] are developing a county wide collection system and is incorporating vacuum sewers.

In Germany, several hundred well-working systems are operating since the 1970’s. Especially in the Middle East (United Arab Emirates, Qatar, Bahrain, Oman), vacuum sewer systems become more and more important due to easy and fast installation along with water saving effects and easiness of maintenance.

The world most famous vacuum sewer project is currently the Palm Island Jumeirah, located in in front of the coast of Dubai City, United Arab Emirates. Approx. 23.000 people will be connected to this vacuum sewer system with only 1 central vacuum station once the Palm Island has been completely finished. Construction for the project was done by Corodex Electromechanic, whereas the technology supplied was by Roediger Vacuum GmbH. Also completed in the U.A.E. by Corodex/Roediger are Al Raha Gardens in Abu Dhabi and the Palm Deira Sales offices. Many other projects are currently under development or construction. Flovac Vacuum Sewerage Systems have also installed vacuum systems in Bahrain at Reef Island and Busaytin, in Qatar at The Pearl of the Gulf, in Dubai at Dubai Festival City, and at Jumeirah Golf Estate.

Amwaj Island in Bahrain is serving more than 15.500 people with a vacuum sewer system and running for more than 3 years, supplied by QUA-VAC BV.

The biggest installation in Europe and considered to be one of the biggest installations world wide (several vacuum stations) can be found in Gerasdorf (near Vienna), Austria, where many benefits of a vacuum sewer system helped to overcome difficult conditions in this mountainous area.

Good examples can be found on the Maldives, the post-tsunami WATSAN project UNICEF - UN, where on several islands vacuum sewer systems have been the best option. Several other project, mainly for resorts, have already been realized on the Maldives.

Vacuum sewer systems are not only used in the Middle East but even in low developed third world countries. Several vacuum sewer systems have been already built or are currently under construction in Africa (South Africa, Botswana, Namibia) for townships and rural areas where the benefit of fast construction time, cost saving trenching and high flexibility have come to full effect.

Australia has been one of the largest users of vacuum sewer systems due to the low installation and operational costs (primarily using Flovac/Airvac systems, based in Australia). The largest system to-date has been at the Tea Gardens development in New South Wales, which will ultimately handle over 4.500 houses. The Water Corporation in Western Australia is considered the largest single owner of vacuum systems in the world with over 30 schemes now under their operational control.

The United Kingdom is well served by Vacuum Sewerage Systems, the region most extensively served are the low lying fenlands of the East of England. High water tables (in some cases less than 1metre below the surface) and poor ground conditions have meant that the local Water Company Anglian Water has embraced the use of Vacuum Sewerage, taking advantage of the system's requirement for small bore sewer pipes laid in shallow trenches, dramatically reducing the requirement for pumping stations as would be required by conventional gravity sewer systems. The largest Vacuum Sewerage scheme in this region serves the villages of Outwell and Upwell, 4 vacuum collection stations serve some 1500 homes in this agglomeration. The system was installed by Flowvac Environmental in partnership with Anglian Water and Barhale Construction. On initial costings for a conventional gravity sewer to serve the area, previously served by domestic septic settlement tanks the site would have required the installation of 32 pumping stations. Using vacuum this number was reduced to 4 collection stations. The system operates successfully today.

Ruling technical guidelines and norms

*EN 1091

*DWA-A 116-1 (also known as ATV-DVWK-A 116, Part 1)

*WSA 07 (Australian Code)
*AS 4310 - 2004 (Vacuum Interface Valve Standard)

* WEF (Water Environment Federation) Alternative Sewer Systems (Second Edition -2008)

External links

*vacuum sewage systems Europe/Middle East [http://vacuum-guide.com/process_vacuum/vacuum_sewage/vacuum_sewage_europe.htm Suppliers]
*vacuum sewage systems USA [http://vacuum-guide.com/process_vacuum/vacuum_sewage/vacuum_sewage_america.htm directory of suppliers]
*vacuum sewage systems UK [http://vacuum-guide.com/process_vacuum/vacuum_sewage/uk_vacuum_sewerage.htm UK Vacuum Sewerage Supplier]

References