Vacuum sewer

Illustration from Liernur's 1887 patent application. Fig. 5 is an end view of the main receiving apparatus at the pumping station.

A vacuum sewer system is a method of transporting sewage from its source to a sewage treatment plant. It 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.

Vacuum sewers were first installed in Europe in 1882 but until the last 30 years it had been relegated to a niche market although it has reamined in use on trains and airplanes. The first applied use of negative pressure drainage (so called vacuum sewerage) was the Dutch engineer Charles Liernur in the second half of the 19th century. Technical implementations of vacuum sewerage systems were started after 1959 in Sweden. Nowadays several system suppliers offer a wide range of products for many applications.

Basic elements

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.

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. 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 opens. 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 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. 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 trouble shooting 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 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, 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. 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 collection 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 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

• 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 stormwater infiltration, therefore less hydraulic load at the wastewater treatment plant
• 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
• In combination of vacuum toilets it creates concentrated waste streams, which makes it feasible to use different waste water treatment techniques, like anaerobic treatment

Limitations

• vacuum systems are not capable of transporting sewage over very long distances, but can pump long distances from the vacuum station to the next sewage treatment plant or main gravity sewer.
• 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 the central vacuum station.
• odours close to the vacuum station can occur, a biofilter may be necessary
• integrity of the pipe joints is paramount
• grease can clog the sensor tube (if set up incorrectly) thus requiring preventative maintenance cleaning
• mechanical controllers (float switches) require preventative maintenance for worn parts and seals
• vacuum valves can get stuck open leading to pressure drops in the entire system. However, a monitoring system can help identifying stuck open valves immediately

Application fields

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

• Especially difficult situations as ribbon, peripheral settlements on flat terrain with high specific conduit 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 priority 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 tracks in Shanghai and Abu Dhabi are using 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-sur-Ozerain, in Oman at the township of Khasab and Al Seeb.
• 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 is why this technology becomes interesting for such kind of applications.

Vacuum sewers connected to biogas sanitation

Vacuum sewer systems can be set up so that they collect concentrated blackwater (toilet wastewater) only and treat this wastewater in an anaerobic wastewater treatment process with the production of biogas. This design has the potential to increase sustainability of water infrastructures.

Examples

• The county of Sarasota, Florida and the city of Carnation, Washington are developing a county wide collection system and is incorporating vacuum sewers.
• In Germany, several hundred systems are operating since the 1970s.
• 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's most famous vacuum sewer project is currently the Palm Island Jumeirah, located at 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. The vacuum station is considered to be the biggest vacuum station in the world.
• The biggest installation in Europe (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 Europe or 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. 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. 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 a vacuum sewer system, this number of pumping stations was reduced to 4 vacuum stations. Other companies in the UK such as Southern water operate vacuum sewer systems, too.
• Lately, vacuum sewer systems become popular for industrial and commercial projects as well, where only little domestic waste water occurs and where the flexibility of a vacuum sewer system allows easy coordination with usually plenty of other utilities in the ground. Good examples can be found again in the Middle East, such as some small industrial areas in the Emirate of Ras al Khaimah or the newly built Qatalum Aluminium Plant in Qatar, the world's largest primary aluminium plant.
• The eco-city of Masdar, U.A.E., uses a vacuum sewer system as well to separate grey from black water.

Ruling technical guidelines and norms

• EN 1091
• DWA-A 116-1 (also known as ATV-DVWK-A 116, Part 1)
• WEF (Water Environment Federation) Alternative Sewer Systems (Second Edition -2008)
• WSA 07 (Australian Code)
• AS 4310 - 2004 (Australian Vacuum Interface Valve Standard)