Vacuum sewerage

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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.

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 onto the market by Electrolux. Nowadays several system suppliers offer a wide range of products for many applications.

Introduction[edit]

  1. Collection chambers and vacuum valve units
  2. Monitoring system for collection chambers and vacuum valve units
  3. Vacuum sewer lines
  4. 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. Besides 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 systems 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. Valve and collection sump (waste water) preferably 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. 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 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[edit]

  • 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.
  • Capital costs can be reduced by 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;
  • 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
  • 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[edit]

  • 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, a biofilter may be necessary
  • Integrity of the pipe joints is paramount
  • Mechanical controller requires preventative maintenance for worn parts and seals
  • Vacuum valve can get stuck open and requires a procedure to locate the stuck open valve

Application Fields[edit]

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 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, 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's why this technology becomes interesting for such kind of applications. As PE or PVC pipes are used, no solids from ageing 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.

Collection chambers / vacuum valves[edit]

Raw sewage flows by gravity from one or more lots into a sealed collection sump. A vacuum interface valve is installed, which is controlled and operated pneumatically without electricity. When a certain amount of sewage has accumulated the controller opens the valve. It is important to understand that the valve shall open only, if the low pressure inside the vacuum sewer line is strong enough to ensure reliable transport; otherwise, an alarm is sent to the control center indicating low vacuum. A minimum value of 0.15 bar for the existing low pressure in the adjacent vacuum line.

When the valve opens, between 20 and 40 l (depending on adjustment and valve) portions of effluent are sucked into the sewer line. Air entering via the incoming gravity line or air vent will be sucked into the sewer line due to the pressure difference to push the sewage. The interface valve will close again after a few seconds. The exact time should have an option to be adjusted and must be long enough to make sure that enough air can enter in order to push the sewage efficiently. This depends on the negative pressure conditions: Generally, the volume of air-stream should be lessened as far as possible, so that the pumps do not have to work unnecessarily. On the other hand minimum ratios of air-to-liquid should be guaranteed to have reliable transporting conditions. Usually the systems work with air-liquid ratios of about 4:1 to 15:1.The restricting minimum diameter of the system should prevent the interface valves and the vacuum sewers from clogging. So, the connection from the sump to the interface valve should have a diameter of 75mm so that no blockage point is created. Usually, larger particles do not arrive in the sump, even though it still can occur. Large particles can be easily removed from the sump by an operator if required. But generally anything that can fit down a house service line should be able to enter the vacuum system and then the vacuum pump station without blockage.

Vacuum Lines / Description of Hydropneumatic Transport[edit]

Flow situations in vacuum sewers cannot be simply described with hydraulic laws. Instead of it a two-phases-flow transport has to be considered (e.g. hydropneumatical). Conveyance takes place by means of a two-phase regime, air (compressible) and effluent. Because of this the continuity equation becomes very complicated.

As it was mentioned before, the main characteristic of vacuum sewerage is the necessity to lay the sewers in the form of a distinct saw-tooth or stepped profile. An effective transport of sewage can only be guaranteed, if the hydraulic losses are agreed to by an approved supplier.

Doses of sewage enter the vacuum line from the collection chambers. As sewage arrive at a low point of the sewer line, sewage is collected there, - until valves upstream open and arriving air will increase the pressure gradient again. Air moving at high velocity into the direction of the vacuum station will exert a strong impulse on the developing sewage.

In this way sewage will be shifted with almost the same velocity over the next peak down the line. The transport of sewage will continue along the sewer line as far as the pressure gradient remains. In a horizontally laid pipe air would stream over water without moving it further.

High flow velocities in the low points of up to 5 m/s avoid any kind of sedimentation, since during the starting movement this kind of flushing effect would take away all hypothetical deposits. Sedimentation problems have never been reported for vacuum sewerage systems.

Prevailing diameters in vacuum sewers are in range of DN 80 and DN 315 (inner diameter). Usually HDPE or PVC pipes are applied in vacuum systems due to their low costs of installation and flexibility. Vacuum sewers have to be absolutely tight. Therefore, DIN EN 1091 requires a thickness of at least PN 10. Leakages do not appear in vacuum systems due to an absolute tightness of installations (each construction company is easily able to install vacuum pipes).

Vacuum Station[edit]

The vacuum station consists of rotary vane vacuum pumps (generate vacuum in the sewer lines), a collection tank, and duplicated sewage pumps (duty/standby) that discharge sewage away from the collection tanks to a wastewater treatment facility. The vacuum pumps maintain a negative pressure of between -0.4 and -0.6 bar in the collection tank. When the tank pressure falls under a preset limit, the vacuum pumps will start working to restore the pressure. As such, vacuum pumps run only for a few hours a day and do not need to run continuously.

Collection tanks are mostly made of steel and not of stainless steel due to the risk of local element chemical corrosion. Vacuum tanks are sized according to flow rates and vacuum suction capacity, with typical volumes ranging from 5 to 12 m3. About 75% of the tank’s volume will be required as a vacuum reservoir. With this vacuum reserve, the vacuum pumps are prevented from too high a starting rate, which is normally limited to 10-15 starts per hour (worst case).

Design[edit]

Planning a vacuum sewerage system seems to be initially a question of design. There is never only one solution at sewer networks in general, but vacuum systems can be designed in many different ways (e.g. connected area, location of the vacuum station, choice of the length profile etc.). A good design needs a perfect overall picture on all the system’s parameters! Some suppliers of vacuum system components help seriously during the design with their assistance. The use of such experienced help is recommendable and preferable.

In the guidelines mentioned above the control of the following parameters is demanded:

  • air-liquid ratio (depending from distances and population density
  • energetic loss (derived from the maximum trunk length in between the vacuum station and the furthest interface valve as well as from geodetic steps due to topography)
  • network-length (sum of all trunks leading together)
  • flow-rate
  • vacuum reserve volume (considering also the sewer network)
  • maximum tolerable distances in between air inlets (interface valves).

The most significant step in designing a vacuum sewerage system is the choice of a good pipe-routing. System boundaries such as maximum trunk length and additional elevations of the pipe length-profile do not have to be surpassed. As this kind of work requires iterations, design-diagrams have been developed. The maximum trunk length is restricted to 4000 m in absolutely flat terrain. A longer distance can be handled must be done in consultation with a system supplier.

While the norms do not give sufficient information about checking and dimensioning of design parameters, it shall be emphasised, that vacuum sewerage systems could become remarkably larger than the norms show it!

Hints about Operation[edit]

Unjustified prejudices against “new” technologies still prevail. Highly assumed maintenance/operational costs are the main obstacle against further expansion of vacuum sewerage systems on the market. Problems, especially at collection chambers, and frequent system break-downs (drowning) were the birth labour of first vacuum sewerage systems. Nowadays, vacuum systems are reliable when their design is based on special knowledge of professional companies.

A monitoring system is an option to indicate the status of the vacuum valves and collection chambers.

Vacuum stations should be visited at least once a week to carry out a visual inspection. Experiences have shown that a well-designed vacuum station will not need more than one visible control and short check once a week (similar to a pumping system). Operating hours and power consumption of the pumps should be checked regularly. Mechanical and electrical maintenance, cleaning of the vacuum tank, briefly a total check of the vacuum station, should at least be done once a year (oil-change and filter change of the vacuum pumps).

Conclusion[edit]

Highly estimated operation costs and fear of malfunction have been the main prejudices and obstacles in the past against an expanded use of vacuum sewers. For an unprejudiced choice of a sewerage concept, it is necessary not to overestimate operational costs of alternative wastewater collection systems. Further, more difficult conditions during construction have to be considered for conventional gravity sewerage! When a vacuum sewerage system is well designed, operational reliability will be guaranteed.

Vacuum sewerage seems to become more and more important as capital costs could be reduced remarkably. Good references from communities seem to show satisfaction. Especially in cases of sparse population density, flat terrain, and high specific costs of pipe-laying, alternative sewerage systems could become much more economic, also in the long run.

It is significant not to overestimate the operation costs of alternative wastewater collection systems, in comparison with the costs of a conventional gravity system (which constitutes work under more difficult conditions). When a vacuum sewerage system is duly designed and built, its operational reliability is guaranteed.

As engineers and municipal officials become acquainted with the advantages of vacuum sewers, the use of this technology will probably expand more and more worldwide.

It is hoped that the use of alternative sewerage concepts will allow designers and regulators to find ways of keeping project costs at a minimum.

Frequently, a combination of different alternative systems together as well as conventional sections will become the most feasible and the most reliable solution for the collection of wastewater.

External links[edit]

References[edit]

[1] CEN : European Standard DIN EN 1091 “Vacuum Sewerage outside buildings”, (1992)

[2] ATV Arbeitsblatt A 116 : “Besondere Entwässerungsverfahren, Unterdruckentwässerung – Druckentwässerung”, Hennef (1992)

[3] ATV Arbeitsgruppe 1.1.2 : “Fragen des Betriebs und der Nutzungsdauer von Druck- und Unterdrucksystemen”, Korrespondenz Abwasser (1997), P. 921-922

[4] ATV-Handbuch : “Bau und Betrieb der Kanalisation”, (1995)

[5] Ciaponi, C.: Fognature Nere in depressione”, Sistemi di Fognatura, (Centro Studi Deflussi Urbani), Milano (1997)

[6] Ciaponi, C.: Un’Esperienza di applicazione del sistema di Fognatura Nera con funzionamento in depressione, Università di Pavia (1986)

[7] Garnier, C., Brémond, B. : “Assainissement sous-vide, étude technique-économique”, CEMAGREF, Groupement de Bordeaux, Division Hydraulique Agricole (1986)

[8] Ghetti, A.: “Prove Idrauliche e technologiche relative alla fognatura di Venezia”, Padova (1970)