Vacuum Sewerage Systems
The New Concept of Wastewater collection
The disposal of sewage to a central treatment plant in sparsely populated, and flat areas often fails. In this situation vacuum sewerage system is a sophisticated and reliable alternative. In regard to higher initial investment, vacuum sewerage system offers more feasible solutions in the long run. Planners and decision makers often hesitate to take such technology into consideration, due to lack of information and prejudices concerning operation of sewerage system. Objective of this paper is to introduce the new concept and provide basic information necessary to consider the system as an alternative to conventional system.
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Subhash DeshpandeExecutive DirectorM/s Deolalikar Consultants Pvt. Ltd, Pune |
Vacuum technology has grown enormously in the past few years. Vacuum Sewerage System is widely used in USA, UK, Germany and relatively small countries like Australia, Dubai, Poland and Malasia. The importance of the vacuum sewerage system is increasing and the next few years will witness wide applications in Asia.
This article is based on the training offered by Roediger Vakuum - und Haustechnik GmbH to the few selected candidates. The author was a part of the team who received this special training.
traditional way used even in Babylon. Today, the rural and satellite areas need to be connected to central sewage plants. In areas of low population, unfavourable subsoil, high groundwater table and flat terraces, gravity sewerage systems leads to high investment costs due to deep excavations and several lift stations.
A Dutch engineer Liernur first applied the negative pressure drainage (so called vacuum sewerage) in the second half of the 19th century. It was used only on ships, trains and airplanes for a long time.
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Technical implementations of vacuum sewerage systems were started after 1959 in Sweden by J. Lilijendahl and afterwards brought on the market by the Electrolux. Today in Germany, for example, more than 300 systems are operating since1970.
Vacuum Sewerage Systems
Vacuum sewer systems are wastewater collection systems. An air stream generated by differential air pressure drives the wastewater toward a vacuum station. In contrast to conventional gravity systems, the pressure within the system is maintained below atmospheric pressure (Negative pressure).
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Schematic Vaccum Sewerage Technology
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Vacuum sewerage systems have three elements:
- Collection chambers at each house
- Vacuum sewer lines
- Central vacuum station
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| Discharge of Effluent in a Vacuum System takes place via a Collection sump with an Interface Valve |
Vacuum technology is based on differential air pressure. Rotary vane vacuum pumps generate an operating pressure of -0.4 to -0.6 bars at the vacuum station. Vacuum pump is the only element of the vacuum sewerage system, which need electricity. Interface valves installed inside the collection chamber work pneumatically. Sewage flows by gravity into collection sump at each house. After a certain level is reached, the interface valve controlled by a controller opens. The resulting differential pressure (1.4 to -1.6 bar) between atmosphere (one bar) and vacuum (-0.4 to -0.6 bar) becomes the driving force, and transports the wastewater towards the vacuum station using air (instead of water) as carrier. Therefore, no inspection manholes are required in vacuum systems.
The whole vacuum sewers are filled with air at a pressure of -0.4 to -0.6 bar. Air-to-liquid ratio is the key for a reliable operation. When a system is well designed, the sewers contain only very small amounts of sewage. Sewers should never be totally filled with sewage since the hydraulic friction loss would be too high. This is ensured by using the interface valves. The proper selection of equipment ensures the automatic optimisation of the air-to-liquid ratio. It also offers highest operational security, optimisation of hydraulic transporting conditions, and thus reduction in energy consumption of a plant.
Considering that the vacuum is the means of transport, 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. Expensive trenching, as in case for gravity sewers having 1:100 slope, is avoided.
Once sewage arrives in the vacuum storage tank at the vacuum station, it is pumped to the discharge point, which could be a gravity sewer or the treatment station directly.
The Components
In the European norm (DIN EN 1091 - 4) for vacuum sewerage systems, general requirements about technical equipment, material, security volume of the collectors, alarm systems, pressure tests, etc., are stated. A German norm for vacuum systems (ATV A 116) was introduced in September 1992. It is more stringent than the European norm. Detailed regulations about the particular units and equipment of vacuum systems are well described in the norms.
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1. Collection Chambers / Interface Units
Wastewater flows by gravity from a house or commercial building into a collection chamber's sump. After a given quantity (batch) of wastewater accumulates in the sump, an interface valve automatically opens. The batch volume plus an additional volume of air flows through the open valve and is transported to a centralized vacuum station. The energy for transport is the differential pressure between the chamber sump and the vacuum sewer line, and from the vacuum sewer line to the vacuum station. The pipeline system is maintained under vacuum at all times by vacuum pumps located at the vacuum
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station. The interface valve uses differential air pressure (atmosphere & vacuum) as its energy source. No electrical power is required at the collection chambers.
Raw sewage flows by gravity from one or more lots into a sealed collection sump. Located in a separate and hygienic valve chamber, 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. The European norm specifies a minimum value of 0.15 bar for the low pressure in the adjacent vacuum line.
| When the valve opens, between 20 and 40 lit. (Depending on adjustment and valve), portions of effluent are sucked into the sewer line. Air entering via the covering lid is also sucked into the sewer line, due to the pressure difference, to push the sewage bulb. The interface valve will close again after a few seconds. The exact time can be adjusted and must be long enough to make sure that enough air enters in order to push the sewage bulb efficiently. Minimum of air-to-liquid ratio of about 4:1 to 15:1 should be ensured to have reliable transporting conditions. Vacuum technology is a very reliable and tested |
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technology when the right equipment is used. Therefore vacuum interface valves are recommended to be membrane-type to protect against sand clogging and have optimising controllers.
The restricting minimum diameter of the system should prevent the interface valves and the vacuum sewers from clogging. The connection from the sump to the interface valve should have a diameter of 50 - 60 mm. The free passage between the collection sump and suction pipe is smaller than the one of the interface valve, which prevents clogging inside the interface valve. Large particles can be easily removed by sump using a clean out.
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| The Starting Movement of a Sewage Bulb. When the subpressure created by the vacuum station is strong enough,with air arriving from above the line, sewage portions will be shifted over the next peak in the form of a bulb... until they will have to be reformed at the next saw-tooth. |
For reasons of security, the European norm dictates minimum volume (including the gravity drain line of the house) of the collection sumps. It should be large enough to keep back at least 25% of the daily sewage flow per house in order to have flexibility against power cuts at the vacuum station.The separation between valve and collection sump provides dry and hygienic conditions, ease in handling without smell and nuisance.
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| The Saw-tooth Profile of a Vacuum SewerLine. Sewage is shifted sequentially in theform of bulb. |
Most communities and operators all over the world appreciate the option to have one collection chamber per house (responsibility principle). The collection chamber should be made of plastic material. In this way infiltration is avoided and installation is eased.
2. Vacuum Lines / Hydro-pneumatic Transport
Vacuum sewer pipe is arranged with high and low points in specific elevation profile. Batches of wastewater come to rest at low points in the pipe. When air is introduced through an upstream interface valve, the wastewater is pushed in the form of a bulb to subsequent high points toward the vacuum station. The sewer height profile is essential for adequate momentum transfer from the air to the wastewater. Velocity in vacuum sewers prevents deposits. Properly designed and constructed vacuum lines are maintenance-free.
Flow situations in vacuum sewers cannot be simply described with hydraulic laws. Instead, a two-phase flow has to be considered (e.g. hydro-pneumatics). Conveyance takes place by means of a two-phase regime, air (compressible) and effluent.
The main characteristic of vacuum sewerage is the necessity to lay the sewers in the form of a distinct saw-tooth profile. An effective transport of sewage portions can only be guaranteed, if the low points and high points along the network are provided as per design experts.
Doses of sewage enter the vacuum line from the collection chambers. As sewage doses arrive at a low point of the sewer line, sewage is collected there, reducing/sometimes totally blocking the pipe diameter - until valves upstream open and the arriving air increases the pressure again. Air moving at high velocity towards the vacuum station will exert a strong impulse on the developing sewage bulbs. In a horizontally-laid pipe, air would stream over water without moving it further.
In this way sewage portions in the form of bulbs 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 large enough to move the bulb. Since more and more air permeates during movements, liquid bulbs are not permanent and will have to be re-formed in another low point. So, usually sewage is moved only gradually, "tooth by tooth", to the vacuum station.
There are always many sewage bulbs along a sewer line. The movement of each bulb changes the pressure situation for the next bulbs at upstream and downstream creating new pressure gradients. Due to the pneumatic effects, sequential movement of the bulbs takes place towards the vacuum station. High flow velocity (of up to 5 m/s) in the low points avoids any kind of sedimentation.
Prevailing diameters in vacuum sewers are in range of DN 80 and DN 250 (inner diameter). Usually HDPE or PVC pipes are used due to their low costs of installation and flexibility. Vacuum sewers have to be absolutely tight. Leakages along the pipes would cause a break down in the system. Therefore, minimum thickness of pipes should be as specified by the codes. Any construction company can easily install vacuum pipes.
As an optional choice, inspection pipes shall be installed at every 100 m along the vacuum line. In this way a total supervision of a sewerage system becomes possible, and leaks can be found much faster. With a system of 'test balls', the vacuum system gets flexible "division valves". The vacuum pressure situations can be analysed exactly, to check the installations before commissioning the plant or to locate leakages.
3. Vacuum Station
Wastewater and air is driven to a collection tank (vacuum tank) at the vacuum station. Vacuum generators (vacuum pumps) maintain vacuum in the tank and sewer pipe. Conventional sewerage pumps forward the collected wastewater from the collection tank to the treatment plant.
The vacuum station is the only place of the complete system, where energy is required. Vacuum station should be located at a low topographic altitude. The vacuum station consists of rotary vane vacuum pumps (to generate vacuum in the sewer lines), enclosed underground collection tanks and sewage pumps to discharge sewage from the collection tanks to a wastewater treatment facility.
The vacuum pumps maintain a (adjustable) negative pressure inside the collection tank in a range of -0.4 to -0.6 bar. When the negative pressure inside the system falls under a certain limit, the vacuum pumps will start working. Vacuum pumps run only for a few hours a day and do not need to run continuously since the vacuum interface valves at collection pits are normally closed.
Collection tanks are generally made of steel. Collection tanks must be watertight. 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 on-off of the vacuum pumps is limited to 10-15 starts per hour. A stand-by vacuum tank is desirable.
Design
Planning a vacuum sewerage system depends on the design of the system. Vacuum systems can be designed in many different ways (e.g. connected area, number of collection chambers, network of vacuum sewer pipes, location of the vacuum station, etc.). There is never only one solution.
The design guidelines control the following parameters.
- Air-liquid ratio, depending on distances and population density,
- Energetic loss 3ªHi derived from the maximum trunk length from the vacuum station, the
farthest interface valve and topography,
- Network length i.e. sum of all trunks,
- Flow rate of 0.008 l/s @ Inhab. is suggested accounting for security, in the case of several valves
opening at the same time,
- Vacuum reserve volume including sewer network, and
- 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 limitations such as maximum trunk length and tolerable elevations of the pipe-length-profile must be considered. As this kind of work requires interactions, design-diagrams have been developed. Table below shows maximum lengths of sewer sections, depending on chosen pipe-diameters. The maximum trunk length is restricted to 4,000 m in absolutely flat terrain.
Inhabitants per m Maximum length of
of Trunk length [I/m] each trunk part
DN 125
- 0.04 - 0.06800 m
- 0.06 - 0.12900 m
- 0.12 - 0.20800 m -300 m 800 m
Guidelines for dimensioning vacuum sewers depending on the density of inhabitants for wave-profiled systems / flat terrain / no ditches to surpass
Operation and Maintenance
Unjustified prejudices against "new" technologies prevail due to assumed high maintenance and operation costs. Nowadays, vacuum systems are reliable when the design is based on the expertise of professional manufacturing companies. Most parts of a system are nearly trouble-free. Service connections, vacuum lines and valve pits do not need any continuous maintenance.
Small problems at collection pits can sometimes occur, especially if the interface valves are not separated from the wastewater sump. Every 2 years, a short visual inspection of chamber and interface parts has to be undertaken. If necessary, sump should be washed. Wearing parts (rubber membranes) at interface valves should be replaced in periods of about 4-5 years. This can be done within only a few minutes, at low price, and does not disturb the operation of the system.
Vacuum stations should be visited at least once a week to carry out a visual inspection. Experience shows that a well-designed vacuum station does 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 (oil and filter change of the vacuum pumps), cleaning of the vacuum tank, etc., should be done at least once a year.
Advantages
In comparison to conventional gravity or pumping system, the vacuum system offers the following important advantages to residents and operators.
1 Low investment costs due to simple trenching at shallow depths and for smaller diameters.
2 Small diameter sewer pipes of HDPE, PVC materials 6 savings of material costs.
3 Flexibility of piping, obstacles (as for example, open channels, underground cables/pipes,
railway lines etc.) can be overpassed or underpassed.
4 No pumping stations required for lifts.
5 Aeration of sewage results in less development of H2S gas. No dangers to workers/inhabitants.
Corrosion of the pipes may be avoided. Sewage is kept fresh.
6 No odours along the closed vacuum sewers.
7 Manholes are not required.
8 No sedimentation due to self-cleansing velocity. Spooling, maintenance of the sewer lines is not
necessary.
9 No infiltration, thereby less hydraulic load at discharge sewers and treatment plant.
10 Absolutely no leakages (vacuum avoids exfiltration), thus ensuring no contamination of
groundwater. Sewers may be laid in the same trench with other mains, even potable water or storm water, as well as in water protection areas.
11 Reduced installation time.
12 The sewer lines form a truncated network, with a central vacuum station. This way, energy is
needed only at one point of the system.
13 No disturbance to inhabitants.
14 Leaks in the system would be shown immediately
Limitations
1 Vacuum systems are not capable of transporting sewage over long distances. The lines can reach
only up to maximum 3 - 4 km, if laid in flat area.
2 Restrictions towards maximum static lifts (3 - 4 m).
3 Restrictions towards maximum diameter of the pipes.
4 External energy is required for collecting sewage.
5 Odour problems close to the vacuum station can occur, because of exhaust air and may need a
bio-filter eventually.
6 Training of the technician, manpower necessary.
7 Vacuum sewerage systems are only reasonable for the collection of wastewater within a
separated system and not for the collection of stormwater.
8 System needs to be designed with help of an experienced manufacturer. (ROEVAC® offers this
service free of charge).
Fields of Application
Vacuum sewerage is most suitable in the case of following circumstances.
1 Especially difficult situations such as ribbon, peripheral settlements on flat terrain, with high pipe lengths of longer than 4 metres per inhabitant. In the case of sparse population density, the influence of the costs for the collection chambers and vacuum stations is less in comparison to the costs of long and deep gravity sewers.
2 Missing incline of the ground, adverse gradient, unfavourable soil (rocky or swampy grounds) and high groundwater table (with the necessity of dewatering trenches) lead to enormous investment costs in gravity sewerage systems. Vacuum sewers offer feasible solution.
3 Vacuum sewers can pass through water protection areas and areas with sensitive high groundwater tables, because there is no danger of spoiling groundwater resources (vacuum sewers are leakproof due to their material; moreover it does not allow exfiltration).
4 Wastewater flows are highly variable in seasonal settlements such as recreation areas, camping sites, farmhouses or holiday resorts, etc. With conventional gravity sewerage, sedimentation problems can easily occur in certain periods of the year or certain lengths. High flow velocity within vacuum sewers prevents clogging.
5 Rural area where houses and buildings are not close to each other or even in old and historical villages, the use of vacuum systems becomes more and more important due to a fast-growing traffic/tourism.
6 Vacuum sewerage system is the only solution for refurbishments of sewer systems without causing any disturbance to existing system and users.
7 For the sewerage system at a larger settlement, it can become necessary to divide the area with several different vacuum stations. Sometimes this also becomes necessary due to topography. Very often it seems to be reasonable and most feasible to combine both vacuum and pressure sewerage systems together.
Conclusion
This article briefly introduces the principles and applications of vacuum sewerage systems. This new methodology has high potential in future. In fact, vacuum sewerage system is spreading all over the world, as vacuum technology becomes the clean solution for the future.
High installation and operation costs and fear of malfunction have been the main prejudices and obstacles in the past. Of course, vacuum sewerage systems will never replace gravity systems in general. However, vacuum sewerage seems to become more and more adaptable as good references from various communities show satisfaction. In Poland, the largest vacuum system has now been installed for more than 30,000 inhabitants with 4 pre-mounted vacuum stations. A vacuum sewerage system is being installed on the man-made island PALM JUMEIRAH in Dubai. At this project more than 2,000 villas will be connected by approximate 1,000 vacuum km of vacuum sewer lines to 1 central vacuum station. The vacuum station is located inside the STP, which is installed underground.
As engineers and municipal officials become acquainted with the advantages (as well as limitations) of vacuum sewers, the use of this technology will probably expand. It is hoped that the use of alternative sewerage concepts will allow designers and regulators to find ways of providing environmental-friendly sewer system, keeping project costs at a minimum. Frequently, a combination of different alternative systems together with conventional sections will become the most feasible and reliable solution for the collection of wastewater.
References
- Project work (thesis), by Dipl..-Ing. Lars Späth, ROEDIGER Vakuum- und Haustechnik GmbH, Hanau (Germany) which has been accomplished at the Institute of Hydraulic Construction at the University "La Sapienza", Roma as well as at the "Institut für Siedlungswasserwirtschaft", Universität Karlsruhe in 1998.
- Vacuum Sewerage-A modern system aiding to reduce the costs of Communal Wastewater Collection - Paper by Dipl..-Ing. Lars Späth, ROEDIGER Vakuum- und Haustechnik GmbH, Hanau (Germany).
- All figures and photographs courtesy Roediger Vakuum - und Haustechnik GmbH.