Vacuum sewage system: Indoor - Outdoor
Vacuum system can be differentiated between "Indoor" and "Outdoor".
Indoor vacuum system is also known as Vacuum Sanitation System and used within buildings. Vacuum toilets require only one litre of water per flush saving significant quantities of water. The volume of water required for urinals can also be reduced considerably.
More infos on indoor vacuum system can be found here
Present situation
A most important aspect of appropriate Sewer Systems, is to collect different waste streams in a concentrated and not diluted state. Years ago several countries implemented separate collection systems for solid (household) waste. Different fractions like paper, tins, (coloured) glass, plastics and others are collected separately in different bins, bags, containers and other collectors. The aim is not the collection itself but the subsequent separate treatment of the varied items. The same is possible, and feasible, with different waste water fractions.
The first and most important step is to separate waste water from households and rain (storm) water. To this day, in many countries, sewerage systems are installed which do not separate between surface runoff and household waste water. The consequence is a significant increase of the waste water quantity and a significant dilution of the pollutants. The STP has therefore to be designed accordingly, for the high volume of liquid and operates at low efficiency. Conversely, the efficiency of the STP increases as waste water becomes more concentrated.
Vacuum Sewer Systems have many decisive advantages as follows:
Economical advantages:
- Only one central Vacuum Station
- no further Pumping Stations required
- Flexible System lends itself to modular design.
- No electrical connection except at the Vacuum Station
- Small Diameter Plastic Pipes (DN 75 to DN 200); HDPE or U-PVC.
- No Storm Water Infiltration
- Smaller Sewage Treatment Plant required
- Higher Efficiency of the STP
Ecological advantages:
- Closed System ? no Leakage, no odour
- No Ground Water Pollution ? No Exfiltration
- No Flushing Tanks required ? Significant Water Saving. Air does the flushing.
Other (technical) advantages:
- No Blockage ? therefore no rodding required
- No Clogging / sedimentation due to high velocity in the sewer pipes
- No manholes required ? no possibility to throw rubbish into the sewer
Applications for Vacuum Sewage Systems (VSS)
- Flat Terrain ? no natural slope
- Municipalities, Residential and Commercial Areas.
- Where water is scarce or costly to the community ? VSS requires a lot less to function
- effectively. It will function well with low volume flush toilets providing the liquid.
- High Ground Water Tables ? lagoons, estuaries, river frontage, wetland margins
- Ecologically Sensitive Areas ? Ground Water pollution is avoided
- Trenching in Rocky or Sandy Environment
- Rural communities, peripheral settlements, isolated suburbs, and town outer limits.
- Where conventional sewage systems become very expensive i.e. areas of low population
- density and difficult terrain.
Vacuum Sewage Systems are
- An alternative to Gravity Sewer
- Ideal for Municipal and Industrial Waste Waters
- Less costly than Conventional Systems (wherever they have been installed)
- Standardized Technology, as per Norm European Standard EN 1091
- Recommended by many Authorities and Institutions that have them in use.
The vacuum system
?Vacuum? is not the correct word, it should be called a ?Negative Pressure? Sewerage System as the operating pressure in the system is 60 KPA below atmospheric pressure.
The heart of the System is the central Vacuum Station. Two or more Vacuum Pumps will generate the necessary negative pressure in the Vacuum Tank, as well as in the complete Reticulation Network. Air is evacuated from the system by these pumps, which enters the system via the Collection Chambers located underground, in convenient locations, nearby the houses.
Waste water flows via gravity pipe from the houses to the Collection Chamber/ Sump. When a predetermined volume is collected (30 litres), the Interface Valve opens, and the WW will be evacuated into the Vacuum Pipe by atmospheric pressure. Together with the quantity of water, about 6 times more air will be sucked into the system. This air is needed as the transport medium for the water. Thus the transport velocity in the system is about 4 - 6 m/s.
Air and waste water are ultimately ?sucked? into the Vacuum Tank. The Vacuum Pumps evacuate the air from the Tank into the atmosphere, through a Biofilter. Discharge Pumps transport the waste water from the tank, via pressure line to the STP or a sewer main.
The pipeline network consists for flexible small diameter PVC or HDPE pipes. They should be located in small trenches into the frost free zone (about 1,2 m deep), and are laid in a Saw-Tooth-Profile. Limited lift is therefore possible.
So, the contour of the piping is more or less horizontal and is according to the topography. The pipe follows the surface in a depth of about 1,2 m. No more deep and dangerous trenching is necessary.
Comparison: Gravity system vs. vacuum system
| Gravity System | Vacuum System | |
| Pipe Diameters | Large (>160) | Small (80-250) |
| Pipe Material | PE or stoneware or concrete | HDPE or PVC |
| Trenches | Deep (up to 8m and more) Wide | Shallow (0,6 - 1.2m) Narrow |
| Excavation | Complicated and long term | Simple and fast |
| Machinery required | Heavy machinery | Simple or even no machinery |
| Lifting stations | Required to avoid deep trenches | Not required |
| Leakage/Exfiltration | Possible | Impossible |
| Infiltration | Possible: Storm water, ground water | Not possible, closed system |
| Traffic (construction) | High impact on local traffic | Low impact on local traffic |
| Pipelines for fresh water and waste water in the same trench | Not allowed | Possible and allowed |
| System | Open | Closed |
| Manholes | Required | No manholes, inspection pipes only |
| Dry sewers | Possible | Not possible |
| Transport medium in pipe | Waste water | Air/Atmosphere |
| Fouling of waste water | Possible | Not possible |
| Flushing of pipelines | Sometimes required | Not required |
Influence on Sewage treatment plant (STP)
Quantity
The Vacuum Sewerage System as a collection system of (mostly) community waste water (from households, residential areas, restaurants, commercial areas etc.). The waste water will be transported, ideally, direct to the next Sewage Treatment Plant or via local gravity sewer.
Because the Vacuum Sewerage System is a closed system, no storm or surface water will infiltrate. Conditions for the effective operation of the STP are, therefore ideal: The quantities are relatively easy to calculate and the layout of the STP will be determined as with reduced average as well as peak capacities.
No storm water will dilute the sewage. Storm water does not have to be treated in an STP so there is no need for over design, efficiency of the STP is higher, and more pollutants will be extracted.
Quality
In addition the ?quality? of the waste water arriving at the STP is much better than with gravity systems. It is obvious that movement of sewage through the vacuum system is faster as the negative pressure is strong (about ? 60 KPA) and the velocity in the pipe higher (about 4 m/s).
Furthermore the air in the system (about 5 ? 6 times more than water) which transports the waste water to the vacuum station, keeps the wastewater ?fresh?. Oxygen in the air prevents the water from fouling.
Cost reduction
A significant cost reduction is possible if the VSS will is installed in areas containing the a.m. criteria. An example shows the relevant cost savings. A village in Botswana has been equipped with our Vacuum System. The cost reduction was as a result of several factors:
- Two central vacuum stations have replaced 10 lifting stations in the area.
- The small diameter PVC pipes (dia 75 to 200) with a total length of 55 km are much cheaper as bigger diameter gravity pipes.
- The trenches for the vacuum pipes are only about 1.2 m deep and very narrow. Only a small quantity of soil has to be moved. Small TLBs and manpower substitute larger machinery. This is even more important in difficult ground conditions (rock). 2 meters of trenches for gravity pipes were excavated in the same time as 30 meters for vacuum sewer pipes.
- More local employment has been created.
- Maintenance for two VS is less than for 10 pumping stations.
- No additional flush tanks were needed
- 10 tanks have been cancelled. Thus no precious fresh water will be spoiled just for flushing gravity pipelines frequently.
- No ?dry sewers? will appear. Thus there is no wear in the pipes.
- In the case of installing a new reticulation network for fresh water as well as for sewerage the pipes may be installed into the same trench. This reduces the cost of a separate trench!
Conclusion
For the planning and layout of a sewerage system both systems should be considered. There is the use for conventional gravity systems in the presence of sufficient slope and in densely populated areas. But in rural communities and town extremities, the VSS should be regarded as an appropriate solution for the installation of a sewer network.
Vacuum Sewage System have been sucessfully running for 25 years. More than 300 installations in Europe, America and Asia are in operation. The first in Africa is now completed. They demonstrate their potential and their reliability, each passing day.
The VSS helps not only to protect the environment, but at the same time to reduce the costs for the installation. It improves the hygienic conditions and will raise the living standard of the population.
