A new BRE guide has been published for designers and installers who may be considering their use.
The building sector has been slow to adopt this new technology, but with the restrictions placed on new projects such as small conduit, ceiling voids and service ducts, and the growing awareness of the need to limit water consumption, the system is finding a place in the building sector.
A DETR funded project led by the BRE has resulted in the publication of Vacuum drainage systems guidance. The guide, to be used in conjunction with EN12109: Vacuum drainage systems inside buildings, is aimed at designers and installers who may be considering using a vacuum drainage system.
How it works
The differences between conventional drainage systems and a vacuum drainage system are summarised in table 1, while table 2 contrasts the advantages and disadvantages of vacuum drainage systems.
When conventional gravity drainage systems are extended as in refurbishment work, the existing gravity drainage system can be fed into the vacuum drainage system. This may be achieved by the use of a sump into which the waste water from the gravity system drains. When sufficient water has accumulated in the sump, an interface valve will open allowing the wastewater to enter the vacuum drainage system. This arrangement, as shown in figure 1, can also be used to collect rainwater or as an interface between a building with conventional drainage and a vacuum sewer.
Vacuum systems are designed to operate on two-phase air to liquid flows. The air in the pipework is not, as in a conventional horizontal gravity system, flowing above the wastewater but is entrained into the wastewater where its expansion propels the wastewater and lowers its bulk density. These factors enable the wastewater to behave more like a gas than a liquid and in particular flow uphill.
An analogy of a vacuum drainage, or sewerage, system is shown in figure 2 as an inverted water supply system in which the water flows backwards.
Installation of vacuum pipes and fittings follow current water system practices. Isolation valves are installed in branches and mains to allow portions of the main to be isolated for repairs or troubleshooting. The usual operating vacuum range of a vacuum drainage system is -0.5 to -0.7 bar gauge.
The vacuum transport process
When an interface valve opens, the differential pressure between the vacuum in the system and atmosphere, forces the wastewater into the vacuum pipework. The magnitude of the propulsive forces starts to decline noticeably when the interface valve closes but remain important as the admitted air continues to expand. The vacuum drainage system transports wastewater by means of atmospheric pressure acting against vacuum.
Once the interface valves have operated, the discharge travels to the vacuum station, normally located at ground or basement level. Air is discharged to atmosphere only from the vacuum station. From the vacuum station, the wastewater is pumped automatically to the building outfall connection, to discharge into the external drainage system by gravity.
Vacuum toilets
A vacuum toilet uses air instead of water to remove the contents of the bowl, and is a form of interface valve. Usually, it includes a flushing rim and the toilet's controller may have a memory function so that it will operate as soon as there is sufficient vacuum available. A typical vacuum toilet is shown diagrammatically in Figure 3.
Unlike most gravity drainage systems, vacuum drainage systems require electrical power for operation. Hence, there may be a requirement to provide an alternative power supply to operate the system in case of a primary power supply failure.
Basic design criteria
In order to design a vacuum drainage system the following basic parameters should be determined and obtained:
- service life expectancy
- type of building
- number of people the system is to serve
- types, number and location of appliances to be connected
- wastewater temperature range (high temperature greywater discharges shall be specified concerning temperature, flow, batch volume and frequency)
- ambient temperature range which the system will operate
- minimum vacuum level required to operate the interface units and vacuum toilets
- air to water ratios required for the interface units
- air consumption of vacuum toilets
- permissible leakage factors
Although most drainage systems only have relatively short distances between the appliances and vacuum station, long vacuum pipelines are laid with a series of reforming pockets. When the propulsion effect of the air has diminished, the wastewater remaining within the pipework will drain under gravity into the reforming pockets. When the next interface valve opens the air movement will remove the water from the pocket and transport it as a slug further towards the vacuum station. This enables the wastewater to be transported to the vacuum station in a series of interface valve operations.
For a larger building it is customary to divide the system into smaller sub-systems, possibly with a crossover, so that in the event of failure of part of the system, each sub-system could operate as a standby for the other.
Downloads
Figure 1: Sump receiving waste eater from gravity systems
Other, Size 0 kbFigure 2: Analogy of a vacuum drainage system
Other, Size 0 kbFigure 3: Vacuum toilet controls and operation
Other, Size 0 kb
Source
Building Sustainable Design
Postscript
Vacuum drainage systems guidance costs £20 while the video (PAL version) is £35. Both are available from BRE on: 01923 664514. Other project partners included CIBSE, the Institute of Plumbing, EBARA (UK), EVAC (UK) and Jets Vacuum AS. John Griggs is senior consultant of the BRE Water Centre.