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Last edited 29 Mar 2019
Passive water efficiency measures
Passive water efficiency measures are those which require no behaviour change by the user.
- Water Saving Taps (efficient versions are as low as 1.7 L/min) are often the same price as less efficient models.
- Appropriate sink sizing. If a sink does not need to be completely filled to use it, then a smaller sink could be specified.
- Foam soap requires little water to remove, and generally means less soap and water are used providing further cost saving.
- Toilets are typically the largest water user in non-residential developments, allowing significant savings to be made.
- Modern standard cisterns have a volume of 6L. Efficient designs can bring the average flush volume down to 3L.
- Siphon mechanism. Low flush (4.5L) versions are available, but they are slow to refill the cistern and are only available in single flush.
- Drop valve mechanism. Allows quick cistern refill (good for high frequency use) but the valve will eventually leak, and it is not as robust as the siphon. It may also require maintenance to remove scale deposits.
- Delayed action inlet valve. This prevents water flowing into the cistern before the flush is complete.
- Interruptible flush. The user stops the flush by releasing the lever when the pan is clear.
- Composting. A tank or chamber is installed below the toilet bowl to collect waste. Liquid waste can be collected separately, diluted and used as fertiliser for trees and flowers. The solid waste can be collected after 6months to a year and used as compost.
- Vacuum. These use air rather than water to flush, and can be used where gravity drainage is problematic. Some designs combine an air and water flush.
Urinals are often set to flush regardless of use, typically 3 times an hour. A number of devices can be used to control flush frequency:
- Hydraulic valve. This can be fitted to the inlet pipework of the urinal system. When water is used elsewhere in the washroom, the pressure decreases temporarily, the valve opens, and allows a pre-set volume of water to pass into the cistern. When the cistern is full, an auto-siphon will discharge and flush the urinal.
- Passive infrared (PIR) sensor. This can be installed in the washroom to detect use of the urinal system. The sensor controls a solenoid valve which allows a pre-set volume of water to pass into the cistern. When the cistern is full, an auto-siphon will discharge and flush the urinal. A PIR sensor can be battery operated. Out-of-hours it can be set to deliver a 'hygiene flush'.
- Manual shut-off. If usage hours are predictable, a single valve can be installed in the pipework supplying the washroom. This can be closed manually each day.
- A timer can be installed so that the water supply is shut off during periods of non-use. This is a more flexible and reliable alternative to manual shut-off.
- Waterless urinals. Can work effectively with the correct maintenance regime. There are three main types:
- Siphonic Trap. These contain a barrier fluid which the urine passes through.
- Deodorising pad / biological cartridge are fitted to break down bio-film.
- Air-flush. Individual traps are replaced by a single trap at the drain end. A low wattage DC fan provides airflow down the bowl to prevent odour. There are no chemicals and they are easy to clean.
 Water Efficient Appliances
Typical washing machines use >50 L/cycle, and dishwashers >15L/cycle. Efficient versions use much less (40L/cycle and 10L/cycle), and modern washing machines often have a 'half load' cycle or intelligent monitoring to only use as much water as needed.
NB Passive water efficiency measures may qualify for tax allowances under the ECA Water Scheme.
 Find out more
 Related articles on Designing Buildings Wiki
- Approved document G.
- ECA Water Scheme.
- Flood and water management act.
- Marine energy and hydropower.
- Passive building design.
- Rain garden.
- Rainwater harvesting.
- Sanitary accommodation.
- Sustainable urban drainage systems.
- Sustainable water.
- The State of the Environment: Water Resources.
- Types of water.
- Water consumption.
- Water engineering.
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