Grey water - Cambridge Water

Grey water

The average person currently uses 150 litres of drinking water per day. The government would like to see this reduced to 130 litres per person per day by 2030.

One way of cutting down on the amount of water we use is by using grey water. 

Grey water is wastewater from showers, baths, and wash basins. If treated correctly it can be used for flushing the toilet, watering gardens and washing cars, helping to cut down water consumption by more than a third.

The use of grey water technologies in our homes is still in its infancy. This is because the cost of fitting such systems is high. It has been estimated that a grey water recycling system would add £5,000 to £6,000 onto the cost of a new property.  This would add 2.5% to 4% to the cost of new properties in this region (for flats the cost is significantly lower at 1%).

There are also concerns that the quality of the water may pose a health risk.  While grey water from baths, showers and basins is usually clean enough for flushing toilets, there are concerns about what happens when nutrient rich waste water is left untreated for a period of time. To prevent bacteria levels from building up it is therefore advisable to only have a small storage tank, which in turn can cause problems for providing a reliable supply during periods of low rainfall.

Generally it is more efficient to reduce water consumption by fitting water efficient appliances and changing your water-use behaviour before fitting a grey water system.

Having said that Cambridge Water is keen to keep abreast of issues relating to grey water and is encouraging developers to incorporate grey water systems in new builds where appropriate.

Factors to consider when installing a grey water system

How does it work

A typical grey water system collects waste water from baths, showers and washbasins. The water is then treated and pumped to a tank for storage until it is required for flushing the toilet or watering the garden.

Larger grey water systems that supply more than one property tend to use more sophisticated "biological" treatment methods. This works in a similar way to the processes used at sewerage treatment works. Small-scale systems tend to use "physical" treatment methods which rely on a basic filtration process. They tend to require the use of chemicals similar to bleach to stop the growth of bacteria while the water is in storage. Some systems use a combination of the two giving a "bio-mechanical" process.

Is it right for you?

Before you install a grey water system you should consider whether or not you will benefit from one. To do this compare how much grey water you are likely to generate (which will depend on the number of baths and showers taken) with your demand for reclaimed water. This will depend on the number of toilet flushes and the volume required for your garden.

Systems installed in larger buildings tend to be more efficient than those installed in individual properties.


All installations must comply with the Water Supply (Water Fittings) Regulations 1999 and the Building Regulations (parts G & H).

Installing the system

Your will need to set aside space to include collection, treatment and holding tanks. Each tank needs to be sized to produce a balance between supply and demand.

The grey water will need to be collected at a low level before being pumped to a height where it can refill toilet cisterns.

All pipes containing grey water need to be clearly marked.


All grey water which is being reused inside the property needs to be biologically treated and disinfected to control bacterial growth


Regular cleaning and maintenance will be required. This will involve visual inspections, regular checks on disinfection levels and cleaning of filters

Costs and benefits to you

Installing a grey water system typically costs in excess of £3000, in addition to ongoing running and maintenance costs.

If your property is metered using a grey water system will reduce your water bill.

Systems that are installed in new buildings are typically more cost-effective than those retrofitted into existing buildings. Likewise, commercial systems are more cost-effective than those installed in individual properties.

Some studies have shown the payback period can be as long as 10 years - this can be longer than the life expectancy of the system.

Environmental impact

A correctly installed, well-maintained grey water system could reduce water taken from a domestic supply system by up to a third. Widespread adoption of such schemes could have a significant impact on the environment, reducing pressure on water resources and the quantity of sewerage requiring treatment. Having said that, grey water systems also use energy which works against some of the environmental benefits of saving water. Cheaper, less energy-intensive ways of saving water include low flush toilets, low-flow taps and showers and changing our behaviour, such as turning off the tap when cleaning teeth.

For more information on grey water visit the Environment Agency website