REUSING GREY WATER

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Introduction

Grey water is the water from sinks, baths, the washing machine etc which has already been in contact with us humans and our germs, and reusing it for any purpose other than garden watering is difficult. It cannot be stored for any length of time without some fairly heavy duty treatment, as otherwise it will very quickly start to smell. Also, research into the impact of systems that treat grey water for reuse in the home have shown that the environmental impact of such systems (using chemicals and/or energy) usually far outweighs the benefits.

It's always best to first reduce the amount of grey water you produce, and there are many simple ways to achieve this. A shower should use much less water than a bath, and fitting spray-head taps and a low flow shower head can make a big difference to water consumption. If you can get on to a water meter then you'll make monetary savings too. See our Water Conservation in the Home tipsheet (available through our pay-per-view page) for much more advice.

What greywater can I re-use?

Pollutants such as salt and grease in grey water can be damaging if it is used for irrigation. However, reducing the levels of these substances is fairly easy, and It is fairly easy to reduce the levels of pollutants (such as grease and chemicals) in greywater, enabling you to safely use it to directly to irrigate the garden. Water from the bath or shower is generally the easiest to re-use, as shampoos, soaps and so on are fairly mild and well diluted. Simple diversion kits are available which enable you to channel grey water from your downpipe into the garden.

If you want to re-use greywater from a washing machine on the garden, then do try and use detergent that is low in sodium, as it can damage plants and degrade soil. Also avoid phosphorus, which causes algal blooms if it collects in ponds or rivers. Otherwise, the water has very small quantities of other undesireables, such as pathgens or grease. Liquid detergents usually contain less salt than powders. Ex-kitchen water can be very dirty, and contain lots of undesireable things, like grease, oil, and chemicals. As it is generally produced in small quantities any, it's best not to reuse it.

There is much more advice in our Making Use of Grey Water in the Garden tipsheet (available through our pay-per-view page). It's also cheap and straightforward to collect rainwater for garden watering. See our Harvesting Rainwater information page for details.

Why not re-use it in the home?

Grey water contains bacteria and a nutrient source, and is often discharged warm, so you have an ideal situation for pathogens to multiply. Storing this water for re-use involves lots of treatment, and even then it will only be suitable for non-potable purposes such as toilet flushing.

Commercially available grey water recycling systems use disinfectants that are often very energy intensive to produce - and which may also cause problems if you have a private sewage treatment system. All independently published case studies of installed systems have shown running costs higher than that of mains water supply. Given the infrastructure requirements and the disinfectant doses needed, it is very difficult to see these systems as environmentally friendly, especially for individual households.

Grey water recycling is still a relatively young technology, and as such may improve with time, but shortages of water in the UK would have to become much more acute to justify domestic grey water systems. Advances in water treatment technologies may in future allow heavily contaminated water like grey water to be cleaned to suitable standards fairly cheaply - but the environmental impacts of this treatment might still be high.

If you are building a new home in an isolated area with no mains water or drainage and your main water source is insufficient for anything other than drinking, cooking and washing, you might consider recycling for some of your non-potable water uses. Even then, the impact of a complicated water treatment system is such that you would probably do better to minimise these non-potable uses by installing a composting toilet instead.

FURTHER INFORMATION AND CONTACTS

All our Tipsheets are available as pdf files through our pay-per-view system; alternatively, put a couple of stamps per tipsheet in the post - addressed to CAT Information Service, Machynlleth, SY20 9AZ - and we'll send them off to you.

CAT's new Water Book includes detailed chapters on rainwater and grey water reuse, including the design of garden irrigation systems.

See www.cat.org.uk/shopping (or tel: 01654 705989) for Tap Magic attachments, the Interflush retrofit kit (to reduce the water used to flush an existing siphon toilet), rainwater diversion kits, grey water downpipe diverters, direct bathwater diversion siphon hoses, and soaker hose kits for garden irrigation.

Green Building Store
Supply low flush toilets, waterless urinals, water efficient taps and showers, and flow restrictors.
Tel: 01484 854 898; Web: www.greenbuildingstore.co.uk

Please note: mention of a company does not necessarily constitute a recommendation. Potential customers are advised to contact several companies to compare products, services and prices.