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Grey (and Black) Water Recycling Design

By: Dr Gareth Evans - Updated: 29 Oct 2016 | comments*Discuss
Greywater Recycling Design Systems Water

Between shifting rainfall patterns and the burgeoning demands of our increasing population, the pressure on water resources in the UK continues to rise, despite our often notoriously wet climate - which puts water reuse and recycling high on the sustainability agenda. Perhaps more to the point, at a time when one in six of the planet's inhabitants has no regular access to a clean water supply, and as much as 80% of the developing world's disease can be attributed to water-borne pathogens, the case for our continued profligate use of potable water is hard to make.

For Britain as a whole, around 30% of water use is to flush toilets, while baths and basins account for a further 21%, and 12% is used in showers. Unsurprisingly, a growing number of people are seeing greywater recycling as an obvious way to cut down on the waste.

Greywater Recycling

'Greywater' is the name given to waste water from baths, showers, wash-hand basins and washing machines - some people also include kitchen sinks - as a result of its typically cloudy appearance, and the fact that its properties lie between 'white' (potable) and 'black' (sewage) water.

Although it is normally slightly contaminated with a range of chemicals, such as soap or detergent, grease and microbes, it can be successfully reused for a range of purposes which do not require drinking water quality purity, including:

  • Watering the garden
  • Flushing toilets
  • Car-washing

Anything other than the most simple of methods - manually emptying your bath tub with a bucket, for example - will require a separate plumbing system to be installed to collect the greywater, and the detail and extent of this will depend on the intended use and the level of sophistication desired.

Products to Avoid

In addition, it will also be necessary to be aware of the types of household chemicals and toiletries used, and there are some products in particular to avoid.

  • Bleach
  • Bath salts
  • Caustic substances
  • Chemical dyes
  • High-salt cleaners
  • Solvents
  • Strong acids
  • Strong alkalis

If in doubt, steer clear of anything marked as harmful, toxic (especially to plants or aquatic life) or corrosive, and your greywater recycling system should run pretty well. That said, ammonia-based cleaning agents are usually fine in normal quantities if the water is destined for irrigation, as plants can make use of the nitrogen.

Watering the Garden

As a general rule, untreated greywater cannot be stored for very long as the bacteria it contains will begin to breakdown the grease and other available nutrients, typically making for bad smells and affecting water quality, so it needs to be used fairly quickly. Never-the-less, simple systems without any form of water treatment can still be very useful so long as a few simple guidelines are followed:

  • Wait until it has cooled
  • Ideally, only use on ornamental plants or trees
  • Avoid contact with the plants themselves
  • Introduce the water at or below the soil surface
  • Do not spray - to avoid the risk of people or pets inhaling any droplets

Greywater Treatment

More complicated systems typically involve some form of purification system, which can help improve the quality of the water for reuse:

Physical or mechanical - the likes of sand filters, dual-medium filtration and UV sterilisation.

Biological - such as activated sludge tanks, biofilters, bio-rotors and aerated filters

Two such systems which are widely available in UK seem to be ideally suited to new-build projects, namely the EcoPlay and the HydroCyc.


In this simple to install system, bath and shower water is diverted to a cleaning tank, which is equipped with a skimmer to remove floating contaminants such as foam, hairs and soap-scum, while heavier unwanted particles sink to the bottom of the tank and are washed out to the waste drain. The partially cleaned greywater produced in this way then enters a 100 litre storage tank where it is held for toilet flushing. A full tank provides around 20 flushes, and an automatic control system ensures that the water is not stored for too long, releasing it to drain, as necessary, to avoid it going bad.


Bath shower and wash-hand basin water flows by gravity to the HydroCyc unit - typically located in the kitchen or utility room - where the system's patented hydro-cyclonic technology forces out impurities. It then pumps the improved quality greywater back up into a header tank, where it remains until needed for toilet flushing.

Generally simple to install in a new build, and with kits starting at under £1,000, greywater systems can meet level 6 of the Code for Sustainable Homes, are unobtrusive, cheap to run and largely maintenance free - which probably goes a long way towards explaining their growing popularity.

For the sustainably minded builder, however, water recycling doesn't have to end there.

Rainwater Harvesting

Probably one of the simplest other options is to incorporate rainwater harvesting (RWH) into the design - and unlike greywater installations which are really only suited to new-builds, there are RWH systems available which can be retro-fitted to existing homes.

There's nothing particularly new about the idea of collecting and using rainwater - gardeners have, after all, been doing it for centuries - but the household systems available these days propel the idea into a completely different league.

A number of rooftop systems are available to collect the rain, and the cost of installing basic RWH for toilet flushing in a typical three or four bedroomed house would probably start around £2,000, though obviously the more sophisticated versions cost rather more.


Collecting the water is, of course, only part of the story. A typical full RWH installation will also consist of a large storage tank - often underground - with a filter system and, in some cases, a UV disinfection unit too. Many also have control mechanisms of varying degrees of sophistication, which automatically switch over to mains if the tank runs dry, or divert excess rainwater harmlessly to a drain if necessary.

Arguably one of the most exciting recent developments in this field for the sustainable builder is the newly launched Rainstream RXL high-volume water storage tank from Polypipe. A modular design, it allows storage capacity to be matched to need, and it can be installed using a single-size granular backfill in most cases, rather than calling for the construction of a more costly concrete support structure. However, perhaps its most unique feature is the optional 'Biomaster' protection - a silver based anti-microbial agent which safely and sustainably cuts the growth of a range of potentially harmful pathogens - including E. coli, Legionella, Listeria and MRSA by up to 99.9%, according to the company.

Blackwater Recycling

The ultimate recycling challenge is, of course, black or sewage water - though opinions are very divided as to how practical a proposition this is for the individual household, and indeed how wise it is to attempt to reclaim water rich in faecal matter and potential pathogens in a domestic setting, at all! It is an approach used fairly extensively in Australia, where cyclical droughts prioritise all forms of water conservation, but it is much less popular in the UK.

The idea behind it is very simple; blackwater flows to a collection tank, where it is allowed to settle and an initial population of microbes begin to break down the solid materials within - much as they do in a conventional septic tank - and then after around 24 hours, the water enters a treatment tank.

What happens next depends on the type of system being used, but a typical treatment would include a period of aeration, which gives the bacteria present a supply of oxygen as they breakdown more of the organic chemicals in the water, followed by another period of settling. This usually takes place in a separate chamber of the main tank, so that the injection of air doesn't stir up the settling sludge.

Reed Beds

The water that results from this kind of treatment is rich in nitrogen and other valuable plant nutrients, and it is often clarified at this point, sometimes disinfected, and then used for irrigation. If cleaner water is desired, however, it is possible to treat it further and one of the best ways of doing that involves reed beds.

This simple, 'green' technology makes use of the fact that certain species of bog plants - most notably the Phragmites reeds - have the ability to 'pump' oxygen down from their leaves, into their submerged roots - and so out into the mud surrounding them. This, in turn, can support large numbers of very active microbes, which can continue the job of cleaning up the water to a very high standard, with the Phragmites themselves benefitting from the abundant nutrients released.

Constructing a reed bed involves digging a sloping pond-like excavation, lining it, filling it with bog-garden compost and planting the reeds. It is important to ensure that the size and flow rate along the bed is appropriate for the required through-put, so specialist advice will be necessary - and the reeds themselves may take a year or two to get established. Although the amount of ground taken up often makes them impractical for individual houses, they can be particularly effective for small developments where access to conventional sewers is impossible - with the added bonus of the enhanced local biodiversity they support, once they are growing happily.

Water recycling systems range from very simple, DIY kits, right up to sophisticated commercial models - and while they are not all appropriate for every project, the good news is that there's always something you can do to cut water waste - even if that only means fitting a rain butt!

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[Add a Comment]
interested in discussing grey water treatment /rwh for a propsed sailing club shower installation near conservation area
thegus - 29-Oct-16 @ 1:16 PM
I really liked your web site we need to spread the word
Eucalptus - 25-Sep-16 @ 7:31 PM
Lodza- Your Question:
Hi, I would like to install a filter system at a hand washing point at a rural school, how best can I go about it

Our Response:
Was there a part of the above article that you needed more information about?
SustainableBuild - 15-Aug-16 @ 12:09 PM
Hi, I would like to install a filter system at a hand washing point at a rural school, how best can I go about it
Lodza - 12-Aug-16 @ 3:58 PM
Does anyone know if rainwater can be used directly to flush? I want to mount a suitable water-butt on the wall at 1st floor height to flush the bathroom and cloakroom and wonder if this is permissible. I would also like to understand better why bathwater is not immediately reusable as flushing water as it is a lot of water... I feel that water companies are rather profligate with their control of H2o to keep profits (excuse to charge) high... My bath water always goes down the loo...
MKING - 24-Feb-16 @ 3:48 PM
Hello, I am research student from Kerala state of India. May I Know how we can collect black water for lab analysis. Thanking You Joice
Jose - 22-Feb-16 @ 5:41 AM
I'm leaving for the Philippines in October to build a prototype home based on the solar decathlon that takes place in Irvine, California 2015.My idea is this...to build a village of solar homes, commitment with vertical farming.I will build the first one, and the Filipinos will build all the rest. (120 homes.)These homes would be built by the Filipinos, for the Filipinos.Is it possible to build a self contained green solar home,and make it affordable for the maharip. We shall see and that will be my desire.Anyone out there that can shed ideas or pitfalls and most of all prayers, please respond. Shanon Burns. 949 651-0044
Shannon - 23-Mar-15 @ 3:53 AM
@buck. We've passed this on to our writer to see if he has any suggestions, stay tuned!
SustainableBuild - 6-Jan-15 @ 12:51 PM
Hi, hope you can help, a couple of years ago my wife and I moved into a zero carbon footprint Eco living house, one of the many features was a grey water recycling system that uses the bath and sink water from the family bathroom on the 3rd storey to flush the toilet in the ensuit in the master bedroom below, the grey water tank is situated in the wall behind the toilet and for 2 years we have had no problem, however lately we've noticed a foul odour akin but not exactly the same as sulphur, it smells vile and it's getting stronger and stronger, especially when we flush the toilet, I'm not fully conversant with the way the system works, I assume that bacteria in the tank breaks down the nasties in the grey water and then it's ready for use, could it be that the bacteria have shuffled off this mortal coil? Is there anything I can do (short of ripping open the wall and cleaning out the tank in the master bedroom) any additives I can drain down the bath? (One forum suggested orange cordial might solve the problem but I don't see how) I've searched high and low on the Internet to little avail. I really hope you can help
Buck - 2-Jan-15 @ 7:21 PM
Hi I am the founder of the Hideaway sauna/spa on Phuket, 27 years ago. I am planning to build a new sauna/spa on Phuket and from past experience, I realise that the ability to recycle shower/bath water to be used again as shower/bath water would be a great cost saver and eco friendly. Estimated maximum water usage to be approx. 5,000 liters per day. Will reverse osmosis or any other filtration system with UV, clean shower/bath water good enough so as to recycle for drinking(?), shower/bath water? If so, could you please send all info and costs for your recommendations. Kind Regards John Buckley
Johnno - 4-Jan-14 @ 4:10 AM
@selfbuilder999 - great idea, let us know how you get on. We've also posted this on our facebook page to see if anyone has any advice for you.
SustainableBuild - 3-Jul-13 @ 12:19 PM
Anyone tried forming a rainwater collection tank with surplus 6" soilpipe hidden in the soffit/fascia? About to give this a go
selfbuilder999 - 2-Jul-13 @ 8:50 PM
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