What is Greywater?

The definition of greywater is (excuse the pun) something of a grey area. The Environment Agency in the UK describes it as ‘wastewater from showers, baths, washbasins, washing machines and kitchen sinks.’ Some exclude washing machine and kitchen sink water, or call it ‘dark grey water’, but as long as it’s not water from the toilet, it fits the general grey category.


In the US, a family of four uses around 400 gallons of water per day (around 375 litres each); it makes a lot of sense to recycle some of this. Why? We’re a thirsty world. Water use has been growing at more than twice the rate of population increase in the last century; water withdrawals are predicted to increase by 18 per cent in developed countries by 2025; and by 2025, nearly two billion people will be living in countries or regions with absolute water scarcity[3]. These are pretty good reasons to try and recycle some of that greywater to help ease pressure on fresh water supplies, reduce the burden on the sewage system, and lower our water bills into the bargain.

The main uses for domestic greywater recycling are for toilet flushing (you can reduce consumption by around a quarter to one third) or watering the garden. But before you jump out of the bath, scoop up that bucket and rush to your lettuce patch, consider this…

Your greywater will likely contain bacteria and viruses as well as bits of skin, hair, soap and detergents, all floating around in a warm fluid. This heady mix provides the ideal conditions for bacterial growth, so it’s wise to consider how you use it. Because of the possibility of contamination it’s not advisable to water fruit or vegetables with greywater, especially if they’re eaten raw. As a rule of thumb, use greywater for ornamental flowers or fruit trees[4].

It’s also difficult to store for more than a day without it starting to smell, so if you want to keep things really simple, go for direct reuse without treatment. Bathwater can be collected in a bucket, siphoned off through a hosepipe, or collected in a water butt then used immediately.

If you decide you do want to store greywater, you’ll need to invest some cash in a treatment system. The simplest method is to skim debris off the surface and let particles settle to the bottom of the storage tank. The next step up is a physical and chemical system that filters the greywater then uses disinfectants like chlorine or bromine to prevent bacteria growing; biological systems use bacteria to remove contamination, similar to a sewage treatment plant. And the most advanced domestic systems use a combination of filters, microbial cultures and UV disinfection.

There’s no doubt that recycling greywater reduces water consumption, but will it save you money? If your water is metered, direct reuse will save some cash. But installing, running and maintaining treatment systems is costly, and the payback period long. Certainly at present you won’t earn a quick buck by investing in this equipment. The best advice to the domestic consumer is to reduce consumption and directly recycle/reuse where possible

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