Roughly a third of household water goes down the toilet and never needed to be drinkable. Another large share is bath, shower and laundry water that has been used once and is still perfectly capable of doing a second job.
In an emergency this is the cheapest water you will ever find, because you already carried it home. Using it properly can halve what you need to filter, boil or fetch.
It also has a small number of rules that are not optional, and one that almost everyone gets wrong.
Which water counts, and which does not
Greywater is not one substance. The risk varies enormously by where it came from, and the ranking is consistent across the research.
Kitchen water is the one people get wrong. It looks like the same stuff, and it is not. It carries food particles, grease, raw meat residue and the bacterial load that comes with all of that, and it is consistently ranked the highest-risk household source. It also goes putrid faster than anything else. Use it for flushing if you like, but keep it away from anything you will eat.
Nappy washes, and any wash containing soiled clothing or bedding, are blackwater. Do not treat that load as laundry greywater. Send it to the drain.
Water that has been used to clean up after illness is blackwater. In a household with a stomach bug going round, stop greywater reuse entirely until it has passed. That is exactly the situation where recycling water spreads it round the house.
The 24 hour rule
The practical consequence: greywater is a flow, not a store. Plumb it, bucket it or divert it to where it is needed the same day. If you cannot use it today, send it down the drain and take fresh tomorrow. There is no shortage of it.
If you genuinely must hold it, hold it for hours rather than days, keep it covered against insects, and use it on the ground rather than on anything edible.
The best use by far is the toilet
If you do only one thing from this page, do this one.
A toilet flush is 6 to 9 litres of drinking-quality water for a job that has no quality requirement whatsoever. In a mains failure, flushing is also one of the first things to become a problem, and a blocked or unusable toilet turns into a sanitation emergency quickly.
Method: lift the cistern lid and pour greywater in until it reaches the fill line, then flush normally. Or, faster and better in a genuine outage, pour 6 to 8 litres straight into the pan in one decisive go, which triggers the siphon and flushes without touching the cistern at all.
That second method matters because it works even when the cistern is broken, frozen, or the mains is off entirely. Practise it once so you know what "one decisive go" means for your toilet, because a hesitant pour just fills the pan.
Bath water is ideal for this. A single bath flushes a toilet perhaps five or six times.
Watering the garden
Greywater is good for a garden and, in a long emergency, keeping food plants alive may matter a great deal. The rules exist because the risk here is ingestion.
The principle: get it into the soil, and keep it away from people, animals and the edible parts of plants.
- Water the soil, never the leaves. Sub-surface or at the base of the plant. Never spray or sprinkle, because that creates aerosols and puts greywater on the parts you eat.
- Never on anything eaten raw. Salad leaves, strawberries, radishes, herbs. The risk is direct: contaminated water on a surface that goes into a mouth without cooking.
- Fine on fruit trees, and on crops that are cooked and grow well clear of the soil. Beans, sweetcorn, top fruit.
- Best of all on ornamentals, hedges, lawns and trees, where the question does not arise.
- Rotate where it goes. Do not put every bath onto the same square metre. Salts and detergent residues build up.
- Do not let it pool. Standing greywater on the surface attracts insects, smells, and is available to pets and children. It should soak away within a few minutes.
- Never use kitchen greywater on food crops at all.
Rest the ground periodically and give it plain water or rain when you can, which flushes accumulated salts down past the root zone.
Soap, and what it does to soil
This is the part that decides whether greywater is good for a garden or slowly poisons it.
Sodium is the problem, not soap in general. Many detergents use sodium salts, and sodium accumulates in soil, destroys its structure, and over a season or two turns a workable bed into something that sets like concrete and drains badly.
If you expect to use greywater on a garden for any length of time:
- Choose low-sodium and low-phosphate detergents. Products sold for greywater or septic systems are the right thing.
- Avoid anything with boron or borax. Boron is toxic to plants at very low concentrations and it does not wash out easily.
- Avoid fabric softener, which is among the worst things you can put on soil.
- Avoid bleach and any chlorinated cleaner in water destined for the garden. It kills the soil biology doing the actual work.
- Use less soap than you think. Halving detergent dose makes almost no difference to how clean the washing gets and a large difference to the water coming out.
Plain soap, in modest quantity, is close to harmless. Greywater is also mildly alkaline, so if you are growing anything acid-loving (blueberries, rhododendrons, most heathers) keep it away from them entirely.
Setting it up
You do not need plumbing. In an emergency, buckets are the system.
The bucket method, which is what most people will actually do:
- Put a bucket in the shower and stand in it. A short shower fills it.
- Bail the bath out with a jug into buckets before pulling the plug.
- Put the washing machine outlet hose into a butt or bucket rather than the standpipe.
- Move it to the toilet or the garden the same day.
A slightly better version: run the bath waste to a diverter valve, so you can send it either to the drain or to a hose that reaches the garden. A simple valve on the waste pipe, reachable, labelled, and turned back to "drain" by default so nobody accidentally sends a nappy wash onto the vegetables.
One legal note. Diverting greywater to your own garden is ordinarily fine. Discharging it to a surface water drain, a ditch or a watercourse is not, and the same applies to a soakaway close to a watercourse or a borehole. If you have a private water supply on your land, keep greywater well away from it, and see well and borehole disinfection for what to do if you suspect it has been reached.