Greywater is used wash water from sinks, basins, baths and laundry, and blackwater is any water that has touched toilet waste, raw meat or blood, or chemical cleaners; a family of four makes about 100-200 litres of greywater a day. Greywater is never drinkable and turns foul within about a day, so use it fresh on soil or discard it, never store it, and never join its pipes to a drinking-water pipe.
Part of Greywater Recycling Systems
Greywater is household wastewater without toilet waste; blackwater is wastewater that has met faeces, blood or hazardous chemicals. The line between them decides which water can be reused on a garden and which must be kept out of every reuse system.
The Dividing Line Is Contact
The parent page sorts household water by what it has touched.
Greywater includes:
- kitchen sink water, with precautions (a grease trap first)
- hand-basin water
- bathing and shower water
- laundry water, if mild or plant-based soap is used
Blackwater (not greywater) includes:
- toilet waste, which goes to the composting toilet (Composting Toilet Systems)
- water that has touched raw meat or blood
- water containing chemical cleaners, bleach or solvents
- diaper rinse water
A household with a composting toilet sends no flush water and no toilet waste down its drains, so every drain that remains can be sorted at the fixture.
How Much Greywater A Household Makes
The parent page puts it at 100-200 L per day for a family of four. Divided across four people, that is 100 ÷ 4 = 25 L to 200 ÷ 4 = 50 L per person per day. Kitchen water, laundry and bathing make up most of it, and the amount rises on wash days.
Greywater Is Not Clean Water
- It carries faecal organisms. Hands washed after the toilet, bodies in the bath and laundry all put some faecal bacteria into the water. Treating it as clean is how an outbreak starts.
- Kitchen water carries food organisms and fat. The fat clogs and the food scraps rot.
- It carries soap, salts and detergents that harm some plants and soils (Suitable & Unsuitable Plants).
It Spoils Within About A Day
Fresh greywater is lukewarm, rich in organic matter and full of bacteria. Left standing, the bacteria use up the oxygen, and the water turns septic: it goes dark, smells of rotten eggs and breeds more harmful organisms. A working rule is to use greywater within about a day of making it and discard what you cannot use. Storing it also lets any pathogens it carries multiply. This is why the system sends the water straight to the soil rather than into a tank.
Quick Sort
| Water | Class | What to do |
|---|---|---|
| Shower, bath, hand basin | Greywater | Grease trap or filter, then soil |
| Washing-up water | Greywater, with precautions | Grease trap first |
| Laundry, plant-based soap | Greywater | Filter, then soil |
| Laundry with diapers or soiled clothes | Blackwater | Do not reuse |
| Water used on raw meat or blood | Blackwater | Do not reuse |
| Water with bleach or solvents | Blackwater | Do not reuse |
| Toilet waste | Blackwater | Compost system only |
Common Mistakes
- Calling shower water “clean”. It is cleaner than kitchen water, not clean.
- Putting the toilet’s drain into the greywater line. The parent page says the toilet has no connection to greywater.
- Storing greywater “for later”. It spoils in about a day.
- Drinking or cooking with it, even after a filter.
Safety
Greywater is never drinkable, and a join to a drinking-water pipe can poison a whole supply
Filtering does not make greywater safe to drink. Keep greywater pipes and containers separate and marked from every drinking-water pipe and tank. A join, even a temporary one with a hose, lets contaminated water be drawn into the drinking supply when pressure drops (Plumbing Separation). A closed greywater tank is a confined space: never enter or lean into it, because hydrogen sulphide deadens the sense of smell.
What’s Next
- Learn next: What Qualifies as Greywater — sort each drain
- Combine with: Plumbing Separation — build the two systems apart