Treated greywater still carries faecal organisms, fat, soap residue and salts, and it turns septic within about a day, so reuse it fresh, below the soil surface or under mulch, on fruit trees and non-leafy plants only, and well away from wells, springs and streams. Treatment lowers the risk but never removes it, so stop reusing the water and send it to a dry well or soak pit whenever anyone is ill, the ground is frozen or heavy rain has saturated the soil.
Part of Greywater Recycling Systems
The contamination limits of greywater are the biological, chemical and physical hazards that remain in wash water after it is sorted and filtered, and the rules that keep them from reaching people, food and drinking water.
Five Hazards That Survive A Filter
| Hazard | Where it comes from | How it harms |
|---|---|---|
| Faecal organisms | Hand-washing, bathing, laundry | Cause diarrhoeal disease if swallowed or if they reach food |
| Fat and food particles | Kitchen sink | Clog soil, turn anaerobic, attract flies and rodents |
| Soap, detergent, boron, sodium | Washing | Harm plants; sodium damages soil structure (What Qualifies as Greywater) |
| Salts | Cumulative irrigation | Build up in the root zone of a garden watered every day |
| Standing-water breeders | Ponding, open tanks | Mosquitoes, whose egg-to-biter cycle is 7 to 12 days per Mosquito Prevention |
Time Is The Hazard You Control Most
Greywater is fresh, warm and full of dissolved food for bacteria. Within about a day in a closed container it runs out of oxygen, turns septic and starts to smell of rotten eggs; bacteria that arrived in it can multiply during that time. So the rule is: use it fresh, and never store it. If a tank is needed at all, it is a short-term buffer emptied each day, not a store.
The Reuse Limits
- Below the surface or under mulch. No spray, no sprinkler, no puddle. Contact with the water is the way people get ill.
- Away from leaves and edible parts. Fruit trees and non-leafy plants only; see Suitable & Unsuitable Plants.
- Away from water sources. The wiki gives several distances: Greywater Management sets the soak pit at least 15 m from any water source; Source Protection lists a washing or bathing area within 30 m upstream of a source as a moderate risk and a latrine within 100 m as critical. Use the larger distances for a well or spring.
- Distance alone is not protection in fractured rock or karst. Water can run quickly through cracks and channels with almost no filtering, and a source far away on the surface can be close underground. The source-protection page makes the same point for sandy and gravelly soils and says to add 50% to every distance when in doubt. In fractured rock or limestone caves and sinkholes, no distance is safe without a measured flow path; do not irrigate near such a source.
- Not over a high water table or frozen or saturated soil. The soil cannot filter what it cannot absorb.
- Not when someone is ill with diarrhoea. Send it to the dry well or soak pit.
Worked Example: Is The Distance Enough?
A well sits 20 m downhill of a proposed mulch basin on sandy soil. The first-month greywater page sets 15 m for a soak pit, which looks satisfied. But the source-protection page lists washing and bathing areas as a moderate risk within 30 m upstream, notes sandy and gravelly soils carry contamination farther, and says to increase distances by 50% when in doubt: 30 × 1.5 = 45 m. The basin at 20 m fails the cautious test, so move it to at least 45 m away or use a different reuse route. This is a judgement from the wiki’s own figures, not a measured result.
Common Mistakes
- Trusting a filter to make the water safe. It lowers the load; it does not sterilise.
- Using the water in a spray irrigator. Aerosols carry organisms onto crops and people.
- Applying water daily to the same spot. Salt and soap build up; rotate beds.
- Ignoring the water table. A shallow table puts the water straight into the well’s source.
Safety
Greywater is never drinkable and must never meet a drinking-water pipe or source
Keep greywater at least the distances above from wells and springs, in karst or fractured rock keep it away altogether, and never allow a cross-connection to a drinking-water pipe or tank (Plumbing Separation). A closed greywater tank or grease trap can hold hydrogen sulphide, which deadens the sense of smell at dangerous concentrations, and is a confined space: never enter or lean into one.
What’s Next
- Learn next: Plumbing Separation — keep greywater out of the drinking supply
- Combine with: Filtration Systems — the three stages that reduce the load