Greywater is a resource you can reuse, but only if you handle it sensibly, keep harmful chemicals out of the soil and check what the local rules allow.
Greywater and blackwater
The book divides household water into two kinds. Greywater is usually the water from washing dishes, clothes or the kitchen sink. It needs minimal treatment compared with other waste water. Blackwater is toilet water, which is a health hazard and must be disposed of carefully (see composting toilet planning and managing household waste).
Check the law first
Before disposing of any waste water, check what your area requires. The book notes that some countries regulate these matters and that breaking the rules can carry strict legal penalties. Rules can also differ between a garden, a watercourse and a septic system, so ask your local authority.
What goes into the water matters
Greywater often carries chemicals from dishwashing soap and laundry detergent. The book warns that boron, bleach and salt, if poured into the soil, can prevent or even damage healthy plant growth. Choosing cleaning products with this in mind makes greywater far more useful in the garden.
A simple strain-first method
The book recommends treating greywater before it goes back to the soil, and describes the easiest, least expensive method: strain it by passing it through a steel mesh to remove food scraps and other solids. Purpose-built greywater treatment systems exist, but the book notes that many are expensive and not beginner-friendly.
A mulch basin for trees and shrubs
The sanitation chapter describes the simplest greywater irrigation system: sink and shower water runs through a sediment filter into a mulched basin around trees or shrubs. It needs no pumps and no holding tanks and very little maintenance. The water soaks slowly through the mulched soil, giving soil organisms time to break down soap and organic material before it reaches plant roots. The book says such a system can supply a significant share of an ornamental garden's water. It also advises keeping any waste system at least 100 feet (about 30 metres) from your water source and downhill from it.
Planning checklist
- Confirm local rules for reusing and disposing of greywater.
- Choose soaps and detergents that are gentler on soil and plants.
- Strain the water through mesh before it reaches the ground.
- Spread it around rather than letting it pool, and keep it away from wells and watercourses.
- Do not reuse greywater from washing anything that could carry harmful contamination.
Greywater sits within the wider Off-Grid Water guide. For the full DIY approach to water, sanitation and waste together, see The Off-Grid Survival Manual.