Greywater

Closing the water cycle

In a conventional house, water follows a straight line. It is pumped, treated, used once and piped away to a treatment plant, often far from where it once fell as rain. Using greywater bends that line back into a loop. The water from the shower and the washing machine stays on the land where it was used. It soaks into the soil, is taken up by roots, rises through the leaves as water vapour and cools the air around the trees, and what the plants do not use seeps deeper and can contribute to groundwater recharge.

This counts most in dry climates. Around the Mediterranean, rain falls in winter, and in summer the land dries out just when trees need water most. Greywater keeps coming in exactly those months, as reliably as people shower and wash their clothes. In his garden in Tucson, in the Sonoran Desert, Brad Lancaster leads the greywater of his house into the same mulched basins that catch the rain, so that both feed one planting. Rainwater harvesting holds water before it runs away; greywater gives it a second life after its first use.

How greywater is used

The principle is simple, and Art Ludwig has described it in detail in Create an Oasis with Greywater, the standard work on the subject. The first step happens in the house: the greywater pipes are separated from the toilet, so that only greywater goes to the garden. From there, the simplest systems work by gravity alone.

The most common form is the mulch basin, a shallow depression around a tree or a group of shrubs, filled with wood chips or other coarse mulch. The greywater flows into the mulch and soaks into the soil beneath, out of sight and out of reach. Ludwig's branched drain divides the flow of a house among several such basins through a series of splits, so that every tree receives its share. The mulch holds back hair and lint, keeps the surface dry, prevents odours and slowly turns into humus, while soil life breaks down the soap residues.

A second way is the planted soil filter, also called a constructed wetland: a bed of gravel or sand, planted with reeds or yellow flag iris, through which the greywater passes before it is used. Roots and the microorganisms living on them clean the water. In a trial in Israel, a small recirculating filter of this kind removed almost all suspended solids and almost all of the readily degradable organic load from greywater within eight hours, and the cleaned water then went to irrigate the garden.

Permaculture has its own well-known example: the banana circle, a round pit filled with mulch and ringed by bananas, papaya or other plants that love water and nutrients, into which greywater is led.

Three habits run through all these methods. Greywater is used fresh and not stored, because standing greywater quickly loses its oxygen and begins to smell. It is applied below the surface or under a layer of mulch, so that nobody comes into contact with it, and it is kept away from the edible parts of plants; application below the surface and keeping water away from edible crops are among the barriers the World Health Organization recommends in its guidelines on greywater use in agriculture. And it goes where it does the most good: to fruit trees, shrubs, hedges and large perennials rather than to lettuce or vegetables eaten raw.

What it brings

The first benefit is the most obvious: every litre of greywater that waters a tree is a litre of drinking water saved. Where a garden would otherwise hang on the tap all summer, this makes a real difference.

Greywater also brings something with it. Besides soap residues it carries small amounts of nitrogen, phosphorus and potassium and some organic matter on which soil organisms feed. What would be a load in a treatment plant becomes food for soil life.

Above all, the soil stays alive through the summer. Under the mulch it remains moist, so that fungi, soil organisms and fine roots keep working in months when bare ground lies hard and dormant. Trees supplied with greywater keep growing in the dry season and shade the ground themselves.

Less water flows into the sewer or the septic tank, which eases the load on treatment plants and small on-site systems. And the water stays in the landscape: what passes through the root zone fills the soil's reserves and, further down, can feed the groundwater.

Good practice

A few simple habits keep greywater use good for the soil. Biodegradable detergents that are low in salt and free of boron are the most important, because soap residues are broken down in the soil, while salt and boron stay behind. Liquid detergents usually contain less salt than powders; bleach and drain cleaners stay out of the greywater.

Fat deserves attention. In a container trial in Israel, untreated greywater rich in fats and surfactants made sandy and loamy soils water-repellent, while greywater that had first passed through a planted filter did not and proved suitable for irrigation. That is why many people keep kitchen water apart or filter it, and why the mulch layer is so useful: it catches what would otherwise end up in the soil.

Finally, the amount should match the plants. Greywater is spread over several basins so that the soil can dry between applications and never stays waterlogged. Changing the areas from time to time, and letting the winter rain wash through the soil, keeps salts from building up over the years.