Water is remarkable stuff. Reflecting the clear blue sky you’d swear it’s blue, but we all know that it really is clear and colourless. So what do people mean when they talk about ‘grey’ water?
As a starting point, clean, clear tap water is regarded as ‘white’ by people judging water quality. It has been collected from rain or rivers and has undergone treatment (including chlorination to kill bugs), making it fit for all uses, including drinking.
Sewage effluent is called ‘black’ and it must go through treatment before release into the environment. The quality of this treatment can range from the simple septic tank to sophisticated large-scale tertiary treatment at a sewage farm or treatment plant. Any water that may have been in contact with sewage (eg, bidet, urinal or even washing nappies or animals) is automatically considered to be ‘black’.
However there is a lot of water that’s used around the house that isn’t so badly contaminated that it’s black, and it’s definitely not white, and this is ‘grey’ water.
Shades of grey
The lightest shade of grey (water) comes from your bathroom handbasin, shower and bath. This type of water represents about 38% of water used in the house. Untreated, the soaps, shampoos, toothpaste and cleaning agents stay in this water and may build up in soil over time. This build-up of particles in soils is more pronounced in heavier soils, such as clay, but with all soils you should also regularly use clean, white water, to help flush out the built-up particles.
Laundry water(representing about 23% of household use) lightens up the further along the wash cycle you go. The first wash cycle water is chemically laden and should never be used in gardens. For example, boron from perborate (oxygen) bleaches can actually kill plants. Detergent products (including soap) all contain lots of sodium and this can build up in soil. Some detergents may damage car paint enamel, so washing cars with it is not recommended. However, water from the later rinse cycles is a much lighter shade of grey.
Kitchen waste water is always considered to be dark grey (and in fact may be as ‘black’ as sewage!). Food and fats feed bugs and dishwasher detergent is so alkaline it can change soil pH dramatically. It must be treated to remove the contaminants, before re-using in the garden.
The problem with soils
When people talk about ‘grey water’ they usually concentrate on the quality of the water, and that’s only half the story. It’s where you put this grey water that really matters, and garden soil makes this topic even more complicated.
Heavy soils (ie, clay soils) will tend to hold onto all the particles floating around in grey water, and these particles can build up to harmful levels over time. However, light soils (ie, sandy soils) drain very freely, and the same grey water applied to a light soil will cause a slower build-up of the same harmful compounds.
Using grey water
- Only use very ‘light’ grey water on your garden, if you must (ie, the last rinse from a wash cycle, or handbasin or bathwater).
- Never rely solely on grey water to water your garden, as it will harm your garden.
- You should always flush away the harmful nutrients from your garden soil by applying clean tap water more often than you use grey water.
- Never use grey water on food plants of any sort: ie, vegies, herbs, fruit trees etc.
- And don’t even consider using grey water near native plants, as the phosphates from detergents will kill them in no time.
- Spread your grey water around the property, don’t always dump it in the same spot.
- Take into account your soil type when using grey water, and be particularly wary if your soil is heavy clay. The chance of harmful compounds building up in clay soil is much greater than with light, sandy soils.
- The lowest risk for grey water is in subsurface irrigation pipes, while hosing it onto the surface of the soil is considered a high-risk option, and spraying or misting with grey water may be even worse.
- Your local council and health department will have a lot to say about what you can and cannot do with your grey water. This will differ from region to region. Always consult them first.
- Special low-phosphorus and no-phosphorus laundry detergents are available (for example the Planet Ark range of detergents), and these will lower the amount of harmful residues in grey water from the laundry.
Storing a no no
Please don’t store grey water, because it’s illegal. Untreated grey water contains nutrients that can support the growth of microbes, some of which may be dangerous. Storage creates offensive odours and attracts flies and cockroaches. And don’t let pets drink grey water. Immediate use of your grey water is recommended, but a small surge tank with an overflow drain connected direct to the sewerage system may be acceptable. And grey water should not flow onto neighbouring properties or into stormwater drains.
The bottom line
There are so many problems associated with grey water that we don’t like its use on gardens. We suggest you should try all the other alternatives (such as installing rainwater tanks, or even changing the plants you grow) before you consider using grey water. Always ‘flush’ your soil regularly with clean tapwater to help get rid of the residues, and never apply grey water to food plants.
Text by Ben Selinger
