07.Grey water for the garden
The water from washing machines contains several chemicals that aren’t good for plants or soil. If the concentration or total load of these chemicals is too high, you’ll eventually end up harming your plants and wrecking the soil.
How we test suitability for grey water
Results were considered on a per-wash basis, assuming intermittent use only in your garden (interspersed with clean water) and over an area of no less than 150 to 200 square metres.
It's safe to use the rinse water from any detergent, but the wash water from those with one to three stars in the 'OK for the garden' column in the Compare detergents table can also be used.
Note that the amounts used were those recommended for 'normally' dirty clothes — if you use more or less detergent the results will be different. The less you use the better for the environment, and you'll probably still get a good wash result.
Factors that affect suitability
Phosphorus Small amounts of phosphorus can be useful for plants, and it’s a major component of fertiliser. Australian soils are typically low in phosphorus, and some native species can’t tolerate high levels. Higher levels of phosphorus can be acceptable if you have clay soil, because it binds to clay minerals and doesn’t leach away. On sandy soils, excess phosphorus can leach into groundwater. When it gets into waterways it can contribute to excessive algal growth, leading to toxic algal blooms. Ideally, the amount of phosphorus per wash shouldn’t exceed 1g.
Salinity All laundry detergents contain salts, typically sodium salts such as sodium nitrate, sodium sulphate, sodium phosphate and sodium silicate. (Table salt — sodium chloride — isn’t used in detergents.) Salinity can be determined by measuring the electrical conductivity of the solution (in deciSiemens per metre, or dS/m). All laundry detergents are highly saline, and frequent long-term use would likely harm your garden, unless it was spread over a large area.
Sodium The sodium in the salts harms not only plants, but soils as well. It effects the soil’s permeability and causes a loss of structural stability. Manufacturers could use potassium salts instead of sodium, but they're slightly more expensive.
pH Laundry detergents are highly alkaline (that is, have a high pH) to help dissolve organic dirt, such as grease, oils and food scraps. Most biological systems prefer a pH between 6 and 9, and grey water with a high pH is likely to harm many plants and soil organisms.
Total alkalinity Laundry detergents contain chemical "buffers" to help prevent pH changes in the water/detergent solution, thereby maintaining optimal pH level. A measure of total alkalinity tells us that many are very highly buffered, so would require enormous amounts of acid to counter the alkalinity. If you put this water onto soil, the pH of the soil would be more likely to change than that of the grey water.
Sulphur and boron Sulphur and boron are important plant nutrients, though they also can be a problem if there’s too much of them. The small amounts of sulphur in laundry water shouldn’t cause any problems unless the soil is prone to waterlogging for long periods. None of the detergents had levels to cause concern.
Accumulated chemical load If you collect the wash and rinse water before using it, the contaminants in the wash water will be diluted. However, the criteria we used in our chemical testing considered the total load of problem chemicals that will accumulate in your garden over time, not just their concentration when you first put them on, so the recommended irrigation area (150 to 200 square metres) still applies. Potential impacts on your garden are very dose-dependent — you could try reducing the amount of detergent you use, providing it still gets your clothes acceptably clean.
Problems There have been reports of grey water hoses acting as a siphon and pumping water from the washing machine while it’s still washing or rinsing — something to consider if your washing machine doesn’t seem to perform as well as usual.
About the claims
Some products are marketed as "green" detergents. Presumably we’re meant to think their impact on the environment is less than that of regular laundry products. In reality, all detergents, no matter what their formulation, affect the sewerage system and aquatic environment.
A laundry detergent is a complex product made up from numerous chemicals. To determine its full environmental impact, you’d need to assess the physical, biological and chemical effects of its formulation on the environment.
This is the ability of chemicals to break down naturally in the environment. Manufacturers tend to make claims about biodegradability in reference to the Australian Standard. All laundry detergents must comply with this standard anyway, though it’s limited. It states that 80% of the surfactant must break down within 21 days. There’s no mention of the other 20%, or of chemicals produced during the breakdown process which could be toxic or non-biodegradable.
About grey water
What’s your soil?
If you don’t know what type of soil you have, this quick test will give you an idea. Take a handful of soil, add a little water and make it into a ball.
Loam soil will form a moist ball with an "earthy" smell. It’s usually brown, and holds and drains water well.
Clay soil forms a hard, smooth ball. Its fine, dense particles inhibit water movement, and when it dries it resists water. Its colour can range from white to red to dark brown.
Sandy soil is soft and crumbles easily — it probably won’t form a good ball. It’s light in colour, has little or no smell and is low in nutrients and organic matter.
- Greywater can be treated and stored and used on the garden (or even in toilets or washing machines), or else it can be diverted to the garden with a plumbed-in diverter (with a switch so that if it’s raining, it goes into the sewer instead). Conditions may apply in the area where you live; contact your local council for advice on options available.
- DIY options include attaching an extra-long flexible hose from the washing machine to the garden or using a bucket.
- If it’s untreated, limit usage to water from the shower or bath, and the rinse water from the washing machine. Kitchen water contains fats and solids that might damage soil and plants.
- Don’t store untreated greywater for more than 24 hours; if you can’t use it (because it’s raining, for example) don’t keep it.
- If someone in your family is sick with gastro or flu or another contagious disease, stop using the greywater.
- Don’t water herbs or vegetables.
- Keep the greywater underground, or under mulch — this helps prevent evaporation, as well as keeps it away from kids and pets.
Specifically for washing machine water
We tested only the wash water for chemicals that could harm your garden. What comes out of your machine may also include dirt and bacteria and viruses from the dirty clothes.
If you collect the wash and rinse water before dispersing it, the contaminants will be diluted and you can spread them further. However, the criteria we used considered the total load of problem chemicals that will accumulate in your garden over time, not just their concentration when you first put them on, so the recommended irrigation area still applies.
Potential impacts are very much dose-dependent — you could try reducing the amount of detergent you use, providing it still gets your clothes acceptably clean.
The larger the irrigation area, the more you’ll spread the chemical load. Don’t water pot plants. Washing machines account for almost a quarter of household wastewater or — depending on your machine — about 60-180L per wash. We tested the wash and rinse water for chemicals that could harm your garden. What comes out of a washing machine will also include dirt, and perhaps bacteria and viruses from the dirty clothes. These are all good reasons for not storing untreated water.