Making the most of used water

Saving water is important, but re-using waste water will help too.
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  • Updated:20 Mar 2007

01.Making the most of used water

Field of flowers in front of washing machine

Greywater is the waste water from showers, baths, spas, hand-basins, laundry tubs, washing machines, dishwashers and kitchen sinks. It doesn’t include water from toilets — that’s called black water. With water restrictions in many areas, using greywater on your garden could help save hundreds of litres of water a day.

Washing machines account for almost a quarter of household wastewater or, depending on your machine, about 60–180 litres per wash. So washing six times a week could send more than 1000L down the drain in one week alone. Add to that your dishwashing, shower and bath water, and you’re soon up to 4000L a week for the average family of four.

Please note: this information was current as of March 2007 but is still a useful guide to today's market.

Using greywater

  • Greywater can be 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.
  • Don't leave buckets lying around if you have small children because they're a drowning hazard.
  • Don’t store untreated greywater for more than 24 hours: if you can’t use it (because it’s raining, say) don’t keep it.
  • If it’s untreated, limit usage to water from the shower, bath and washing machine (preferably rinse water). Kitchen water contains fats and solids that might damage soil and plants.
  • 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, or pot plants.
  • Keep the greywater underground, or under mulch — this helps prevent evaporation, as well as keeping it away from kids and pets.

Specifically for washing machine water

  • In our latest laundry detergent report we tested the wash water for chemicals that could harm your garden. See our test report for which detergents we’d recommend for grey water reuse.
  • The components most likely to cause problems are phosphorus, salinity, sodium, and pH.
    • Small amounts of phosphorus can be useful for plants, and it’s a major component of fertiliser. When it gets into waterways, however, it can cause excessive algal growth, leading to toxic algal blooms. The effect on your soil is varied depending on your soil type. Clay soils can deal with more phosphorus because the phosphorus binds to clay minerals and doesn’t leach away. On sandy soils, excess phosphorus can leach into groundwater. Australian soils are typically low in phosphorus, and some native species can’t tolerate high levels.
    • All laundry detergents contain salts, typically sodium salts such as sodium nitrate, sodium sulphate, sodium phosphate and sodium silicate. 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 in the salts is particularly detrimental not only to plants, but soils. It affects the soil’s permeability and causes a loss of structural stability.
    • Laundry detergents are highly alkaline (that is, have a high pH): a pH higher than 10 helps dissolve organic dirt, such as grease, oils and food scraps. Most biological systems prefer a pH between 6 and 9, and greywater with a high pH is likely to harm many plants and soil organisms.
    • Our test 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.
    • The larger the irrigation area (minimum recommended area is 150 -200 m2), the more you’ll spread the chemical load.
    • Potential impacts are very much dose-dependent — try reducing the amount of detergent you use, providing it still gets your clothes acceptably clean.
    • Some laundry detergent products whose names imply they’re environmentally friendly could in fact cause problems if used on your garden. See our report for more.

    If you want to install a grey water pre-treatment system, do all of the above, plus:

    • Consult your sewage removal authority if you intend to redirect all or a major part of your used water.
    • Consult a licensed plumber for advice on the best system for your needs.
    • Inform your water supply authority of any changes to your plumbing.


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