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Water Conservation in the Garden - Greywater

Wendy van Dok

How much water do we use in the garden?

In the average Australian home, about 34% of all water is used outdoors, 4% is used in the kitchen, 16% in the laundry, 20% in the toilet and 26% in the bathroom. The average total household water use is about 300 kilolitres (kL) per year and there are about 5,000,000 urban homes connected to mains water around Australia. Therefore, about 100 kL a year per home, or about 500 gigalitres (GL) Australia-wide (assuming all homes have a garden), are used to water urban gardens. That does not include gardens irrigated with private water supplies.

[Note: 1 kL = 1,000 litres and 1 GL = 1,000,000,000 litres]

What does water efficient mean?

   water drop

My definition of water-efficient is to grow plants suited to local conditions so that most, if not all, of their water requirements can be met by rainfall. If plants do need to be irrigated in the dry season, greywater and rainwater tanks are used instead of tap water and applied with the most efficient irrigation techniques. Tricklers and soaker hoses placed underneath mulch, and capillary irrigation are some efficient techniques.

What is greywater?

Greywater is wastewater from bathroom, laundry and kitchen. Blackwater is wastewater from the toilet.

Is it Iegal to use greywater?

Most authorities do allow greywater to be used for subsurface irrigation but they often have an unnecessarily restrictive and expensive permit approval process. Most States have plumbing regulations that require a licensed plumber to make any changes to the existing plumbing.

How safe is greywater?

Most greywater is safe for garden irrigation so long as garden-safe cleaning products are used, and so long as the water is distributed 2-4 cm below the surface. The exceptions are: kitchen wastewater (high grease content), washing machine wash cycle water (high salt content in many detergents), dishwasher water (very caustic), nappy washwater (high faeces content), water softener backwash (high salt content), or greywater that contains a high concentration of bleach, drain cleaner (caustic soda), acid, hair dye, disinfectant (including high concentrations of herbal products like eucalyptus or tea-tree oil), fat and enzymes.

Greywater does contain some human pathogens but the risk of contracting a disease by using greywater in the garden is equal to the risk of contracting a disease from soaking in a bath, or standing over a washing machine. We come into contact with human pathogens every day from people coughing and sneezing all around us, from touching handles from drinking glasses and cups not cleaned properly, from handling money and from swimming pools and spas. Yet most of us don't get sick from these exposures. Common sense applies here so if someone is sick with a waterborne disease, extra precautions need to be taken.

Chemicals in greywater, and the harm they can do to soil and plants, often pose a greater risk to the garden than do pathogens to people. Soaps, detergents and cleaners contain many synthetic and organic chemicals that can wreak havoc in the garden. The main problem chemicals come from laundry detergents, laundry soaps and surface cleaners. Personal soaps, shampoos and conditioners are less problematic because they have a lower pH and are fairly well diluted. Nevertheless, any product should be used sparingly.

Is greywater a problem for Australian native plants?


Australian Plants and Phosphorus

Many of the commonly grown Australian native plants have originated in phosphorus deficient soils and high levels of phosphorus can be detrimental to the health of these plants. Phosphorus sensitivity is a particular issue with members of the Protea family (Proteaceae) which includes, Banksia, Grevillea, Hakea and Isopogon.

However, phosphorus sensitivity varies and it should not be assumed that all Australian native plants will not tolerate phosphorus.

For further information on this issue, refer to "Phosphorus Needs of Some Australian Plants" by Kevin Hendreck, from the December 1997 issue of Australian Plants online.

The four issues of most concern with regard to greywater and natives are:

  • Phosphorus
  • High pH
  • High alkalinity
  • Salinity.

Many natives do not like much phosphorus so yes, greywater containing phosphorus - eg. from laundry detergents - may not be suitable (see box).

Most laundry greywater also has a high pH so if you have natives that do not like basic soil, do not use greywater with a high pH. The pH of greywater can be reduced with household white vinegar.

pH is different from alkalinity although the two terms are used interchangeably. Alkalinity measures the hardness or concentration of calcium carbonate, whereas pH measures the hydrogen ion concentration and ranges from basic to acid. Some detergents may have a high alkalinity as well as a high pH, but they can have a high pH without having a high alkalinity.

Salinity is mainly a problem with washing machine water as most detergents have a high salt content. Preferably do not use saline water, especially with clayey soil, unless your plants can tolerate it. You could add gypsum to the washwater if it is saline (ie high sodium concentration).

What precautions should I take when using greywater?

  • Don't drink greywater, feed it to your pets or allow children to play with any parts of the system.
  • Don't spray the edible parts of plants with greywater and don't use it for root/tuber vegetables.
  • Don't use a sprinkler with untreated greywater: use subsurface trickle irrigation.
  • Don't store greywater. Match the amount of greywater you use with what the garden needs.
  • Don't allow greywater to pool and stagnate.
  • Don't let your greywater go beyond your property and cause a nuisance for your neighbours.
  • Don't allow hot water to run straight out to the garden, allow it to cool first.
  • Don't connect all the drains to the greywater irrigation system. Leave the bathroom basin and laundry trough connected to the sewer so that garden-unsafe water can be disposed of there.
  • Do condition the soil and add a lot of compost before using greywater.
  • Do test clay soil for sodicity (sodium) every year and if necessary, add gypsum to help eliminate any build up of sodium and magnesium.
  • Allow treated spa water to stand overnight before discharging it to the garden.

From the March 2001 issue of "Growing Australian', newsletter of the Australian Plants Society (Victoria).

Wendy van Dok has a PhD in biology and is a landscape designer and greywater irrigation specialist. Wendy can be contacted at 5 Shirley Ave, Glen Waverley, Victoria, 3150. Tel: 9802 7211. Email: wvandok@nemesis.com.au

Further information on the use of greywater on gardens can be found on Wendy's website. There is also a large section on greywater in Wendy's book, "The Water-efficient Garden" - available from the Australian Plants Society (Victoria) book sales service.


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Australian Plants online - June 2001
Association of Societies for Growing Australian Plants