What is greywater?

Greywater is simply the cleaner waste water from inside your home:

Source % of usable greywater Is the water reusable?
•  Showers, baths & spas 50% Yes
•  Laundry 40% Yes
•  Wash basins 10% Yes
•  Kitchens  0% No *

*  Reuse of kitchen wastewater (including from the dishwasher) is not recommended due to the likely presence of food scraps and fats which may clog the system.

Greywater does not include wastewater from toilets and urinals. This 'blackwater' has too great a hygiene risk for safe treatment

Why use greywater?

A drying climate in Perth since the mid 70s plus continued population growth is putting increasing pressure on our supplies of drinking water. Half is already produced by energy-hungry desalination whilst most of the remainder comes from groundwater. Whilst there’s still water in our dams, inflow from streams is now sometimes exceeded by evaporation whilst some dams simply store water pumped up from the coastal desalination plants.

Installing a greywater diversion system is a low-cost way ($1500-$5000 fully installed) to substitute drinking-quality water with cleaner waste water for dripline irrigation. There is no treatment of the water, and it can’t therefore be used for flushing toilets or doing laundry inside the home. That needs a Greywater Treatment System at $12000+.

If you already use a garden bore for retic then the advantage of greywater is reduced, but remember that restrictions on sprinkler use also apply to garden bores whereas they don’t for greywater.

Advantages of greywater

Using greywater for irrigation has a number of benefits both for you as a homeowner as well as for the environment, including:

  • Reduces your water bills
  • Helps reduce overall demand for drinking water
  • Reduces discharge of wastewater
  • Improves soil quality and plant growth when used appropriately
    • The small amounts of nitrate and phosphates in the water from soap and detergent provide the essential nutrients needed by plants
    • The soap in greywater acts in the same way as soil wetting agents by helping the soil absorb more water

 Are there risks in using greywater?

It's not safe to assume that greywater is clean and safe for reuse for all applications so long as it doesn't contain “blackwater” (toilet water). Of course the government has a duty to point out the health risks in greywater reuse and you may think that these are over-emphasised in official documents. However, your greywater may contain high levels of:

  • Disease causing organisms (such as bacteria and viruses)
  • All kinds of solids and organic matter, including dirt, lint, food, hair, body cells, fats and oils, and potentially also traces of faeces and urine
  • Chemicals derived from soaps, shampoos, dyes, household cleaning products etc.

Garden soil, microbes and plants can degrade and adsorb many of the contaminants found in greywater so long as they're not overloaded. Many nutrients can even be beneficial if they do not exceed plant requirements.

Issues associated with overwatering

Inappropriate use of untreated greywater (i.e. the type of water from a standard diversion system) has the potential to harm your local environment by:

  • Overloading the garden with nutrients or salt. This causes degradation to the soil structure, decreased permeability and changes to soil pH (acidity) levels which significantly affects some plants
  • Because detergents are alkaline by nature, some plants may show signs by the end of a summer irrigated with greywater that the increased pH from is limiting their uptake of essential trace elements. Increasing the amount of organic matter in the soil will help, since compost for example naturally buffers pH change and improves soil health
  • Causing the soil to become permanently saturated, which not only starts to smell but prevents plants from growing

The amount of water and frequency of watering may be too much for the capacity of the soil and plants. If the ground is already moist from rain it may well not need irrigating. In winter a greywater system can be easily turned off (i.e. the water diverts to the overflow and sewer) by flicking off the pump power at the wall socket or turning the diverter valve. This gives the soil a rest and allow rainwater to naturally flush out salt or phosphorus. Then restart the system as the weather warms up

Management of Risks

Many of the environmental risks associated with the use of greywater can be managed by sensible selection and use of detergents and other household chemicals. Look for “eco” branded washing powders and cleaning products that say they’re good for the environment.

To reduce health and pollution risks it’s sensible to try and reduce the concentration of hazards in your greywater by:

  • Not collecting water from the laundry after washing nappies or other laundry items soiled by potentially infectious matter, such as faeces or vomit;
  • Not disposing of household or garden chemicals into greywater systems; and...
  • Excluding kitchen waste

Use of untreated greywater

A Greywater Diversion Device (GDD) does not treat the water in the sense of decreasing any health risk from exposure to it. Although you may believe that your shower water is perfectly good for use all over the garden, washing the car or even flushing the toilet, government regulations prohibit you from using it for anything else than subsurface irrigation.

How can I use this water?

The quality of untreated greywater is variable with time of day, time of year and where it comes from. Wastewater quality is highest from bath, shower and washing machine final rinse, and lowest from the kitchen. Somewhere in the middle is laundry wash water.

If you install an automatic pumped diversion system to save the effort of collecting your wastewater in buckets, then the table below shows you where this water can be used:

Use it? What for? How is the greywater used?
Yes! Sub-surface or drip irrigation installed 100-150mm below the surface Letting the greywater flow through the soil adds an extra level of filtration and treatment. Greywater dripline must be separate to any existing mains water drip irrigation to prevent cross contamination of drinking water and because the smaller holes in standard dripline will clog up from the solids in greywater
Yes! Covered surface irrigation Using dripper line on the surface but covered by thick mulch, for example around fruit trees or vines
With care! Watering of native plants Our beautiful native plants are not only waterwise but rather fussy about what’s in their water. High levels of detergent (for example from ‘unfriendly’ washing powder) could kill them
Check! Right across your property Setback limits apply from property boundaries, buildings, pools, paving etc. and there are guidelines for the area of garden required to dispose of the greywater. These must be followed in order to get a permit granted
No! Sprinklers and sprayers Spraying water over lawn or garden is not permitted. In any case, this is a very wasteful method of watering due to high losses from evaporation and windage
No! Surface watering of underground or leafy vegetables or low fruit Contamination risk from the water being directly in contact with the skin or leaves of the fruit or veggies. Washing your produce before you eat it will significantly reduce any risk. No impact on fruit trees where only the roots are in contact with the soil

Selection of detergents and chemicals

It is important for the occupants to be aware of the potential impact of the chemicals they use (detergents, cleaning products, shampoos etc) which will end up in the greywater can have on their garden. Specifically, it makes good sense to:

  • Use detergents that are less unkind to the environment, with lower levels of phosphorus and salt. These are typically sold as ‘eco’ or ‘garden-friendly’ products. Liquid laundry detergents are generally better for greywater than powders due to the reduced salt level required
  • To avoid overload (and save money!) don’t use more detergent than necessary for the size of the wash and hardness of the water
  • Avoid the use of bleaches and disinfectants as far as possible as these kill beneficial soil micro-organisms
  • Avoid putting any fats, oils and paints down the drain as these can cause clogging and reduce soil permeability

How much is greywater worth?

The average Perth household uses over 40% of their total water consumption on the garden, which currently costs around $200 per year in water charges. Of course every household is different in terms both of size and their water consumption, and exactly how much of the water you buy can be replaced with free greywater depends on several factors including the size of your household and your garden area.

It would be reasonable to assume $150 of water savings at current prices for the average household. Taking into account rising water prices and the cost of purchasing and installing a mid-range greywater system, the break-even point could be reached in around 15 years. Let us calculate savings for you!

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