Greywater Re-Use

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Greywater re-use

Currently 44% of scheme water supplied is used outside the home (in Perth). There is a preconception that greywater will be insufficient to meet garden watering needs but this figure indicates that internal use exceeds external which is encouraging for the greywater recycling industry.*

Re-use of waste water in amenity horticulture has been practiced around the world for more than 50 years (and much longer on a smaller, less regulated scale). Countries like the US, Israel and Australia have many large scale systems in operation and greywater re-use is becoming more popular among householders. Most of the systems in Australia have been operating for more than 20 years with no impacts on human health or
the environment.

Greywater should be considered, not just to save water, but to reduce the inherent energy costs of transporting and treating water.

In the US, the reduction of CO2 emissions involved in the supply of potable water is a major reason cited for the promotion of wastewater re-use.

Water re-use is the use of effluent water for beneficial purposes in landscaping, agriculture, industry and domestic systems.

All waste water can be recycled via onsite or septic systems which treat waste water including ‘black’ or toilet water and disperse the water close to the source. Greywater systems can treat or re-use water from the laundry, bathroom and kitchen to irrigate lawns and garden beds.

As a landscape garden designer you will be required to work with greywater applications and it is important to familiarise yourself with the systems available and suitable methods of dispersal.


‘The Code of Practice for the Reuse of Greywater in Western Australia’ provides information on the requirements for the reuse of greywater in residential gardens. It provides guidelines for householders and persons wishing to design and market systems including comprehesive information on approvals, setbacks and site suitability.

Developers interested in large wastewater reuse schemes for commercial or subdivision purposed are advised to refer to the ‘Australian Guidelines for Water Recycling: Managing Health and Environmental Risks.’

Types of greywater systems

The Department of Health provides a list and information on approved greywater systems which can be found at:

There are two types of greywater systems for the domestic market that both require local government approval and state government guidelines for installation:

  1. Greywater Diversion Devices (GDD)

    These are either gravity or pumped (laundry and bathroom only) and only to be installed in single residential dwellings

    Gravity diversion device

    A gravity diversion device incorporates a hand activated valve, switch or tap and is fitted to the outlet of the waste pipe of the plumbing fixture such as a laundry tub. Greywater is diverted directly to a subsurface irrigation system in the garden.

    Pump diversion device

    A pump diversion device incorporates a surge tank to cope with sudden influxes of greywater for distribution of the greywater directly to a sub-surface irrigation system in the garden. The surge tank does not operate as a storage tank.

  2. Domestic Greywater Treatment Systems (DGTS)

    These upgrade water quality and provide more flexibility in terms of re-use applications (can be used for kitchen water as well as laundry and bathroom). These systems are obviously more versatile and provide a more
    reliable source of quality water but the price of treatment can be prohibitive for many clients at the present time.

Calculating greywater volumes

The volume of greywater generated by any household will vary according to the dynamics of the household. This is
influenced by the number of occupants, the age of the occupants, their lifestyle and water usage patterns.

The method of calculating greywater volumes will depend on the greywater reuse system and the type of building. The Simplified Greywater Volume Calculation Method is as follows:

Sample estimate of greywater volumes
per individual.

Sample estimate of greywater volumes per individual.
Recommended estimates for greywater flows 1, 2, 3

Bathroom 60 420
Laundry 40 280
  1. Adapted from AS/NZS 1547:2000, Onsite Domestic Wastewater Management.
  2. The estimates assume a top loading washing machine and no water saving devices in the bathroom and laundry. Where water efficient appliances are used, this may affect the estimates.
  3. These volume estimates are recommended average figures generated by an average house (three-bed house and four occupants). Local governments may upon considering the public health and environmental conditions for the particular site vary these estimates to reflect local conditions.

Greywater flow calculations are based upon the number of bedrooms rather than the actual number of people who currently occupy a dwelling, because the number of bedrooms will remain constant, while the number of people may vary over time. To estimate the quantity of greywater generated in a household, follow the steps below:

  1. Calculate the number of occupants of a house as follows:
    • 2 persons for first bedroom
    • 1 person per additional bedroom
  2. Calculate each person’s daily greywater flow allocation as per ‘recommended estimates for greywater flows’ to the left, then calculate the volume of greywater generated using the following formula:40 litres (laundry usage*) + 60 litres (bathroom usage*) = 100 litres x no. of people in house x 7 days = litres of GW/week* per person per day, based on top loader washing machine and no water saving devices in bathroom/laundry
    Sample calculation

    A three bedroom house reusing both bathroom and laundry greywater.

    Occupants: 4 persons

    Greywater volume (L/week) = 4 persons x 100 litres/person/day x 7 days = 2800 litres/week

    To calculate irrigation area

    This is a simple formula where the volume of greywater in litres per week is divided by the Design Irrigation Rate (DIR) in millimetres per week to get the size of the irrigation area in square metres.

    Note: the optimum DIR varies depending upon soil type, but in terms of the current standard recommended drink of 10mm per application for WA, twice a week, the weekly DIR for Perth would be 20mm.

    Site assessment

    To determine whether a site is suitable for greywater irrigation consider the following:

    • **Will the land be susceptible to ponding and run-off?
    • **Is the greywater likely to seep through to adjoining properties?
    • **Is the greywater likely to seep into the underground water table?
    • **Is the greywater likely to seep into areas, on premises, that will affect the stability of buildings?

    Restrictions on greywater use

    In soils where the phosphorous retention index (PRI) of the soil is less than five, the greywater systems should be installed more than 100 metres away from any wetland, stream flow or other water sensitive ecosystems. Information about local water features can be obtained from the Department of Water.

    Greywater systems within proclaimed public drinking water source areas, located in designated Priority 1 areas, wellhead protection zones or reservoir protection zones (as defined in published drinking water source protection plans or land use and water management strategies) must be approved in writing by the Department of Water. Information on these areas and zones is available online at

    If within a Priority 1 Drinking Water Source Protection Area, the PRI of the soil will need to be assessed by a soil scientist. Soil tested must be collected from the soil in which the greywater is to be irrigated. The testing
    procedure must be conducted by a NATA registered laboratory. System flow rates on coarse sandy soil/gravel should be carefully designed to avoid greywater directly entering surface water bodies.

    Irrigating with greywater

    • * Drip dispersal systems are used for greywater, delivering a slow and precise application of water throughout the soil.
    • Drip irrigation eliminates health risks associated with wastewater and reduces run off and ponding due to a slow application rate.
    • Specially designed dripline is available for greywater irrigation. Pressure compensated dripline can be used
      for treated greywater and nonpressure compensated for greywater diversion systems.
    • The principles for installing a drip irrigation system for greywater are the same as for general dripline apart from the specialized, colour coded pipe specific to greywater.
    • A multi-indexing valve which operates volumetrically allows different sections of the garden to be watered, rather than dispersing greywater to the same area
    • It is essential that household laundry detergents etc. are compatible with greywater re-use, ie. low in salts; phosphorous, sodium, boron and chloride.

    Information on salts, nitrogen and phosphorous contents of various washing detergents in Australia can be found at:

    For details of phosphorus free and alternate cleaners see:

    Important considerations when using greywater

    • Good dispersal design is essential. Greywater volumes and averages should be matched to the area to be irrigated.
    • To ensure water balances are achieved in high and low evaporation periods of the year and with varying occupancy of the dwelling, supplementary water may be required to ensure security of supply.
    • When using greywater in heavier soils where saturation of garden soils of low permeability may occur during periods of heavy rainfall there should be a facility to divert the greywater to a sewer or alternative disposal method.
    • Regular maintenance of the drip system is essential – check the manufacturer’s recommendations.
    • Treated rather than diverted greywater is recommended for turf. Although some paspalum varieties such as ‘Velvetene’and Zoysia are able to handle poor, low quality or recycled water, the dripline required to accommodate larger particulate matter in diverted greywater can be prone to root intrusion.
    • Greywater is not for use on leafy edible crops – herbs, vegetables etc.
    • A licenced plumber is required to modify any plumbing work.
    • Occasional irrigation with rainwater or scheme water to disperse salts from the soil is recommended.
    • Incorporating organic matter and using wetting agents is beneficial.

    Grey water tolerant plants

    Water quality of untreated greywater can vary depending on cleaning products used etc.

    Common concerns are that the alkalinity and salt content of greywater will have adverse effects on plants irrigated with untreated greywater.

    Despite the widespread use of greywater systems there are not many formal studies on plant tolerances.

    The Handbook for Irrigation of Amenity Horticulture with Grey Water by Dr. Daryl P.Stevens, Steven Smolenaars and Jim Kelly is a useful reference, listing plants with a moderate to good tolerance of soil salinity.

    The handbook can be viewed at