How should I use greywater in my garden?

The reuse of greywater for irrigation requires consideration of the following variables:

• Local context: climate, soil, garden type and composition, new home, old home, occupants.

• Calculation of greywater volumes and design irrigation rate to determine required application area.

• Greywater dispersal areas in the garden/ lawn and irrigation requirements.

Once these have been assessed by a greywater professional an irrigation system can be designed that is best suited for the specific site and also matches vegetation requirements for volume and distribution. If the system is designed correctly then everything else should be accounted for.

Calculation of greywater application area

A simple calculation is used to determine the area required for greywater dispersal.

Expected greywater volume per day is determined by the number of bedrooms in the property (maximum likely number of occupants: bedroom 1=2 people; all other bedrooms = 1 person) multiplied by a fixed amount of greywater produced by each occupant daily
in both the laundry (40L/person/day) and bathroom (60L/person/day) (based on AS/NZS 1547:2000).

Design Irrigation Rate (DIR) is determined based on soil type. Sandy soils of the Swan Coastal Plain have a DIR of 10mm/day and heavy clay soils have a DIR of 2mm/day.

Generally, if the available garden area is greater than the application area required then a greywater system will be approved by local government.

Irrigation

The unique context of the Swan Coastal Plain requires an expert with a local appreciation of irrigation for ultimate success of a greywater system. Components of irrigation design include: pump sizing and specification, layout, soil and plant assessment, and maintenance requirements.

Greywater is ideally suited to the irrigation of trees, shrubs and garden beds with dripline buried beneath 100mm of mulch, or in deeper pipes or trenches. Greywater system manufacturers may make specific recommendations for the design of an irrigation system to ensure adequate performance given flow rates of the specific pump used, including: type of dripline, length of dripline between supply and flush and total length of dripline in irrigation area.

The dripline suited to greywater (large emitter orifice) can be used with scheme water, but the opposite is not true due to the smaller sized aperture of the drippers being prone to clogging.

If the ground is already moist from rain it may not need irrigating. As with scheme irrigation, turn the greywater system off in winter and restart it as the weather warms up.