The formula on this page is a summary from the Queensland Plumbing and Wastewater (QPW) code.
To estimate your household's weekly volume of greywater:
1. Calculate area available for irrigation - do not include space covered by buildings or hard surfaces eg. driveways, leaving appropriate setbacks from buildings and boundaries. See the QPW code for setback distances.
2. Calculate volume of greywater that your household will generate with this formula:
- number of people per household x total daily use (litres/per person/per day) x seven days = greywater volume per week in litres
- (an average person generates 95 litres of greywater per day from bathroom and laundry combined).
3. Calculate the area needed to soak up this volume of greywater - this will depend on the soil type and is based on Brisbane's average Design Irrigation Rate (DIR) of 25 mm/week:
- area needed (square metres) = volume (litres/week) divided by 25 (DIR).
Compare the area needed for the greywater generated with the available land. The available land should be larger than the area needed.
Where there is not enough land to distribute all the greywater from a house, it is possible to divert the greywater from just one source such as a laundry, or limit the diversion times.
- Available area for irrigation = 130 square metres.
- In an average four-person household: four people x 95 litres x seven days = 2660 litres/per week.
- Area needed (square metres) = average greywater volume per week: 2660 litres divided by 25 (DIR) = 106 square metres.
When you are sure your property is suitable for greywater use, have all the information required and the volume requires Council approval before proceeding with the installation, submit the documents and greywater application to Council for assessment.