Various estimates show that the average Australian produces almost 100L of greywater per day. You may be able to come up with a more accurate figure by checking the water meter before and after showers, and before and after doing the laundry. Monitor these figures over a week.
By far the easiest way to use your greywater is on the garden. However, it very much depends on the size of your garden: if you produce more greywater than your garden can use, you’ll need to consider other uses of greywater — or only use your 'cleaner' greywater.
When you're calculating how much greywater you can put on your garden, reckon on about 20L per square metre per watering event. The frequency of watering events required depends on the local climate, rainfall and the season, while the amount of water required per square metre depends on soil type (20L is for loam soils).
Using these average figures, though, the average person produces enough greywater to water 35 square metres of lawn or garden once a week.
So — to work out how many square metres of garden you can supply with greywater:
EITHER calculate the amount of greywater produced by your household each week, and divide it by 20
OR take the number of people in your household, and multiply by 35.
Other factors to consider
If your garden is smaller than the calculated figure or you have no garden, consider a treatment system that allows your treated greywater to be used in the toilet (150L per person per week on average) and/or washing machine (190L per person per week).
If your garden is as big as or bigger than the calculated figure, you could get away with a greywater diverter system. However, there are several caveats:
- You shouldn’t store water for more than 24 hours, so you may have to water different parts of your garden each day, and divert greywater to the sewer when it’s raining.
- Keep an eye on the health of your plants: greywater tends to be high in chemicals that alter the structure of the soil, and it also tends to be overused — sick-looking plants could be suffering from overwatering, rather than the greywater per se.
- Give your plants a break by using rainwater (if you have a rainwater tank) or tap water every six weeks.
- Increase the organic content of your soil (with compost, say) to improve its structure and help it survive the chemical onslaught.
- Your greywater shouldn’t escape from your property into a neighbouring one, into stormwater systems or aquifers used for drinking water — in fact it’s illegal.
- Water from front-loading washing machines tends to have a high concentration of detergent, so unless you’re willing to use less than the recommended amount and compromise on the 'cleanliness' of your clothes, we wouldn’t recommend you use the wash water on your garden. The amount of rinse water may not be enough to make it worthwhile (about 30 to 50L per cycle — enough to water about two square metres of garden).
- Don’t use untreated greywater if someone in your household is sick, if you wash nappies and when you use certain chemicals, including bleach and dyes. You might also have to change some of the cleaning agents you use.