Recycling grey water around the home

Drought, dehydrated gardens and global warming have spurred many home owners into seeking alternative water supplies and using water more wisely. Diverting rain or grey water to flush toilets achieves both aims. Gary Workman, of the Master Plumbers and Mechanical Services Association of Australia, says an average household generates about 100 litres per person of grey water daily. But using rain or grey water isn't always straightforward and grey water particularly needs forethought – and a significant investment.

Gary, national manager of the association's green plumbers program, says grey water needs to be treated if it's stored more than 24 hours, and the level of treatment required varies from state to state. "The intensity of treatment also varies a lot, depending on where you're sourcing the water from," he says, adding that bath and shower water needs less treatment than kitchen or laundry water, which is usually highly contaminated.

Treatment generally involves a combination of methods, including heat, UV light, oxygen, chemicals and natural products, such as peat, which is used to filter the grey water. After treatment, the water also needs a storage tank, separate to a rainwater tank if you have one. The average cost is about $8000 all up, says Gary.

Using rainwater to flush your toilet is more straightforward than using grey water. "It's probably cheaper, there's less red tape to work through, and you can use it for anything and everything," Gary says. He estimates the cost at about $4000 for a 3500 litre tank and says each state has rebates for both rain and grey water systems. 

Plumbing Requirements

Both grey water and rain water systems also need pumps and separate plumbing to the toilet to avoid cross-contamination with drinking water.

Rainwater can also have an interconnection device that automatically switches to mains water when the tank runs dry. This isn't an option for grey water, but the amount generated daily means you're unlikely to run dry, particularly if you have a four-star toilet that uses only 4.5 or 3 litres for flushing, Gary says.

To get the best out of your rainwater supply, he recommends keeping roofs clean and using a first-flush device, which diverts the first 10 litres off the roof into the garden, leaving cleaner water to run into the tank. Factors determining the tank size include local rainfall patterns, roof area for collection and planned uses for the water. "We recommend year-round activities, such as toilet flushing, laundry and hot water heating, rather than summer activities such as watering the garden," Gary says. The garden would use about 700 litres an hour, and empty your tank in just one or two uses.