Rainwater tank how-to

By Alex Brooks

There are a few sensible steps you need to take before you start harvesting rainwater.

When I was growing up in Adelaide, every­one seemed to have a rusting galvanised-iron rainwater tank in their backyard, often with pantyhose strung over the tap to keep the wrigglers out.
Wrigglers were the extra bit of protein from mosquito larvae, all part and parcel of drinking crystal clear rainwater when I was a kid. Some people doused the top of their tanks with a layer of kerosene to kill the little mozzie babies. Others relied on the old pantyhose as a filter.
When I told my swish Sydney friends that Adelaideans often drank rain­water with a tang of rust and kero rather than risk choking on the brown, watery mud that came out of the taps, they were horrified. They believed I had grown up during the Depression, rather than 1980s suburbia.
Oh, how drought and water supply problems can change people's tune.
Rainwater tanks are now as sophisticated as city tastes. Some tanks are so skinny you could dress them in a bubble skirt. Others, called "bladders", can be installed under decks.
Tanks have mozzie-preventing screens and first-flush devices to stop the first few drops of rainwater that washed the dirt from a roof entering the tank.
So how do you choose which tank is right for you? Unfortunately, the best rainwater tanks are like haute couture - they need to be tailored to your household size, the roof catchment area and water requirements.
Sustainable materials consultant David Baggs, who founded the green building service Ecospecifier, says there is no point installing rainwater tanks until a house or apartment is water-efficient with low-flow showerheads, tap aerators, dual-flush toilets and pool blankets on any swimming pools.
"Efficiency is the cheapest thing you can do. People are keen to put in tanks but [it's] better to reduce water use inside the house before worrying about what to put outside," he says.
Sydney Water spokesman Brendan Elliott says most people put in tanks because they want the flexibility to water their gardens outside restriction periods. Since the rebate was introduced in October 2002, 29,400 tanks have been installed in Sydney.
Eco Living Centre founder Alastair Duncan says gardeners are keen to invest in tanks but he suggests households implement water-efficiency measures as they may be able to reduce tank costs.
Most people spend between $2500 and $3000 to install a tank. Round tanks are the cheapest to install but take up the most space. Slimline tanks fit more readily on skinny blocks of land but often require concrete plinths as a base, which can add $300 or more to their already higher cost. You need to add the cost of plumbing, gutter guards, installation and extra requirements such as pumps, first-flush diverters and mains back-up devices.
Duncan says retro-fitting tanks to existing houses makes it difficult to maximise the roof-harvesting area to collect the rain. "You can usually only collect water from one side of the roof with a retro-fit," Duncan says. "That means you get only half of what you could harvest."

The tank needs analysis

• Sydney Water says coastal suburbs receive higher rainfall than inland suburbs. Go to the "Recent & Climate" section at www.australianweathernews.com to find the rainfall statistics close to your home.
• Work out your roof's rainwater harvesting capacity by calculating the volume of water it can collect and whether all downpipes can connect to the tanks. Go to www.harvestingrainwater.com for detailed equations and calculators.
• To calculate your household's water storage needs: the litres you wish to use daily from rainwater storage multiplied by the average number of days your area can go without rain equals the litres needed for storage. You then need to work out whether your harvesting capacity can match your storage needs.
• Sydney Water can pay a two-part rebate of up to $800 to people installing large tanks that are connected to washing machines or toilets. See www.sydneywater.com.au for details - but don't expect the tanks to pay for themselves for some years. "If you are a hard-nosed bean counter, then you probably won't want a rainwater tank," says Sydney Water.