Rainwater tanks: a drop in the bucket

G Magazine

As the belts on our water resources tighten, tap into your own water supply with a rainwater tank for your home.

Eco Sac Rainwater Tank

Credit: Eco Sac

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As water supplies become more precious with every drop it seems to be imperative that we catch each millimetre of rainfall that we can. Installing a rainwater tank at home helps to ease pressure on our struggling dams. And while a tank is common place for many rural Australians, they have been popping up in our cities too.

In Australia’s highly urbanised zones up to 90 per cent of rainfall can flow into the stormwater system. In Sydney alone, it’s estimated that 420 gigalitres of stormwater goes straight out to sea every year. That’s the equivalent of almost the entire contents of Sydney Harbour.

“If you’re capturing rainwater, you are stopping water from running into the stormwater drains and flushing out to rivers and to the ocean... If you use rainwater on the garden, it redirects that water into the ground where it should be,” says Colin Nash, Chair of the Rainwater Harvesting Association of Australia.

“Rainwater doesn’t come through sedimentary rocks or across the earth and rivers, so it’s often very low in calcium and magnesium. So, for washing windows or cars it’s brilliant because it doesn’t leave any residue. It’s what’s called ‘soft’ water, and it’s good for washing dogs, people and hair, and it is better for household appliances such as washing machines and dishwashers,” says Nash. “It has low salinity, and in terms of drinking – provided it’s disinfected in a simple way – it’s a pleasure to drink.”

“It’s also good to be able to use water that you’ve captured, there’s a feel good factor there. To watch your rainwater tank fill up when it’s raining, it connects you to the environment, and it’s guilt-free.”

Made to measure

“One of the things people make mistakes with when capturing rainwater is they don’t stop and think what roof area they’ve got and how much roof area they can use for water capture,” says Nash.

To work out how much water you can collect, and therefore the size of tank you should get, first work out the average rainfall in your region. “As a rule of thumb if one millimetre of rain falls on one square metre of roof area you can capture one litre of water. For example, in Sydney with a 2000 L tank, you can catch 68,000 L per year from 100 square metres of roof area.”

Nash advises that people should check that you have enough downpipes to feed into the tank. “There’s no point buying a big tank and only connecting it to one downpipe on one part of the house, it just draws off a small roof area, and you won’t get the most out of your tank.”

Jackie Hammond from Rainwater Harvesting advises to “check your roof surface, make sure that it’s appropriate for collecting water – most people’s roofs are.”

“It’s important that people are aware if they’ve got old roofs that have lead flushings... It doesn’t mean they can’t harvest the water, it just means that you might need to be careful if you were going to drink that water if you had lead flushings, but there’s no dramas to use that water in your garden,” says Hammond.

Choosing a tank

Tanks have changed over recent years, and we’re now spoilt for choice – no longer do they need to be the old bulky concrete monstrosities. A huge variety of sizes, styles and colours are available, allowing you to fit a tank around your home and the space you have available. While traditional round tanks are still around – commonly in plastic or steel – slimline tanks that are typically long and as small as only half a metre wide have become popular for fitting in small urban spaces, while a bladder tank can fit under the house or water walls can double as fences.

Depending on the rainfall in your area, it is possible to become completely self-sufficient on rainwater, but you will need a very large tank. “A typical household with four or five people might be using around 300,000–400,000 L per annum in the house. We do know of cases where people have had a good surface area, much larger tanks, and are virtually self-sufficient,” says Nash.

“Typically though, if you have a smaller tank that’s around a couple of thousand litres and you connect it to your toilets in the house and to your washing machine you can probably save 30 to 40 per cent of your water usage with that.”

Hammond warns that it’s important to think about your rainwater harvesting system as a whole, and not only about the tank. A rainwater system could include devices that keep the water clean (see ‘Clean and healthy’ box), a gauge to show how much water is in the tank, an overflow, a pump, and an air gap to prevent backwash from stormwater drains. There are also devices that switch the water automatically from rainwater to mains water if you’re using a combination of both. “It’s low cost to add these products in, but they can make a big difference,” she says.


Installing a tank can cost anywhere between $200 and $4,000, depending on the system you choose. “Typically, people are installing between 2,000-3,000 L tanks,” says Nash. A fully installed system connected to the house with a 2,000-3,000 L tank costs around $2,000-$3,000, including the tank, pump, accessories and plumbing.

While there were previously rebates available from the Federal government, these were dropped in May, though there are still a number of State government and council rebates around for households installing new rainwater tanks. For information specific to your area check with your State government and council.
Hammond says that DIY-installations are best left in the garden. “Using a tank on your garden is pretty straight forward. It’s just a matter of making sure that the place they’re putting their tank on is level and stable. Normally, you can get a tank positioned where you need it when it’s delievered to you.”

It is possible to put a small tank in on a downpipe and to fit your own pump. Nash says “you can buy do-it-yourself type pumps from pump shops, hardware and plumbing chains and connect them to your sprinkler system.”

“But if you really want to save water and do the best for the environment, you should think about connecting it inside the house.” You will need a licensed plumber to make those connections and to help choose the right pumps and accessories. Ask your tank supplier to recommend a plumber that is familiar with your make of tank.

“The best time to capture rainwater is right through the winter and rainy periods… often in the summer when you want to put it on the garden, there’s no rain in that period,” says Nash, so consider installing your water tank during the rainy period, so it can fill up and quickly become functional.

Clean and healthy

There is constant debate and opinion that surrounds drinking tank water. “This is always an interesting topic because there are so many people that drink and live off rainwater. A lot of people in rural communities have a basic setup and don’t have any dramas,” says Hammond. There’s a lot of research to show that rainwater is as good as mains water, but it always of course comes down to the individual making sure that they’re looking after their tank appropriately.

To keep your water clean make sure the tank is well screened to keep out animals that can potentially contaminate it. Ensure you also keep it free of debris and organic matter such as leaves, and ensure you have an air gap if your tank flows into stormwater, and a first flush device.

“First-flush devices are a really important part of keeping your water clean. They divert the first flush of dirty water away from the tank. When you have a rainfall essentially the rain washes the roof… that has stuff like debris, bird droppings, things like that in it. And so that water should get diverted away from the tank.”
You will also need to do some occasional maintenance cleaning in your tank. Every three years or so check to see if you need to clean any sludge out of the bottom of the tank. You can do this yourself or hire a professional. Additionally you can look into siphon vacuum products such as the Tankvac that automatically keep your tank clean, improving the water quality.

More info

For help choosing the right rainwater tank for your area according to rainfall, household size and water usage, check out the ARID website at www.arid.asn.au. The Save Water website (www.savewater.com.au) is also a good source of information.

Smart Approved WaterMark (www.smartwatermark.org) are Australia's certification label for water saving products and services. They have created, along with the Institute of Sustainable Futures and Bureau Of Meteorology, Australia's first rain tank calculator, available from here. This is also available free on mobile platforms for iPhone and iPad as iSaveH2O – also an Australian first.