Do rainwater tanks save water?
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% reduction in mains water use by households with rainwater tanks plumbed to the house (Source: Moy)
Research published in the latest issue of the journal Geographical Research indicates that residents with a rainwater tank don’t reduce their consumption of mains water by any more than those without a tank.
The study finds households in NSW’s Illawarra region who installed a rainwater tank reduced their consumption of mains water on average by 10.3%. Households without a water tank reduced their consumption on average by 10.8%.
On the face of it, this is a worrying finding given at least one State Government is now promoting the virtues of rainwater tanks as an alternative to the astronomical capital and operating expense of desalination solutions to water security.
Researcher Candice Moy from the University of Wollongong examined water consumption by all households in NSW’s Illawarra region (Wollongong and Shellharbour municipalities) from 2002 to 2009 (you can read the full paper here). Level 1 water restrictions were made law in 2003, Level 2 in 2004 and Level 3 in June 2005. The latter meant gardens could only be watered by hand-held hose on two days per week, between 4pm and 10am.
7,125 households (about 8% of all households in the region) installed a rainwater tank in the three years between 2005 and 2007. Their consumption was documented for two years prior to and after installation and compared against all households in the region. Ms Moy also conducted interviews with a small number of these households and analysed a separate survey of tank-owning households in the Illawarra.
She reckons there’s a straightforward explanation for why rainwater tanks make no difference to mains consumption. The great bulk of households who install a tank don’t do it in order to reduce overall water consumption or to protect the environment. Rather, the tank is a means of maintaining “luxurious water behaviours” in the face of mandatory restrictions.
Most tank-owning households aren’t concerned, she says, with saving water but with using it. Since they collected it off their own roofs, they see rainwater as a private good. It’s a commodity they’re free to use in any way they wish, whether for gardens, pools, or hosing the driveway.
This interpretation is supported by the small proportion of tank-owning households – just 10% – who use rainwater for internal uses, notwithstanding the availability of a connection subsidy. About 5% connected their tank to either the washing machine or the toilet , and another 5% connected it to both. The latter group was the only one to show a substantially larger reduction in mains use (16.1%) than households without rainwater tanks (see exhibit).
The writer says her study challenges the finding by the ABS that 41% of households who install rainwater tanks do so to “save water”. On the contrary, the research shows “that water for outdoor usage was the clear motivation for the large majority of participants”. She also argues her findings suggest the proportion of household water used for outdoor purposes is much lower than the 28% assumed by water authorities.
Her conclusion is tanks have the potential to generate significant water savings, but water-intensive behaviours have to be modified. If tanks are not plumbed indoors:
the potential of this alternative water source will not be met. Currently, rainwater tanks facilitate water users as much as water savers.
I have some misgivings about this paper, though. It’s ambiguous in parts, relevant information isn’t always shown, and some comments seem contradictory. Nor is there much effort to canvass other possible explanations.
It’s not discussed in the paper, but perhaps households with tanks didn’t have the scope to make bigger savings simply because there was insufficient rainfall to fill them often enough. Or perhaps, as cynics often suggest, some households filled their tanks from the mains system.
Another point is households who installed a water tank between 2005 and 2007 were already on Level 2 restrictions from June 2004, limiting them to hand-held hosing on three days per week. Their scope to reduce mains water consumption further would accordingly be limited. There’s also the “rebound effect” – households who install a rainwater tank might now feel it’s OK to (say) take longer showers with mains water.
The immediate threat of drought has receded across most of Australia, but it would be unwise to ignore the possibility it could come back sooner rather than later. Governments see rainwater tanks as having a major role in water security, so it’s important their limitations are properly understood. Notwithstanding some reservations on my part, Ms Moy’s findings certainly warrant close examination by water authorities.
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Maybe our circle of friends are different to the fine people of Illawarra but those that are now harvesting roof water for their gardens claim they have not increased their water consumption elsewhere. The whole water awareness campaign has made us more sensitive to setting consumption targets. We find the State Premier’s attitude to not setting targets regrettable.
Some of us have only tank water, have non-flush toilets, and plan our gardens so we only water vegetables and fruit trees, and mostly use our grey water for that.
I can feel a reality TV show coming on – The Biggest Users 22500 l Challenge.
We installed water tanks (in Melbourne) during the drought so that we could continue to water our garden without using mains (having foliage around the house keeps the summer temperatures down). There were no subsidies for us to install our tanks. To connect them so we could use them for the toilet was going to add considerably to the already high cost of the tanks and their installation. The water in our tanks is not of a quality to use in the house (too much vegetable matter, smells like a festy swamp).
Even during the drought, there was always enough roof run-off to top the tanks up. Now though the drought has broken, we still water the garden from the tanks during summer and autumn dry spells. This means that we do use less water from mains because we very rarely use mains on the garden at all. So we are using less mains water than we would have otherwise.
I installed a 3000 liter tank to use for growing food in a renovated back yard.
It was part of renovation that scrapped a brick extension to a 2004 period home.
I recycled the bricks to build two raised garden beds 8 meters by 2 meters wide and half meter high, in which I have planted 8 fruit trees with veggies growing underneath.
Weeding can be a done by sitting on the double brick outer bed walls which is great for my wife and I with bad backs. In the cool half of the year rain is enough to water the beds and the rest of the time we use the tank water to keep the plantings alive. We eat these home grown veggies and fruit and save water doing so. We do not need to buy water intensive food which uses a lot of energy to transport to local shops.
I wee on the garden beds reducing toilet flushing while providing fertilizer. The falling leaves of the trees, pruning and grass clippings, are blended with kitchen waste in bins and then dug into the beds after 12 months rotting in the bins.
I am only writing this to show one way to make better use of water in urban back yards.
“Rather, the tank is a means of maintaining “luxurious water behaviours” in the face of mandatory restrictions.”
Luxurious water behaviours like keeping the fruit trees and veges alive???
Was the amount of mains water used by houses with tanks compared to the amount used by those without tanks rather than just a % reduction in use? One might expect that those who care enough to install tanks might already be “water savers”, and so unable to further reduce water usage.
There’s a few things wrong with the study:
1 – it didn’t investigate whether there was much rainfall during those periods or if it was still a drought period with little rainfall.
2 – they also used poor data comparison methods for rainwater tank customers vs everybody else. They used water company records for the rainwater tank customers only. For the “everybody else” they used total area average data which is going to severly underestimate usage as it will account for houses with no occupancy, apartment buildings etc so I don’t think we should draw too many conclusions on rainwater tanks on this one.
Most legislation & funding assistance is requiring the use of rainwater tanks to be installed to at least the toilet and not just for the garden so while I agree that average figures likely overestimate garden water usage, if we start getting better installation practices, better filtration methods, we will see increased uptake of toilet & laundry rainwater use. In well installed systems its impossible to see that you’re using rainwater for toilets.
I also know from experience that rainwater installation practices used in the 2005-2007 period were poor, particularly with a frenzy of new suppliers getting involved because of funding opportunities which meant lots of poor installations – giving problems of poor roof collection area (I’ve seen 5000L tanks hooked up to 5m2 of roof area) , poor water quality through non-existent filtering and first flush devices, poor usage throughout the house (rainwater tank in back area not being able to be used to water the garden in the front area etc).
I think familarity with rainwater tanks is increasing so I would expect we will be seeing better results for newer installations.
I think this assertions underpinning the article are a massive generalisation. Many people in Melb put in a water tank so they could continue to water their vegies and fruit trees during almost 15 years of substantially below average rainfall. I don’t think producing your own food means you do it purely to maintain “luxurious water behaviours”.
I live in an outer metro area and effectively only use rainwater, with the mains kicking in if the smaller of the two tanks (30K capacity) drops below a predetermined level. Whenever I drink mains water whilst out, all I can taste is chlorine. Give me tank water anyday.
Microseris: The author is referring to the average punter in the Illawarra. I expect Crikey readers are, on average, not much like your average Illawarra punter. AD
Water is too cheap.
The ‘subsidies’ for connecting a tank to a toilet barely cover the cost of the plumber’s call out fee. For wiring, pipes, compulsory ‘no back flow’ valves and connection to a toilet and washing machine, I couldn’t get a quote under $3,000. This means we might start saving money some time in 2025.. the economic drivers become more important than the environmental.
Water from our 5,000l tank is just used for the garden. How much does a household have to spend to save the environment? That was, for us, too much – we have “better” things on which to spend the money.
Maybe when water is $6 per litre, it will become ‘worth it’.. but for now… $ first, environment.. distant second.
The premise is a bit flawed. If the aim is to save potable urban water, then diverting consumption from mains supply to local supply (rainwater tank) is achieving the goal, and potentially avoiding investment in additional infrastructure whilst maintaining amenity.
In Australia’s coastal cities (I think I’m safe to say) the rain that falls on rooftops would only end up running quickly out to sea as part of the stormwater system. It doesn’t supplement anyone else’s supply. (Canberra and other inland centres are a bit different).
Although even at fully cost reflective pricing, a plumbed rainwater tank isn’t justified compared to the cost of mains water, it becomes a way of shadow-pricing the value to the household of maintaining outdoor watering during restrictions (or assuaging urban guilt about outdoor water anyway).
None of the comments seem to have pointed to the crucial issue:
“This interpretation is supported by the small proportion of tank-owning households – just 10% – who use rainwater for internal uses, notwithstanding the availability of a connection subsidy. About 5% connected their tank to either the washing machine or the toilet , and another 5% connected it to both. The latter group was the only one to show a substantially larger reduction in mains use (16.1%) than households without rainwater tanks (see exhibit).”
The only way to save substantial amounts of mains water is to plumb the rainwater into the toilet (minimally) and w.m.
So obviously people who do not do this are doing it almost exclusively to maintain their garden. However I assume that new houses with rainwater tanks supply the toilet. If not then it should be mandated by law. The “deficiencies” are entirely related to government implementation. In Brisbane they introduced silly laws that insisted that when the rainwater tank was plumbed into the toilet that the rainwater tank itself was plumbed into the mains; it that usual bogey-man of OHS on the notion that it would be unhealthy if the rw tank ran dry and you couldn’t flush the toilet. This I think could be fairly described as Kafkaesque.
Thus the study reveals something but has nothing relevant to say with respect to rainwater tanks on urban water saving–which could be huge if done the proper way.
Incidentally AD you say that government is keen on rw tanks but I think you’ll find in all those cities that have gone the absurdly expensive desal route, that they have been quite cool on tanks–because, having committed themselves (ie. we, the ratepayers) to buying a lot of this super-expensive desal water whether it is needed or not, they hardly want to see us substituting rainwater! Indeed I think it was Melbourne or Victoria commissioned one of those accountancy consultancies to produce a report-to-order that concluded that rainwater was more expensive than the desal option!
Re comments by s Allyn (11:21 am) and Richard Scott (12:20 pm), you correctly point out the silliness that has been imposed by the authorities. Most of these regulations arose at exactly the point all major cities in Australia were planning to get major desal plants and it is no accident that they made it unnecessarily difficult to use rainwater as a substitute for several of the big water-using appliances. As has been pointed out there are still plenty of places, not too far from capital cities, which rely upon rainwater tanks for all water including drinking/cooking, and these do not have the same ridiculous regulations. My sister’s place in a swanky estate on the Sunshine Coast is exactly like that.
When the use of the water is not required to be potable (toilet, laundry, arguably shower but see below) there is no need for inhibitory and expensive regulation. Especially if the water is gravity fed (there is no need for pressure for toilet or laundry).
The absurdity in Brisbane is that mains water must be connected to the rainwater tank so that it fills (with mains water) if the level falls below a certain threshold. But the problem is that this means householders never know if the rainwater has been exhausted or not. (There is the separate but important issue of tank size; one really needs 20kL or more.) Clearly, with mains already provided to all appliances in the house there is no real reason for this silliness. It would be trivial to have a manual switchover valve, and with gravity-fed rainwater the (almost non-issue) of backflow is impossible. (Indeed this is the principle of the slightly peculiar English system where mains is delivered to a small storage tank, akin to the cistern of a flush toilet, that is at the highest point in the house–and the reason for the infamous feeble showerhead flow in Blighty. It is illegal to connect mains water directly to taps in the UK!)
I’m happy for people to have rainwater tanks if they pay for them themselves. Subsidy should not be provided however. Pretty much every major study (including the extensive work done by the productivity commission) demonstrates that rainwater tank subsidies are not a cost effective way of reducing water consumption from a community perspective.
Furthermore it has been shown time and time and again that rainwater tanks have little impact on infrastructure requirements.
Also keep in mind that rainwater tank policy is not always controlled by the water utilities in most cities, but by the development consent authority and health department, which in most cases is the local council and the state department of health. The only thing that comes from the utilitiy generally is the subsidy. Which as I have stated previously makes no sense as it is not cost effective from a community perspective.
And again whilst some people make the point that there are plenty of places that rely on rainwater tanks, they have roof material that are suitable for rainwater collection. Tiled roofs present enormous health risks for water collection and therefore in an urban context, almost all roofs would have to be treated at great expense, even ignoring the greater risk of airborne pollutants in built up areas.
Again this doesn’t suggest that rainwater tanks connected to toilets don’t save water, just that there are probably better options than subsidising rainwater tanks in terms of providing secure water supply to the entire community.
I think many cities have jumped the gun on desal plants but they are useful and I would imagine the probably show a better community cost benefit performance than rainwater tank subsidies.
We didn’t bother with the subsidy (because of the plumbing requirements and restrictions) but were happy to invest in the tanks for future benefit. We have the 2 separate systems so as to avoid our rainwater contaminating the mains supply and have to switch over manually (we also had to buy a pump as it’s not gravity fed). We’ve been off mains water for well over 2 years and only had to use mains for about a month when the tanks were low. It doesn’t save us a great deal of money and will take some time to pay off the tanks but we are very happy with it. If we could afford to we’d go solar power off grid.
Michael R James says “It is illegal to connect mains water directly to taps in the UK!” This may be the case now but certainly was not in the past. It was required that one tap – usually in the kitchen under the sink – was directly fed from the mains. Our house was built in about 1933-35 – just before my time so I was born into it. I suspect that this was to ensure supply was available in periods of frost (even internal pipes could freeze), or when the pipes got clogged up with chalk (very hard water area – less than 20 years to clog pipes.).
I recall that in Canberra water used to be supplied at a fixed charge per annum – no usage charge. This was changed during the late 1970s, much to the disgust of some gardeners, who thought that Canberrans should continue to be supplied with ‘free’ water to enhance its ‘garden’ setting. Rather ridiculous of course in a semi-desert area at best. When the fixed annual charge was replaced by a smaller fixed charge plus user charges, usage dropped. That is the way to go if you want to save water. Abolish fixed charges and charge high prices for usage.
Maybe those are the facts but it goes to show how slack, pathetid and aware that most people are. When I built my low energy mudbrick house 25 yrs ago http://www.fleurcom.org/Owner_Builder.htm I mistakely piped mains water to my house and installed rainwater tanks too. We quickly phased out the mains water and havent used mains for 20 years and have never regretted it as there have been no problems and no mains water use. They insist on billing me a small unfair quarterly charge which Im now trying to get waived
I live in Brisbane and installed a 3,000 litre water tank as the drought deepened. Now this tank is currently full and unused and as such is a stranded asset. My garden has been completely droughtproofed and no longer requires watering. The arrangment of my plumbing (hidden in walls) prevents me from easliy connecting it to the washing machine and toilets. The only viable use is to plumb it to the entire house water supply, and use the town supply to top up the tanks when they get low. I am not allowed to do this.
At the same time, I have a house at Noosa North Shore which is supplied from rainwater tanks, with a top up option from a mains supply. It is brilliant.
Certainly in Brisbane I no longer time my showers, nor collect water in buckets for use on the garden. My water usage has risen, but not wastefully. If I could put my tanks to good use, I would!
I did an expert submission to the Victorian Parliamentary Enquiry into Desalination of 2008. The report is viewable at
http://dl.dropbox.com/u/4385917/DesalHarrison.pdf
and the issues are just as applicable today.
In the paper I noted that the NSW government committed to its multi-billion dollar desal plant without meeting the prerequisite it had set for itself – dam levels falling below 30%. As the Southern Oscillation Index changed to a wetter forecast, the government realised that they had to approve the donors’ proposal before the trigger moved further away. But dry/weter periods are on decade cycles, so by purchasing a desal plant 5-7 years before one needs it, you face interest on a few billion, plus $100m/yr standby costs. Moreover, the technology may keep improving over the period.
I also noted that if the NSW government had intended to achieve water savings, it would have increased the ‘per kilolitre’ usage fee, but instead it increased ONLY the fixed supply fee, fearing that induced water conservation might make its desal plant look unnecessary (as it has done since commissioning, but due to high dam levels, not conservation).
The article talks about percentage savings, but not absolute savings, as a percentage basis favours the author’s argument. I suspect that the people who installed tanks were on average larger water users to begin with. Their 10% savings would be larger when expressed in kilolitres, than the 10% of those who do not have tanks. Besides the tanks have a very long life, and can yet be plumbed into toilets etc, if usage charges rise to the point where doing so becomes attractive. The average 100sqm house has >100,000 litres/yr running down downpipes – collectable at little cost, and we only drink around 1% of the water we use.
I use mains water in the city, but my Southern Highlands property uses only rainwater. The two 27,500-litre tanks collect from c150sqm of tin roof. The issue of terracotta roofing causing water quality issues is sheer rubbish. The far bigger issue is to not drink water collected in areas of poor air quality.
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