Rainwater tanks - not in my backyard!

Rainwater tanks - not in my backyard!

This thought leadership article by Iain Rabbits, a Senior Water Specialist Engineer at Harrison Grierson, is intended to provide you with insights and relevant information about the cost benefits of installing rainwater tanks for supplementary water supply in suburban areas. Our thought leadership articles on topical and specialist issues are designed to present the key points in an easy to digest and interesting manner.

Collecting rainwater from your roof has long been preached as cost saving and environmentally sustainable. The argument that rainwater is free because it falls from the sky is an uneducated approach to both economics and sustainability (of which economics should be a part).

Just to clarify, I am talking about the use of rainwater tanks as a supplementary supply for laundry, toilet flushing and outdoor usage. Obviously, if you live in a rural area with no reticulated supply, it is essential.

Yola Rabe wrote in the New Zealand Herald letter to the editor that her household reduced its water bill to $18.53 for the month after installing a rainwater tank. But, and here is the big BUT, she doesn’t tell us the cost of installing all the rainwater collection, storage and treatment systems. Most people regard the cost of a rainwater system as the price of the tank itself.  However, this is a bit less than half the overall cost, which can amount to over $10,000 by the time you’ve included connecting the spouting, installing a concrete tank base, first flush diverter, pump, filters and pipes connecting all your laundry and toilets. It isn’t cheap. Then, there’s the ongoing cost of maintenance and regular cleaning of the spouting and tank!

In my role as a water and wastewater specialist engineer , I’ve looked at installing rainwater tanks for 6 areas in New Zealand, including the 4 cities in Wellington. My research showed that to make it viable to have a reliable water source, your tank needs to be at least 20,000 litres. Based on real rainfall figures, this will run dry once every two years. And when it runs out, what do you do? You don’t stop going to the toilet. You fill it from the mains, which need to be sized to cope.

And here’s another thing. The cost of producing water is between 85% and 95% fixed cost and only 5-15% variable. The fixed costs are things like operators’ salaries, vehicles, maintenance, overheads and the big one, depreciation. The variable costs are typically power and treatment chemicals that are used to make the water safe to drink.

So let’s assume for a moment that we all install rainwater tanks and cut our usage by 50% - way to go. This means we’ve reduced the variable cost by 50 % - but we still have to pay the fixed costs. We’ve saved around 15% to 7.5%. But the water suppliers still have to pay their operators the same amount and still have the same amount of infrastructure to maintain and so to cover these costs they increase the unit price of water. This means that if we all install rainwater tanks, the price of water would go up and our actual savings would only be between 2.5 and 7.5%. Additionally, would putting in rainwater tanks actually delay the need to build a new water source as Watercare want to do? The answer is probably not and here’s why: There are approximately 440,000 dwellings in Auckland. To make the rainwater tanks a real possibility, you need about half the houses to install rainwater tanks at a cost of… wait for it…$2,000,000,000. Yes, that’s right - over 2 billion dollars!  You can build a lot of plant, dams and pipes for that.

There are other problems too. The Resource Management Act specifically forbids changing the rules on people once they have built their houses. So Watercare and, for that matter, Auckland Council have no means by which this could be implemented and enforced.

Under the Local Government Act, councils have to evaluate their projects for sustainability defined as social, environmental, cultural and economic. Other than the visual eyesore of a tank, there’s no real measurable social impact. Cultural benefit, - there are others better placed to comment than me. Environmental – just where is the benefit? Certainly not in stormwater management. There may be a benefit in not building a dam but we have to drill the oil to construct the polyethelyne tanks or liners. I don’t see the environmental benefit of over 200,000 storage tanks even if we had somewhere to put them. Economic – are you kidding!

My advice if you want to make a real difference to your water usage and defer the cost of a new water source, is to buy a low flow shower head for a couple of hundred dollars. If you really want to save oodles of water, buy a front loading washing machine.  Both options are much cheaper than rainwater tanks and are far more effective.

Back to HG Perspective