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Crisis? What water crisis?

By Ian Mott - posted Friday, 23 June 2006


The average household uses 700 litres a day or 255 kilolitres a year at a cost of 94 cents/kL or $240 a year. If that money was used to pay off a mortgage at 7 per cent interest over the 25 year tank life, then a bank would give you 11.65 times the annual payment. But in this case you are not investing for an additional benefit but simply replacing an existing outlay. Capturing your own expense is one of the lowest risk investments you can make. So it is more appropriate to use the official cash rate of 5 per cent which also matches the average yield of a rental house. And this would set the capital value of your $240 water bill at 14.09 times, or $3,380. Spend more and your tank water will cost more than council water.

That is, until the next price increase. Projected to be 6 per cent a year, they will double in 12 years from $0.94/kL to $1.90/kL and this differential will improve the rate of return on your investment accordingly. But even without these savings, the largest 27,000L polytank will give you $300 change from your $3380 break even point. But this is serious overkill for a house connected to mains water.

The average roof is 250m2 and each millimeter of rain makes a litre of water on each square metre of roof. And all of it can go into the tank. Your existing supplier, on the other hand, has trees and pasture in their catchment so only 5 per cent of their rain will flow into their dam. And their storage has no roof so half of that will evaporate to give a net efficiency of 2.5 per cent compared to your 100 per cent.

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Without recycling, the average Brisbane household needs only 1,020mm of rain (89 per cent of average) to supply all of their water needs. The spread of both rainfall and water use enables smaller tanks to be refilled many times whereas the public water system relies on one big storage that is only filled every five years. And this is a major contributor to public water inefficiency. The annual cost of your tank is spread over the number of times it fills each year, many times its own capacity.

All other factors being equal, the smaller the storage, the more times it will be filled and the lower the apparent cost of a captured tank full. But don’t go out and buy the smallest tank available. The smaller the tank, the sooner it will overflow. A 1,000 litre tank costing $430, connected to 250m2 of roof will overflow after only 4mm of rain so most of those valuable summer storms would be wasted. The tank may fill up 264 times but the projected cost of only 12 cents/kL would be illusory because only a small portion of it would actually come out of the tap.

A 22,000 litre tank costing $2,700 would need 88mm to fill and it would lose much less to overflow. It would only fill 12 times a year to produce a projected cost of 72 cents/kL. But even this size will still lose 1.5 tanks to overflow in an average year and will need a 1.12 tank top up from the mains in a dry season. And obviously, the supply of your own water will drop in a bad year and the cost of the tank will be spread over fewer full tanks. In a year with only 81 per cent of average rain (857mm) the average cost of water, including the additional purchases from the mains, will rise to 89 cents/kL. In an above average year like the past 12 months with 1245mm of rain, the overflow will be greater but the amount drawn from the mains will be lower and the average cost will fall to only 82 cent/kL.

Interestingly, once you have installed an adequate storage capacity there is very little sense in adding lots of additional water-saving devices as these will only reduce the total volume of water against which the cost of the tank is defrayed. These gadgets are fine in very low rainfall zones but on the coast they can make your tank water a lot more expensive than mains water. And let’s face it, it is your water from your roof, so feel free to use as much of it as you like, as you like. Go on, you can even hose your driveway, it will only go down the drain if you don't.

The optimum mix of limited mains extraction in dry times and limited overflow in wet seasons was met by a 13,500 litre tank costing $2,050. This would need additional outlay on foundations and plumbing but this would be covered by the $850 worth of council rebates. Beattie has recently announced a $1,000 rebate but details are still unclear. But even under the old rebate it would deliver an average water cost of 69 cents/kL in a normal year and 74cents/kL in a dry year. And only 13 per cent of total household use would be drawn from the dams, enabling them to perform their proper role as a supply of last resort to an urban community that has taken all reasonable steps to help itself before putting its hand out for a public service.

Our much-maligned farmers have been doing it for decades.

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About the Author

Ian Mott is a third generation native forest owner, miller and regenerator from the Byron hinterland. For more information on the "New Farm States" campaign contact Ian Mott at talbank@bigpond.com.au. Discover more Bon Motts here.

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