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26 November, 2007

Sterilising My Drinking Water The Easy Way

One of the many things about living out here in the bush with which I am unfamiliar, is the way water is collected and treated. My new house has three separate water collection systems. One is a ginormous plastic tank which collects the run-off from the house and shed roofs. The next is a small pond (or 'dam' as we call them here) that collects water that runs off the ground. The third is a pair of large plastic tanks which constitute a waste treatment plant for sewerage and other domestic waste water. The waste water plant generates relatively clean water which it then pumps out into a garden sprinkling system. The dam water is untreated and also has a pump, which we can use as required for garden watering or whatever. Water from the ginormous plastic tank that catches rainwater from the roofs, is pumped up to the house to provide our domestic supply.

The dam water and the treated waste water don't bother me. We only use them on the garden (or will, once we have a garden). It's this rainwater/drinking water system that bothers me. This water comes off the roofs straight into the tank where it sits for very long periods before being pumped into the house. The tank is closed (apart from two fat overflow pipes with a mesh over their ends) but the water that flows into it comes from the roofs and gutters. Apart from whatever airborne dust, smoke, pollen, and other organic matter landing on the roofs, there must inevitably be bird droppings and dead insects falling onto them all the time. Surely this means the water can't be quite sterile and must have quite a lot of stuff living in it?

It's not such a big deal because Wifie and I never drink unfiltered tap water anyway and any other water we consume in our food is always put through some kind of cooking process that would sterilise it. Yet it is just a little bit unsettling that the water we consume has been used to wash a roof with and has then sat in a big tank in the hot sun for weeks or months before we get round to pumping it into the house! Also, the fact that more and more people around the world are drinking re-cycled rainwater from just this kind of system and no-one is jumping up and down and saying what a health hazard it is, is actually quite reassuring.

Yet I have been wondering what to do to remove any risk whatsoever. And I think I have the answer: make rainwater tanks out of clear plastic.

I came across this idea in August 2000 when New Scientist reported on an Oxfam meeting that discussed the use of solar disinfection to combat a shortage of chemical disinfectants in Assam, India. (Issue 2253, New Scientist magazine, 26 August 2000, page 14. You may need a New Scientist subscription to read this online. Otherwise, your library probably has it.) A team of Swiss researchers in Duebendorf has show that filling a plastic bottle with water and leaving it in the sun can effectively disinfect it in as little as one hour. Martin Wegelin, who headed the Swiss research team, says that if you paint half the plastic bottle black and stand it on corrugated iron, it will heat up much faster and cut the time needed for thorough disinfection. The combination of heat (the water temperature goes above 50 degrees C) and ultraviolet radiation kills most micro-organisms, including 99.9 per cent of Escherichia coli, Vibrio cholerae (which causes cholera), and the parasite Cryptosporidium (which causes severe diarrhoea). This last is particularly interesting to me since I remember an outbreak of Cryptosporidium and that other popular faeces-borne parasite Giardia intestinalis that hit the water treatment plants in Sydney one year when I was living there and meant that we all had to boil our tap-water before using it for several weeks until they got the outbreak under control.

So, if it works for a couple of litres in an old Coke bottle, maybe it also works for five thousand litres in a clear plastic rainwater tank. If no-one has done the science yet, remember you heard it here first and don't forget to add my name to the patent, please.

1 comment:

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