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Water, water everywhere…
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15/01/2007
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It has been estimated that more than half the hospital beds across the world are occupied by people suffering from water-borne diseases and that more people will die from such causes this century than from the effects of either AIDS or climate change.
Anybody who has travelled to hot countries will know the problem. Even chlorinated water from the tap is liable to upset the workings of the human digestive system, while drinking from an untreated water supply almost guarantees a hospital visit.
Sterilising tablets are liable to make one nearly as ill as the disease and the only true remedy – which, you will observe, is religiously followed by many of the locals – is to boil any water to be used for drinking for 20 minutes, timed by the clock.
Because of the well-known nature of the problem, and the huge number of people affected, we have seen many “solutions” to the problem in terms of ultra-fine filters and solar stills -- but most are expensive, or need access to a mains electricity supply. One good idea that appealed to us – until we tried it – is to dig a hole in the evening, place a piece of plastic sheet in it weighed down by a stone, place a mug beneath it and see what you find in the mug in the morning. It’s not exactly ‘potable’.
The Challenge
Our challenge this month is to come up with a way of supplying clean water to people either living or travelling in a rural area in a tropical country. It needs to be simple, reliable, rugged and cheap enough to supply to large numbers to relatively poor people. It must not rely on mains electric supply.
The solution offered is expected to solve the problem elegantly and at low cost. However, there must be other ways of solving the problem. November’s challenge prompted an alternative answer from a reader – which is printed in our new ‘Technology Briefs’ section.
Solution to Coffee-time Challenge
The solution to this month’s challenge comes from Professor Colin Humphreys, Goldsmiths’ Professor of Materials Science at the University of Cambridge. He points out that all life has evolved on earth because it is shielded from hard ultraviolet rays by the ozone layer. Hence ultraviolet light at 270nm – as produced by LEDs made of aluminium gallium nitride – disrupts DNA and RNA sufficiently well to kill any living organism. Ultraviolet light sterilisers, using various types of mercury discharge lamps, are commercially available. But they are quite delicate, and use too much power for rural village use. LEDs last 100 times longer and typically use about one tenth of the power – so might one day be powered by solar photovoltaics. However, Prof Humphreys says: “Their efficiencies are still much too low for flowing water.” There is also a need to find some better way of keeping the interface between the light source and water clean. Conventional UV sterilisers with dirty tubes inspire false confidence in the water they process. Research aimed at producing better, commercially viable products continues.
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Author
Tom Shelley
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