The new technology demands a return to the tradition of drinking surface water instead of groundwater, says Prof M Fakhrul Islam, the inventor.

The devise, christened 'Chulli Water Purifier', is made of a hollow aluminium coil and some plastic pipes. It makes use of the heat generated by a clay oven (Chulli), common in rural Bangladesh, during cooking to treat water.

Over 650 households have already adopted the technique and the Unicef has successfully field-tested around 500 purifiers in the last couple of years, says Fakhrul, a retired professor of applied chemistry and chemical technology of the Rajshahi University.

The purification process involves passing surface water through the hollow aluminium coil connected to a clay oven. As water passes through the coil, its temperature rises to 70 to 80 degree Celsius, killing the pathogens that cause diarrhoea and other water-borne diseases.

The hot water is then collected in a traditional earthen pitcher called Kolshi and used for drinking when it cools down.

"The system is simple and inexpensive, and laboratory tests have proved its efficacy. It also helps the poor villagers avoid the extra cost needed to boil water, like the urban people do," the professor notes.

A number of scientists agree that if the new technology is promoted and put to extensive use, it may gradually substitute the 'tube-well technology', which in the past two decades has proved unsafe.

The Integrated Approach for Community Development (IACD), a local non-government organisation that focuses on developing affordable water and sanitation technologies, introduced the Chulli technology last year, informs David B Nunley, representative of Wagtech International, a UK-based organisation that supports such locally developed techniques.

Considering its affordability and prospects of community acceptance, the Department of Public Health and Engineering (DPHE) in association with the Unicef engaged the IACD to field-test the Chulli technology in areas, where 90 to 100 percent tube-well water is contaminated with arsenic above the acceptable level.

The rural poor cannot afford to buy fuel solely to boil drinking water, says Nunley, so the Chulli technique offers a perfect solution by using the heat of cooking ovens.

According to a study of the Bangladesh Council for Scientific and Industrial Research (BCSIR), more than 80 percent of the heat produced by rural ovens is wasted to the atmosphere. Only a small percentage of that heat is needed to kill the bacteria in surface water.

The Chulli system uses simple gravity flow to pass water through the coil. Water is poured into a 25-litre plastic bucket placed on a three-foot high stand made of bamboo.

The bucket contains 12kg sand and is connected to the input end of the aluminium coil by a locally made connector or valve fitted at its bottom. The sand filter frees the surface water of all organics, algae and foreign matters before letting it pass through the coil.

Once the heat from the Chulli reaches the cooking temperature, the plastic valve at the bottom of the bucket is opened, releasing the water into the coil. The water is allowed to run through the coil at the specific flow rate of 500ml per minute. As it flows through the coil, it gathers heat until it reaches the exit temperature of approximately 70-degree Celsius and comes out at the other end of the coil through a plastic faucet.

"In view of the very high production rate of bacteria-free water from ordinary surface water sources, the Chulli technology has potentials of mass application in areas where ground water is arsenic-contaminated but an abundant supply of surface water exists," observes DPHE Superintending Engineer Ihtishamul Huq.

"A major challenge for arsenic mitigation has been to find an alternative water source, which is free of germs and chemicals, can supply safe water all round the year, is inexpensive and easy to operate, and is well accepted by consumers. The Chulli purifier seems to meet all of these conditions," remarks Unicef Project Officer for Water and Environmental Sanitation Rick Johnston.

"It is inexpensive, easy to operate, and has little or no recurring costs. It quickly purifies surface water, generating more than enough microbiologically safe water that a family needs a day," he says.