A new device uses sunlight to disinfect water

Clean water: A remote possibil a water purification device developed by scientists at the Mumbai-based Bhabha Atomic Research Centre (barc) could provide safe drinking water at a low cost even in remote, non-electrified parts of the country.

Capable of producing up to 20 litres of potable water every day, the device requires sunlight and a cheap, abundantly available material -- titanium dioxide -- used as a photocatalyst. In the presence of ultraviolet rays of the sun, titanium dioxide becomes chemically active and kills bacteria.

The device consists of a steel plate (about 1 square metre) coated with titanium dioxide and a plastic tray of similar size. The steel plate can be fixed to the bottom of the plastic tray, which holds the water meant for treatment. A glass plate works as a dust cover.

Though powdered titanium dioxide is used in remediation of wastewater, it is not considered desirable for purifying drinking water as its separation from water is not easy. The barc scientists, led by chemist Arvind Belapurkar, overcame this problem by coating the inside surface of the tray which is meant to hold the water with titanium dioxide. When water in the tray was exposed to sunlight for about four hours, the concentration of Escherichia coli, a common bacteria that causes diarrhoea, dropped from 100,000 bacteria per millilitre to almost zero level, the scientists reported in the July 10 issue of the journal Current Science (Vol 91, No 1). E coli is the toughest of disease-causing microbes found in raw water.

The scientists say that inorganic impurities such as common salt present in the water do not affect the efficiency of the device. But water with organic impurities should be first treated with activated charcoal to reduce concentration of organic compounds to six parts per million for the device to be effective, they add.

The scientists say that the photocatalyst maintained its bactericidal activity at the same level for almost a year. According to Belapurkar, the system is ideal for rural areas that lack electricity and chemicals required for water treatment. "If mass-produced, the price of our water purification system could be less than Rs 1,000 and also there is no recurring cost involved," he adds.