A South African professor has invented a hi-tech "teabag" that can purify polluted water instantly - at a cost of just three cents a litre.

The filtration system is so small it fits into the lid of a water bottle, and has already been hailed as a breakthrough in the battle against waterborne disease in poor areas.

It was developed using tiny nano-fibres - each about one hundredth the width of a human hair and packed together into a teabag-like sac that filters out microscopic bacteria. Instead of tea the nanotech bag contains activated carbon which then kills the harmful bacteria.

"Not only do we filter the bacteria out but we also kill them," said inventor Professor Eugene Cloete, a microbiologist at the University of Stellenbosch.

He said the new technique was a major step towards helping South Africa fulfil its millennium goal of eradicating poverty.

"One of the major problems of meeting the millennium goals is that there is not sufficient infrastructure to pipe water to everybody in the world - so this really is something which addresses the need right now," Cloete said.

The bag had already prompted several inquiries from foundations providing relief to those without running water.

"People are not dying of thirst but because they don't have access to safe water. This is inexpensive. It would cost about three cents a litre to produce water that is the quality of bottled water," Cloete said.

The breakthrough coincides with the launch this week of a major Stellenbosch University initiative called the Hope Project, which hopes to combine the scientific expertise of several departments to address the country's most pressing needs.

The university's rector, Professor Russel Botman, said: "Whether it is renewable energy supply for the region, food security . conflict resolution and leadership or rural healthcare and development, we are looking to throw our weight behind the country and the continent's most pressing needs."