To a corps of hydrologists, the only way that parched regions of the world can meet the surging demand for water is to recycle -- and use -- the stuff that has already been through the human body.

Rather than throwing away water that results from treating urine, faeces and bathwater, the valuable liquid can be harnessed once more, they say.

It could go not just for farm irrigation or industry -- as is already widely the case in many countries -- but also for drinking water.

Presentations at the World Water Forum have been pressing the argument that "used" water, also called rather more gracefully "grey" water, should comprise a percentage of what comes out of our taps.

But specialists also caution that overcoming human repugnance -- could it be called a gut response? -- is a far greater challenge than the engineering.

"People hate the idea of drinking something that could have been sewage," said Gerard Payen, a member of the UN's consultative committee on water and sanitation, which reports to Secretary General Ban Ki-moon.

"There's a major psychological block. But it will go away bit by bit."

Windhoek, the capital of the arid southern African country of Namibia, has been using a "toilets-to-taps" system with success for many years.

But it's a rare example of public acceptance to date.

In Australia, inhabitants of the Queensland town of Toowoomba voted out of introducing such a scheme in a referendum three years ago. In other parts of the drought-wracked country, the idea encounters a shake of the head, but is at least being taken seriously as an option.

"Technically, using modern recycling methods, we are able to turn out water that is perfectly drinkable," explained Antoine Frerot, managing director of Veolia Eau, a French water company that has high stakes in this sector.

"Used water is a resource that is close to cities and its availability rises at the same rate of consumption," he said. "Recycling it uses less energy than desalination and avoids pollution."

According to Frerot's figures, drinking water extracted from an aquifer costs around 10 euro cents (eight US cents) per cubic metre and 70 euro cents (56 US cents) when taken from sea water.

Somewhere in between is recycled used water, at 45 euro cents (36 US cents) a cubic metre.

Faced with public suspicions, water companies are looking at indirect ways of water conservation. They include separation of drinking water and toilet systems, so that sea water can be used to flush toilets.

Another widespread practice is "indirect" sewage recycling. In other words, the sewage, once cleaned by treatment, is poured out into the local river or reservoir, which is drawn up by a different intake pipe as the source for drinking water.

This has been the practice for many years on the River Thames, for instance, where local utilities upstream extract and return the water several times before it reaches London.

Singapore, a groundbreaker in reuse, has a programme called NEWater, in which one percent of drinking water comes from recycled sewage effluent, which is added to the city-state's main reservoir.

"Passing the water through a 'natural environment' is a way of partially overcoming the psychological barrier and also brings in the ecosystem as an additional filter," said Jacques Labre, a specialist with Suez Environnement, another French water services company.

"The psychological barrier is still quite strong, but I think it will change in the future. It's a lot about trust in the technology," said Louise Korsgaard, an expert with Danish consultancy DHI and a researcher at Singapore's Nanyang Technical University.

Jerome Cartillier

AFP/Expatica