"The history of men is reflected in the history of sewers," French 19thcentury author Victor Hugo wrote in Les Miserables. "The sewer is the conscience of the city ... A sewer is a cynic. It tells everything."

Judged by its sewers, the world is not doing well. Only three in 10 people now have a connection to a public sewerage system.

And with the world's population expanding, a goal of improving sanitation by 2015 is slipping out of reach, despite progress in nations such as China and a few big contracts for firms such as Veolia or Suez to build waste treatment plants in cities from La Paz to Rabat.

Experts say a part of the solution, especially to cut water-borne diseases for the rural poor, may lie in renewed and smarter exploitation of nature - for example through plants or soil bacteria that feed on waste.

Novel schemes include a plan to build an artificial wetland at a jail in Mombasa, Kenya, to process sewage from 4,000 inmates that now flows untreated into a creek, or ponds in South Africa where algae purify waste and are then used as fertiliser.

"About 90 per cent of the sewage and 70 per cent of the industrial waste in developing countries are being discharged untreated into water courses," said Achim Steiner, head of the UN Environment Programme (UNEP).

"Understanding the ability of peatlands, of marshes, of wetlands to play an integral part in filtering ... waste water is often overlooked," he said.

The UN set a millennium goal of halving the proportion of people with no access to sanitation - even simple latrines rather than sewers - by 2015 from 40 per cent of humanity or 2.6 billion people now: "Africa is probably struggling the most," Steiner said.

A 2007 scorecard showed the sanitation goal was likely to be missed by 600 million people worldwide on current trends. 2008 is the UN's International Year of Sanitation.

France's Veolia, the world's biggest listed water supplier, says East Asia and the Pacific are progressing best. In Africa, the company's only big contract so far is to supply water and sanitation to three cities in Morocco, with investments totalling €2.2 billion.

"A lot of countries underestimate the effect of sanitation on health," said Pierre Victoria, head of International Institutional Relations at Veolia Water.

UN data show a child dies as a result of poor sanitation every 20 seconds - that is 1.5 million preventable deaths a year from diseases such as diarrhoea or cholera.

In many countries "we are disappointed by the lack of interest of the politicians about water issues," Mr Victoria said. "We'd like to have new contracts in developing countries but we need contractual, legal and financial security."

Proper sewers, with pipelines and treatment plants, are prohibitively costly for many nations. As a sign of low ambitions, the logo of the International Year of Sanitation shows a latrine built above a hole in the ground.

Among lower-cost projects, prisoners at the Shimo La Tawa jail in Mombasa, Kenya, will soon start work on an artificial wetland where plants will act as a sewage processing plant in an experimental $117,000 scheme.

"This technology costs very little both for construction and maintenance," said Peter Scheren, manager of joint UNEP-Global Environment Facility projects in Africa.

The scheme will also include a fish farm - fed by waste water purified by two artificial wetlands each 55 metres long, nine wide and two deep. If it works, the fish can be eaten by prisoners, or even sold.

Such wetlands can have other spinoffs. "There are experiments going on in Tanzania where types of grass for roof thatching and basket weaving are grown on wetlands," he said.

Many scientists say natural systems, such as wetlands, forests or mangroves, are worth more left alone rather than cleared for farmland because they supply free services such as food, water purification or building materials.

"For sanitation it's much better to get nature on your side," said Dag Hessen, a biology professor at Oslo University.

Global warming may aggravate water shortages for hundreds of millions of people, for instance by disrupting Africa's monsoons or by thawing Himalayan glaciers whose seasonal meltwater now feeds crops from China to India.