But the lack of sewage treatment for most of its people is typical of much of Latin America and other poor areas of the world, where improper sanitation poses health risks and destroys valuable resources.

Almost all the sewage from Costa Rica's urbanised central valley is pumped untreated upstream into the Tarcoles River.

"Fifteen years ago...the town lived on tourism. Now it's a ghost town," said Diego Vargas, 39, owner of Crocodile Man Tours, a company that takes tourists up the river.

Now the Costa Rican government hopes to clean up the river by constructing a waste water treatment plant in the capital, San Jose, about 100 km (62 miles) inland.

The plant will handle 3,600 cubic metres (127,100 cubic ft) of sewage a day to cover around 65 percent of the central valley's population, or more than 1 million people.

"Our rivers have become disgusting open sewers," President Abel Pacheco said in a ceremony with Toru Tokuhisa, executive director of the Japanese Cooperation Bank that will provide a loan to help fund the first phase of the US$450 million project.

In Latin America only about 2 percent of sewage is treated, according to the WASH initiative recently launched by the United Nations-backed Water Supply and Sanitation Collaborative Council, or WSSCC.

Over 40 percent of the world's population, some 2.6 billion people, live without access to basic sanitation facilities. Another 1.1 billion have no access to safe drinking water.

Residents of Tarcoles -- set in a lush tropical forest teeming with birds and other wildlife -- have set their hopes for a local revival on the new San Jose disposal plant.

"With the plant people will start to come back," said Manfred Lopez, 29, who waits tables at his family's restaurant, catering to the few tourists who still come through town.

THREAT FROM HUMAN WASTE

Sanitation needs are most severe in Africa and South Asia, where more than half the population uses open pit or bucket latrines with no connection to public sewers or septic tanks.

"Rivers and soils are being degraded. And the greatest problem is not industrial waste but human waste," the WSSCC said in a report.

It said nearly half the people of the developing world suffer at any one time from diseases associated with poor sanitation, noting: "One gram of faeces can contain 10 million viruses, 1,000 parasite cysts and 100 worm eggs."

Although the situation improved between 1990 and 2002, the WSSCC says that at the current rate most countries will struggle to meet goals set by the UN to slash by half the proportion of people without access to proper sanitation.

In Costa Rica's central valley, which contains roughly half the nation's 4 million people, the lack of adequate sewage treatment has gradually turned the Tarcoles into a fetid stream, polluting not only the waterway but miles of coastline around its mouth.

The brackish water of the river, lined in places by tropical mangroves, runs a sickly greenish-brown. Debris, including old tyres, cling to fallen trees and to the banks.

Surfers attracted by good waves at nearby beaches complain about the slicks. But despite its filth, the river hosts a wide array of tropical birds and a large population of American crocodiles.

MODEL FOR CENTRAL AMERICA

The planned sewage treatment plant is billed as the first of its size in Central America. It should clean up the Tarcoles River and tributaries running through Costa Rica's main towns.

"I believe we can give the example to other countries in Central America and the Caribbean that a project of this kind is possible," said Rafael Villalta, president of Costa Rica's Aqueduct and Sewer Authority.

Ground will be broken for the plant in 2007 and the first stage will be completed in 10 years. A second stage, yet to be funded, is to be completed in 2027.

The first phase will include 1,700 km (1,055 miles) of sewerage lines and will be financed by a US$127.7 million loan from Japan and US$103 million from Costa Rican coffers.