HAVANA - For six miles, the Luyano River wends past a panorama of Havana - residences, businesses, industry - before it spills into Havana Bay, one of the busiest but most polluted ports on the Caribbean.

The Luyano exerts a substantial influence on Havana's environment as the largest of three rivers feeding the capital's bay.

Now, after a decade of studies, the watershed's dirtiest river is receiving its first wastewater-treatment plant, funded with $4 million from international partners and $20 million from Cuba's government, according to the international financing body Global Environment Facility.

The waterway is a case study in how Havana's growth has overwhelmed its century-old public water system, forcing the Luyano and other rivers to become dumping grounds, experts and officials say.

Since the late 1990s, Cuba has been cleaning up the bay through closures, relocations and renovations of 15 industries, officials say. Between 2000 and 2005, oxygen was up and contaminants such as phosphorous, nitrogen and suspended solids were largely down, sometimes by more than half, GEF figures show.

Still, tons of untreated sewage and contaminants flow annually through the Luyano.

Ariel Castillo, who has lived the last 25 of his 31 years in a riverbank home, dreads rainfall; it creates a foul odor in his neighborhood and an unnatural plume in the waterway.

"It's a devil of a mess," Castillo said.

Two-thirds of a mile away, crews are laying the foundations of a plant that will treat wastewater from 62,000 inhabitants, but they are behind schedule and experiencing millions of dollars in overruns, according to Cuban and United Nations Development Program officials.

For now, the unfolding plant, which also calls for more than nine miles of new sewer lines, resembles a crater, half of a football field in size, with some concrete footings.

For all its ambitions, the plant is a drop in the bucket against the estimated $30 billion needed to modernize Havana's sewer system and leaky underground water lines, said geography professor Joseph Scarpaci of Virginia Tech University, coauthor of

Havana: Two Faces of the Antillean Metropolis

.

That estimate is based on his discovery of Soviet documents from the mid-1980s that calculated $10 billion to $15 billion was needed to modernize treatment of Havana's wastewater, most of which is now dumped in rivers and the sea, Scarpaci said.

"I salute the government, but the dollar amount they're talking about is insignificant when you look at the larger scope," Scarpaci said of the Luyano project.

Cuban officials acknowledge the challenge. The government newspaper Granma said: "The city's sewer system, built in 1915 and designed to manage the wastewater of an urban area that housed 300,000 inhabitants at the time, now functions with the same capacity for a population of approximately 1 million."

Like its counterparts in Guantanamo and Santiago de Cuba, Havana Bay is notable for its shape - a narrow mouth with a bulbous harbor. While aesthetically alluring, the enclosure exacerbates pollution because nine days pass before water cycles out of the bay, Scarpaci said. As a result, a black ring resembling tar lines the seawalls.