But urban sprawl has blighted the landscape, and bathers prefer less-polluted beaches like Copacabana, on Rio's Atlantic coast.

Home to 12.5 million people, the bay's watershed system has long been a dumping ground for garbage, toxic chemicals and sewage, 54.3 per cent of which goes untreated.

Now, after decades of failed fixes, Rio state authorities say they have a solution.

Last year, they privatized tottering water and sanitation service Cedae, selling the operating rights for Rio city and 26 other municipalities to company Aguas do Rio.

The new operator promises to invest billions to do what no one has managed yet: clean up Guanabara Bay.

'Graveyard of failed projects'

Aguas do Rio, which took over in November, plans to invest 2.7 billion reais ($570 million) over five years fixing broken waste-treatment systems and cleaning sewage from the bay.

The company, a subsidiary of sanitation group Aegea, has promised total investments of 24.4 billion reais across its 35-year contract to bring the sewage-treatment rate to 90 per cent.

"I have no doubt people will start swimming again in the bay," says chief executive Alexandre Bianchini.

Locals are skeptical, given the history of failed plans to save the bay.

In 1994, Rio state launched a clean-up program with international funds, spending $1.2 billion on sewage treatment plants -- but largely failed to finish the pipes connecting them to residents.

Then came Rio's rush to host the 2016 Olympics. As international media ran embarrassing images of the polluted bay, Rio earmarked nearly $1 billion to clean it.

But the state declared insolvency weeks from the Games. It never came close to fulfilling its promise to bring the sewage-treatment rate to 80 per cent.

"Guanabara Bay has become a graveyard of failed projects," says ecologist Sergio Ricardo, a co-founder of environmental group Baia Viva.

Fishermen without fish

On a mucky, fetid bank off the bay's northwest corner, fisherman Gilciney Gomes brandishes two of the myriad plastic bottles he has pulled from the water.

"Am I supposed to feed this to my family?" says Gomes, head of the Caxias Fishing Colony, a fishermen's association.

Gomes lives near the Jardim Gramacho landfill at the bay's edge, once the biggest open-air dump in Latin America. Officially closed in 2012, it continues leaching a toxic slurry into the bay, environmentalists say. An illegal dump nearby still receives trash by the truckload.

The bay's banks here are clogged with plastic, diapers, clothing, tires, furniture and appliances.

Major chemical and oil companies operate facilities nearby that have leaked toxic industrial pollution into the water, say fishermen and activists.

Gomes, a father of four, says there are no longer enough fish and crabs in this region of the bay for fishermen to survive.

"We've become garbage-diggers," says Gomes, who started working as a nine-year-old boy digging garbage at the dump and is now back to selling recyclable trash at age 61.

Biologist Mario Moscatelli, who runs a program to replant mangrove forests at the former dump site, calls Guanabara Bay "a microcosm of how environmental problems are managed in Brazil."

"This disaster has been created by disorganized urban growth, booming slums, lack of public-housing policy for the poor, lack of universal access to the sewage system. And the environment always pays for it," he says.

Still, he says, he is willing to give Aguas do Rio a shot.

"We gave the state 50 years to make this mess. We can give the company five."