From a boat on the Yamuna River that flows past India's capital, it's easy to spot bubbles of fetid gas sent up to the water's surface by rotting sewage -- and that's after at least 350 million dollars has been spent on cleaning it up.
Now, with New Delhi to host the Commonwealth Games in 2010 and the sports village meant to house the athletes being built on the stinking river's banks, India must do in three years what has not been achieved in the last decade.
Boatman Lalla Navwalla, 36, whose brother spends eight hours a day swimming for coins tossed in by Hindu pilgrims who still revere the river, understands perfectly why the river is dirty.
"Once you could look 20 feet (six metres) down and see the coins," he said, looking expressionlessly at the opaque grey water.
"But after the eighties the population started growing and the river became dirty."
More people means more waste, and all of Delhi's three and a half billion litres (950 million gallons) of daily urine, excrement and other waste flow into the Yamuna -- much of it raw as treatment plants struggle to keep up.
"Delhi is growing very fast. The gap is always widening," said R.C. Trivedi, an official at India's Central Pollution Control Board monitoring agency.
Boatman Navwalla says he remembers when there were tortoises living in the water. Now the only thriving life forms are bacteria.
-- Nationwide problem --
What has happened in the Yamuna is happening to rivers all over India, where 30 percent of the population now lives in cities, straining infrastructure that was creaky to begin with.
These teeming cities take more and more water from rivers or groundwater reserves, returning it as barely treated waste.
Water-hungry Delhi dams the Yamuna north of the city and takes about 1.1 billion litres of fresh water, more than a third of what the city uses every day.
After that, only drains carrying thick sewage feed the river.
Delhi has invested a huge amount on waste treatment in the last decade: between 170 million and 214 million dollars according to estimates from the nonprofit Centre for Science and the Environment which put out a study on the river in April.
A national river cleaning programme also spent 175 million on the river, more than a third of that in Delhi.
But the city grew -- from 1991 to 2001 its population expanded by 47 percent -- and now a third of Delhi's waste, the biggest cause of the river's pollution, goes into the Yamuna untreated, the pollution monitoring board says.
The city is playing a game of catch-up, and losing it could be deadly, environmentalists say.
"We can't afford to develop a waste treatment culture where only some part of waste is treated," said Centre for Science and Environment director Sunita Narain. "One hidden cost of a dirty river is bad health" for people living downstream.
Every year almost 400,000 Indian children die because of diarrhea alone, according to UNICEF.
"The government starts throwing money at the problem without really understanding what the problem is," said Shreekant Gupta, who until March headed the National Institute for Urban Affairs think-tank.
"The response has been to put money into sewage treatment plants where half the drains are not connected to them."
-- Sewage pipe dreams --
The new sewage plants sit idle at times, short of waste, while homes and shops that have sprouted unplanned around them send their domestic waste into storm drains that go directly to the river.
The pipes that do lead to the plants are falling apart -- a third are in urgent need of repair -- so sewage gets blocked on its way and has to be pumped out, also ending up in the drains.
The head of the Delhi Jal (Water) Board, which handles sewage as well, said his hands are tied by a court order.
The water board is forbidden from hooking up unplanned neighbourhoods, leaving at least three million people unconnected, Arun Mathur told AFP. Other groups say half the city of 14 million is unconnected.
"You want to clean the Yamuna on the one hand and you won't provide us with a way to move sewage to the sewage treatment plants," said Mathur.
"What do you do for the colonies (suburbs) which have no treatment at all? The presumption seems to be that these colonies will somehow disappear."
But Mathur said the city is now deadly serious about reviving the Yamuna.
"The pressure is on us but I am now confident," said Mathur. "My aim is to put in systems that go on for 50 years no matter who is in the job."
Costly plans are underway to repair old sewage lines and build new ones to take away and treat waste from the three largest drains as well as to hook up unauthorised colonies.
The river will be visibly cleaner in just three years, Mathur said, in time for the Commonwealth Games.
But with the water board now suffering from a "credibility" problem, Mathur said the agency will outsource these public works.
"I am hopeful because we are not going to do the job ourselves," said Mathur. "We'll hire the best outside agency."