For India's devout Hindus, the sacred River Ganges is always clean and always pure -- even if its waters are a toxic mix of human sewage, discarded garbage and factory waste.
The belief that the Ganges washes away sin entices millions of Hindus into the river each year, and huge crowds of pilgrims are currently passing through the town of Haridwar for the three-month Kumbh Mela bathing festival.
But concern over pollution along the length of the 2,500 kilometre (1,500 mile) river is growing, and the city of Kanpur -- 800 kilometres downstream of Haridwar -- is the site of one of the worst stretches of all.
Factories in the industrial city chug millions of litres (gallons) of polluted water into the river daily, rubbish forms into solid floating islands, and a foul smell wafts over the water's murky surface.
The situation is "acute and critical", said D.K. Sundd, executive director of the Sankat Mochan Foundation, a non-profit group working to clean up the river.
"The problem is worst in Kanpur. The city generates nearly half the volume of sewage and industrial waste as compared to the fresh water flow in the Ganges," he said.
Most communities located on the river from the Himalayas to the Bay of Bengal in Bangladesh lack proper sewage treatment facilities, and the river has for years "been misused as a convenient sink for raw waste," said Sundd.
Next to one Hindu temple in Kanpur, domestic waste water spills out of a giant drain, merges with a stream of white foam and flows into the river.
Ganges water is considered by many to be blessed, and has for centuries served as an essential component of Hindu ceremonies, from childbirth to death -- when ashes are often scattered in the river after cremations.
Worshippers like Ram Sharma, who regularly wades in the water for an early morning bath with only a cloth tied around his waist, are proof that for many Indians faith outweighs science.
"How can you call this water dirty?" asked Sharma incredulously.
"For us it is holy water," he said as he dipped his cupped hands in the river and took a slurp.
Further down the banks, Mahinder Pal Singh rolled up his pants and stood knee-deep in the water praying.
"You won't find water this auspicious anywhere upstream," he said proudly.
He may be right -- Sundd points out that the polluted segments are separated by cleaner stretches, one of them being Haridwar, the site of this year's Kumbh Mela.
"Ganges water is well known for its extraordinary resilience and recuperative capacity," said Sundd.
In Kanpur, one challenge that the holy water must overcome is the leather industry, which employs around 50,000 people in more than 400 tanneries using chemicals such as toxic chromium compounds.
Although factories are required to treat sludge and waste water before transferring it to a common effluent treatment plant, environmentalists accuse them of dumping waste directly into the Ganges.
Imran Siddiqui, director of Super Tannery Ltd., one of Kanpur's largest tanneries, said the leather business was unfairly being singled out because it was the city's most prominent and profitable.
"People are making culprits out of the tanneries, only it's not true. Only two percent of total generation of effluent comes from tanneries," he said.
"There is more toxic and hazardous waste compared to tannery waste, like that coming out of the electroplating and the dying industries," Siddiqui added.
The common effluent treatment facility in Jajmau, the hub of Kanpur's leather trade, has a capacity of nine million litres (two million gallons) a day reserved only for industrial waste water.
"Nobody knows how much waste water they generate but everybody accepts that tanneries produce more than nine MLD (Million Litres a Day), probably between 20 to 30 MLD," said Ajay Kanujia, a chemist at the Jajmau plant.
Siddiqui said Super Tannery had set up a state-of-the-art primary effluent treatment plant as part of its "moral responsibility".
But Kanujia dismissed the move, saying smaller tanneries were all dumping their waste water into storm drains.
"One single tannery may be doing primary treatment and chromium recovery, but if 90 percent are not then what is happening?" he said.
Kanujia said chromium levels in the river had not decreased since the common treatment plant was established in 1995, and now stood at more than 70 times the recommended maximum level.
The government has spent more than 160 million dollars to clean up the river since initiating in 1985 the Ganga Action Plan, which uses the Hindi name for the river, but even it has conceded bacteria levels are dangerously high.
In February last year the government established the National Ganga River Basin Authority to monitor conservation efforts, and in December the World Bank announced a one billion dollar loan to support clean-up schemes for the river.
But years of state-funded attempts have "failed miserably," said Rakesh Jaiswal founder of the Kanpur-based NGO Eco Friends.
"We do not have a vision for the river -- what's practical and what's achievable," he said.