'There are places now where the river is more or less septic,' said Veer Bhadra Mishra, head priest of the 400-year-old Sankat Mochan Hindu temple, which overlooks the Ganges. He is also a retired professor of hydraulic engineering and an environmental activist, and his Sankat Mochan Foundation leads the city's clean-river campaign.

Technically, septic water is so polluted that it contains no dissolved oxygen, and the coliform bacteria count in the Ganges in Varanasi is some 3,000 times the safe limit determined by the World Health Organisation.

But that does not stop people bathing in it; to do so is in fact a religious obligation.

For more than two millennia, the Ganges, which flows more than 2,400km from the Himalayas, across the plains of India, into Bangladesh and then the Bay of Bengal, has been revered as a symbol of spiritual purity.

'Man becomes pure by the touch of the water, or by consuming it, or by expressing its name,' said Lord Vishnu in the Ramayana, an epic poem written in the fourth century BC that forms a cornerstone of Hinduism.

Four thousand years later, India's first prime minister, Jawaharlal Nehru, described it as 'above all the river of India, which has held India's heart captive and drawn uncounted millions to her banks since the dawn of history'.

But last year, angered by the state of the Ganges, thousands of holy men threatened to boycott the six-yearly Ardh Kumbh Mela, one of the world's largest religious gatherings, during which millions plunge into the Ganges to wash away their sins.

In the event, millions took their holy dip in the Ganges at Allahabad, a city upstream from Varanasi, though it was speculated many of them might have needed medical treatment afterwards.

But while Indians continue to revere the river, India's rapid growth is killing it. There are more than 100 cities, numerous towns and countless villages scattered along its banks. Some 500 million people are dependent upon the Ganges for water, and as it has been siphoned off for irrigation, its water levels have fallen.

In Varanasi, say inhabitants, the river level has dropped by an easily observable two metres.

Climate change is also taking its disastrous toll. The Himalayan Gangotri glacier, the source of most of the Ganges' water during India's long, hot summer, is shrinking by 40 metres a year, say scientists. By 2030, they warn, it could disappear altogether - making the Ganges seasonal, dependent upon erratic monsoon rains.

But although environmentalists urge India, one of the world's top producers of greenhouse gases, to take urgent action, activists in Varanasi say an opportunity is being lost to tackle the much simpler problem of domestic sewage pollution.

Few of the fast-growing conurbations along the Ganges' banks - indeed, few of India's cities and towns - have sewage treatment plants. The WHO says dirty water is the leading cause of child deaths in India.

But the result is especially gruesome in Varanasi. It is not just that millions of Hindus visit every year, expressly to touch and sometimes drink the water. The banks of Varanasi are also used to cremate human corpses, with the remains disposed of in the river.

More than 3,000 bodies were reportedly seen in the river in Varanasi last year. Mr Mishra, who inherited the role of temple high priest from his father when he was 14 - a family tradition more than 200 years old - first became aware of the dirtying of his beloved river more than 30 years ago.

Over the years, he himself has suffered from waterborne diseases including typhoid and polio and yet, to this day, he sometimes drinks the river's water. 'I am a scientist - how can I not say the water hurts me,' the silver-haired 70-year-old said as he sat barefoot on a floor cushion in his office, which overlooks the murky brown river. 'I have a rationally trained mind. But I also have a passionately committed heart.'

In 1982, mindful of both, he set up his foundation, with the support of religious people and scientists. In 1986, largely due to the foundation's campaigning efforts, there came some good news.

Rajiv Gandhi, then prime minister, launched the Ganga Action Plan (GAP), a multimillion-dollar scheme intended to clean up the river by means of wastewater treatment plants.

But by almost every expert's reckoning, the scheme has been a costly disaster. The plants built under it handle only a small amount of the sewage generated along the river. Because they rely on electrical pumps, during power cuts - frequent occurrences in India - even the small amount of sewage they should theoretically handle flows into the river. And, experts say, when the floodwaters rise, sewage enters the sump well of the pumps, stopping operations for months.

Most seriously, the GAP system is designed to remove solid waste but not faecal micro-organisms from the water.

So more than a decade ago, Mr Mishra set about finding an alternative. Working with scientists from the University of California at Berkeley, he worked on an adaptation of an 'advanced integrated wastewater pond system' developed by William Oswald at the university, which is in operation in parts of California. Experts agree it is cheaper than the current system, sustainable and suitable for a tropical climate such as India's.

Instead of depending on scarce supplies of electricity, the system would use gravity to carry sewage to four big pools, built on wasteland several kilometres outside the city, where it would be broken down by bacteria, algae and sunlight.

An independent assessment found the plan was cheaper and more effective than the existing scheme, which Mr Mishra described as 'supplying medicine without diagnosing the disease'. The scheme was unanimously accepted by the city council nearly a decade ago, but the state and central governments rejected it.

Mild-mannered Mr Mishra continued his tenacious lobbying, writing letters to Sonia Gandhi, the widow of Rajiv and leader of the Congress Party, which leads the central government, and last year he secured a meeting with Prime Minister Manmohan Singh.

Then, last month, he heard what he described as 'the best news in 20 years'. On June 30, the central government wrote to Mr Mishra and the state government, saying it would support a pilot run of Mr Mishra's scheme in Varanasi and suggesting it would hold back support for the much costlier, ineffective state government-led scheme.

'If the result is convincing, it will be difficult for the government to refuse to roll it out,' said Mr Mishra with a broad smile, adding that if the pilot project went ahead, the result would be convincing.

Experts say the amount of domestic sewage pumped into the Ganges has doubled since the 1990s and that it could double again in a generation.

Mr Mishra also hopes the scheme might one day become a model for other Indian towns and cities.

But the Ganges that flows beneath his temple remains his motivation. 'All our rivers have stories,' he said as a wooden boat of pilgrims floated past his window, trailing flickering floating candles in the gathering dusk.

'All our rivers are important. But there is nothing anywhere like the Ganga.'