There is an infectious, impassioned enthusiasm about Bezwada Wilson that is hard to ignore. He laughs, he smiles. He frowns too, but soon he is smiling again. And yet things might have been very different. When he was aged 18, he came very close to taking his own life. The thing that led him to the very edge was the discovery of what his parents really did to scrape together a living.

Growing up in a gold mining area of southern India, they had told him as a child that they mined for ore. The evening they revealed to him that they were actually "dry latrine" cleaners who spent their days covered in the filth of others, he was so horrified, so disgusted, that he came close to committing suicide at a secluded water tower. After hours of weeping, wrestling with his thoughts, he decided he was better off alive, fighting to help people like his parents.

Two decades later, his efforts have been nothing short of remarkable. As head of a nationwide organisation that has confronted head-on some of his country's most ingrained prejudice, he believes he is close to eradicating the dehumanising practice known as "manual scavenging".

Ten years ago, there were around three million people employed as manual scavengers in India; today there are fewer than 600,000. By the end of the year, he believes the figure will be zero. "Once people realise that it's slavery, they want to stop. The problem is that it's never discussed publicly," Mr Wilson said.

Dry latrines are toilets that do not flush. In most Indian homes, such latrines have been replaced with flush systems. In 1993, a law ruled all dry latrines should be destroyed and the practice of manual scavenging abolished. But in pockets of India, especially in poorer states such as Uttar Pradesh, Rajasthan and Bihar, the practice has persisted.

In India, the people employed to clean such toilets have always been members of groups of "Untouchable" or Dalit people. Equipped with nothing more sophisticated than a brush and bucket or pieces of cardboard, such groups have cleaned the toilets of higher-caste people, carrying away the waste, or "nightsoil", in baskets placed on their heads.

Payment for such work is minimal; many scavengers say they do it because they have always done so, because their parents and grandparents did the same. Activists say it is a form of bondage.

The work of a scavenger is filthy and soul-destroying. One morning I accompanied two women, members of the Balmiki clan, as they went about their work on the edge of the city of Ambala, in Haryana.

Cheranji Kaur and Seema clean six dry latrines every morning, using a brush to sweep away the waste into a nearby open drain. "I started this work at the age of 10," said 35-year-old Mrs Kaur. "My parents did it, our ancestors did it. I'd go with my parents; that is how you learn."

The women made their way around a scruffy, higher-caste neighbourhood where they set about cleaning the toilets. One resident threw a bucket of water into the latrine, sending the waste splashing and gushing as the two women used their brushes to clean it away. The air was sour and corrosive. It was a sad sight, made worse by the women's determination to try to retain their dignity as they worked, and keep their clothes clean, gold bangles bouncing on their wrists.

One of the latrines belonged to a family of Sikhs, a religion whose founder utterly rejected the notion of caste differences. The head of the household, Mahinder Singh, said he paid the women 50 rupees (71p) a month and that "it had always been the Balmiki who cleaned the toilets".

Mr Singh added: "Everybody gets it done so we get it done. Those who are supposed to do it, should do it. I did not know it was illegal."

Mr Wilson's campaign has two aims: to raise awareness among groups of scavengers that they do not have to perform such tasks, and then help them find alternative work. A key element of his publicity drives, or yatras, is the symbolic burning of the scavengers' wooden baskets and the destruction of the dry latrines themselves.

In one incident in the southern state of Andhra Pradesh, activists discovered a block of supposedly outlawed dry latrines in a court complex, which were being used by judges. When activists destroyed the toilets, officials did not dare protest, aware that the existence of the latrines had been in breach of the law.

Mrs Kaur and Seema revealed that they laboured every day for around 150 rupees a month. They said the work was difficult, it was hard to get rid of the smell and they suffered widespread discrimination.

"We do it because there is no other work," said Mrs Kaur, who has two young children. "The other communities will not let us inside their homes to work as cleaners or domestic servants. Any work would be better than this; a little shop, a vegetable cart, or rearing cattle."

Seema added: "I don't feel good doing this work, people look at us different, they look down at us."

Largely as a result of the work of Mr Wilson and his group Safai Karmachari Andolan (SKA), there are now just a handful of scavengers in the state of Haryana. One of those who was "rehabilitated" is Saroj Balla. She had worked as a scavenger all her life to earn enough to send her children to school. Thirty years ago, members of her community even struggled to buy vegetables; vendors would put them on the floor rather than hand them over directly.

Once, when Mrs Balla was pregnant, she fell from a ladder while descending from a dry latrine located on a roof, collapsing on the floor in a slew of human waste. The toilet owners refused to help her, instead prodding her with a stick to make her get to her feet.

Five years ago, activists from SKA, which receives support and some funding from Christian Aid, which is based in London, arrived in Ambala and told Mrs Balla and others that they were not required to perform such degrading work. "They told us we had a right to a better life," Mrs Balla said, her voice swelling. "We all knew it was dirty work. We took courage from the people who came to talk with us."

The 50-year-old recalled the day that she and other scavengers went out and demolished 15 dry latrines in the area. The toilets' owners had told them to clean them, but instead the group of women used heavy sticks to break them up. "I felt very good," said Mrs Balla, who now works as a domestic help. "It felt like a rebirth." Her advice to the other two women was simple: definitely stop.

Mr Wilson, 44, who presented a piece of brick from a demolished dry latrine to Navi Pillay, the UN High Commissioner for Human Rights, talks about eradicating all scavenging by 31 December of this year. Volunteers are currently holding rallies across the country and will descend on Delhi at the end of the month.

By then he believes the number of people working as scavengers will have been reduced from 600,000 to 300,000, giving him two months to finish his task.