INDIAN lawmakers have passed a long-promised bill to ban manual cleaning of toilet waste by workers who are seen as the "ultimate untouchables" under the country's ancient Hindu caste-hierarchy.

Already illegal under a largely ineffective 1993 law, the Congress government pledged to make another attempt at stamping out the practice with the new legislation cleared by parliament's decision-making lower house.

The new act modifies the 1993 law which criminalised the scavengers who clean out primitive toilets by hand, collect the faecal matter in bamboo baskets and buckets and take it away in handcarts to dump.

"This dehumanising practice is inconsistent with the right to live with dignity," social justice minister Kumari Selja said after the bill's passage late Friday.

The new measure aims to outlaw employment of manual scavengers and provide retraining and help for their families, Selja said.

"We want to remove the stigma and blot on the society," the minister said.

"Passing the law is a most important thing - this is a subhuman form of work," Bindeshwar Pathak, founder of the Sulabh Sanitation and Social Reform Movement, a social organisation which develops inexpensive, eco-friendly toilets.

"But these people need to be given skills and respect," Mr Pathak, who is a veteran campaigner for an end to so-called "manual scavenging", told AFP.

Scavengers belong to the Dalit caste, once known as "untouchables", and are treated as pariahs even by others at the bottom of Hinduism's officially banned but still existing hereditary caste hierarchy.

Mr Pathak, a follower of India's independence icon Mahatma Gandhi, began his mission more than four decades ago to change the lot for scavengers.

He has opened centres across India to teach them literacy and vocational job skills and now also spends time seeking to break down caste barriers and get upper and lower castes to eat together and worship in the same temples.

"Gandhi had desired that one day a scavenger should be president of India," Mr Pathak, who is from an upper-caste Brahmin family, said.

The new measure also bans construction of non-flushing toilets that must be emptied by hand, and sets out a one-year jail term or a fine of up to 50,000 rupees ($830) - or both - for anyone employing a manual scavenger.

It also contains tough sanctions for municipalities employing sewer cleaners without protective gear.

Workers stripped down to their underpants and equipped with just a hoe and a wooden bar can still routinely be seen clambering into the stinky depths of septic tanks and sewers.

Manual scavenging points to a lack of sufficient investment in modern sewerage systems by a government which struggles to provide basic services, social activists say.

A 2011 survey by India's Central Pollution Control Board showed just 160 out of nearly 8,000 towns had sewer systems and sewage treatment plants.

Over 600 million Indians lack even primitive toilet facilities and practice what is known as "open defecation" in roadways, ditches and fields.

Rural development minister Jairam Ramesh created a stir last year in the deeply religious country when he noted "there are more temples in the country than toilets" and that "India has a godliness surplus and cleanliness deficit."