Millions of pilgrims hoping to cleanse their sins by ritual baths at India's Kumbh Mela festival rely on key lavatory workers to clear up behind them -- those born on the lowest rung of the Hindu caste system.

The millennia-old sacred show of religious piety and ritual bathing, which began on Monday and runs until February 26 in the north Indian city of Prayagraj, is this year predicted to be the biggest yet, and the largest ever gathering of humanity.

Organisers expect a staggering 400 million pilgrims will bathe during the six-week-long festival in the confluences of the Ganges and Yamuna rivers, holy waters for Hindus.

That creates a waste removal and public health challenge of epic proportions, with 150,000 temporary toilets installed across packed riverbank campsites covering an area greater than 2,000 football pitches.

Critical to the festival's running are the 5,000 workers hired just to clean the toilets -- and nearly all of them belong to the lower rungs of an age-old rigid social hierarchy that divides Hindus by function and social standing.

"I clean and clean, but people........

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