When most of Mumbai is fast asleep at 5am each day, 41-year-old Tanu Behre sets out on her hunt.

Armed with a little handbrush, she walks the narrow alleys of Zaveri bazaar, one of India’s largest bullion markets, and dusts the streets for gold. She enters drains outside goldsmiths’ workshops, and gathers the black sludge in her aluminium pan. If she’s lucky, the slime will turn up the precious metal.

“I find a few hundred milligrams every day – gold worth around 1,000 rupees [£11]. At times, it can go up to a gramme, or even two,” says Behre, who travels 40 miles from her home to get to the jewellery hub.

Named after the Hindi word for the pans they use, ghamela, there are hundreds of “ghamelawallahs” in Zaveri bazaar – men and women who scavenge grime for gold.

The 150-year-old Zaveri bazaar in south Mumbai is home to more than 7,000 jewellery stores, and accounts for an estimated 40% of India’s bullion trade. Its importance as a prominent trading centre has made the market the target of three terror attacks since 1993.

Congested, labyrinthine lanes house scores of jewellery workshops and factories, where gold is cut, carved, and shaped. Particles make their way into streets when craftsmen walk out – gold dust gets stuck to their hands, hair or shoes. The particles even get deposited in drains, when goldsmiths wash their hands.

“On an average work day, I spend four hours collecting seven to eight pounds of filth and sludge from the streets,” says Wasim Sheikh, 32, a native of the northern Indian city of Agra who migrated to Mumbai a decade ago to pursue his ancestral occupation. As with most other ghamelawallahs, generations of Sheikh’s family have been in the informal trade.

“Over the years, I’ve studied the bazaar closely; and now, I know exactly where workshops are, and where goldsmiths live. I usually target the crevices on pavements outside their workplaces, and drains outside their homes. On my best day, I found one and a half grammes of gold, worth 4,500 rupees [£49].”

After he collects the waste, he submerges it in water. Gold, being heavier, sinks to the bottom, while the mud and dirt float out. He then carries the residue to a secluded corner outside his rented home in Zaveri bazaar, where he lights a tiny furnace to refine the yellow metal.

“I add mercury to the residue so that gold particles stick to it, and the trash gets separated. In order to filter out the finer waste – magnetic dust and ferrous particles, I carefully run a tiny magnet over the remains. I then add this residue to a furnace, where I cook it with nitric acid for about 10 to 12 minutes. The acid reacts with mercury to form liquid salts, leaving behind a pellet of gold.”

Not every day, however, is lucrative. Mohammed Amir, a 17-year-old, who quit school to join the profession, says the monsoons leave him and his four brothers, also ghamelawallahs, struggling.

“All the dirt on the streets gets washed away with rainwater, while the gutters overflow, leaving no scope to identify sewage flecked with gold. Hence, during monsoons, my brothers and I take up other jobs – construction work, hawking at traffic signals or working as domestic labour in homes.”

Ghamelawallahs usually collect a gramme or two of gold before selling it to small-time goldsmiths in the bazaar. Mohammed Babloo, 34, says the rates are often lower than market prices. “They tend to haggle with us because they think we’re scavengers, living off sewage from gutters. I normally check rates with at least five buyers before parting with my gold.”

But now jewellers, aware of the ghamelawallahs’ work, are increasingly getting cautious about their waste, says Jaymin Zhaveri, a 30-year-old businessman, whose family has been running a jewellery store in the bazaar for 100 years. One shop owner has begun vacuuming and storing the dust.

“Workshop owners have started giving uniforms to their craftsmen, which they have to leave behind after work so that gold dust on their clothes is retained with the employers. Others clean the pavements outside their stores themselves, and some even collect the water after goldsmiths wash their hands or shower. All of this waste is preserved over time, and sold to buyers from all over India in bulk,” says Zhaveri.

For government street sweepers, meanwhile, the ghamelawallahs are a nuisance. Ranjan Mule, 37, who works with Mumbai’s civic authority, says: “Hundreds of these men and women are out every morning, searching for gold. They collect the dust and sewage, but throw everything else on the streets. There have been instances where ghamelawallahs have stolen garbage off our dumpsters, and littered the waste after.”

But ghamelawallahs are undeterred. “Business is good,” says Behre. “In my slum settlement I’m the only woman who wears nose rings and earrings made of real gold.”