These Wetlands Treat the Wastewater of a Megacity
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These Wetlands Treat the Wastewater of a Megacity
Every day, the East Kolkata Wetlands recycle about 900 million liters of wastewater. The result: water clean enough to farm delicious fish.
By:<br>Geetanjali Krishna
August 13, 2026 6 min read
These Wetlands Treat the Wastewater of a Megacity
Every day, the East Kolkata Wetlands recycle about 900 million liters of wastewater. The result: water clean enough to farm delicious fish.
By:<br>Geetanjali Krishna
August 13, 2026 6 min read
Climate + Environment
Credit: suprabhat / Shutterstock
This story is part of:
Waterline
Waterline is an ongoing series that explores how nature-based solutions preserve and protect our water resources. It is funded by a grant from the Walton Family Foundation.
On a humid summer afternoon, Gagan Das mixes groundnut and mustard meal in water in a large drum. On either side of him, large ponds reflect the sun. Mango trees laden with fruit, palms heavy with their payload of yellow dates and shady neem trees line the embankments. It is hard to believe that we are barely 15 miles from Kolkata, the bustling capital of the east Indian state of West Bengal.
Das fills a feeder with the pungent concoction he has mixed and wades into the fresh water. “My fish are fingerlings right now, but this tasty feed I’ve prepared will fatten them up in no time,” he says. Pisciculture (fish farming) is profitable here, he says, as Kolkata’s appetite for the different varieties of carp he rears is prodigious. This year, he leased two ponds, locally known as bheris, for about $940 U.S., and reckons that if things go well, he will be able to quadruple his investment. “There’s just one problem we face every now and then,” he says. “Sometimes we don’t get enough sewage supply from Kolkata!”
Roughly 900 million liters of Kolkata’s wastewater is recycled by these wetlands every day. Credit: suprabhat / Shutterstock<br>This is the East Kolkata Wetlands (EKW), a roughly 12,500-hectare (6,178-acre) Ramsar Site of international importance. A mosaic of sewage-fed ponds, marshes, channels and farmland, it recycles roughly 900 million liters of the City of Joy’s wastewater every day. “This is a purely natural wastewater treatment system, which saves Kolkata from having to build and operate significantly larger, high-tech and expensive sewage treatment plants,” says Susanta Kumar Chakraborty. His 2023 book, Wetlands Ecology: Eco-biological Uniqueness of a Ramsar Site, details the ecological – and economic – services provided by what he describes as “the largest natural waste recycling system in the world.”
Located on a gradient that slopes lower than the heart of the bustling Bengali metro, the wetland is fed by underground pipes connected to pumping stations in East Kolkata. From here, in age-old sewage canals, nature works to purify the water. The solid waste settles into a carbon-rich sludge, which is composted into a potent fertilizer. The sun treats and oxygenates the water, while aquatic plants, plankton and microbes feed on the organic matter, and, in turn, become food for the fish reared by local farmers. The wetlands also absorb floodwater during heavy rains, and by the time Das pumps water from the canal and into his bheri, it looks clear and smells fresh.
Gagan Das makes a feeding mixture for the fish in his bheri. Credit: Geetanjali Krishna
It is a typical pre-monsoon day in Kolkata, characterized by temperatures hovering around 95 degrees Fahrenheit and humidity in excess of 60 percent. Chakraborty, two of his students and I drive about 25 miles out of Kolkata on the Basanti Highway towards the mangrove forest known as the Sundarbans.
In Jamaipara, a little fishing hamlet in the EKW, we visit Das’s shimmering bheri. Chakraborty, rivulets of sweat running down his face, points out pollution-eating plants like water hyacinth and a leafy green often used in Bengali cuisine, kalmi saag (water spinach). Across the embankment, there are fields of blooming sunflowers, paddy and vegetables like eggplant, tomato and chili — all grown with the composted sludge that accumulates on the sides of the canals. Narrow water channels crisscross the landscape. We cross a bridge made simply of a single cane of thick bamboo, and come across a thatched shelter that villagers use to keep watch over their fields and bheris. A group of teenagers offers us tender coconut water. Nineteen-year-old Sumit Mandal is one of them.
Susanta Kumar Chakraborty. Credit: Geetanjali Krishna<br>There are over 254 sewage-fed fisheries in the wetland, which provide livelihood to over 30,000 fishers and fish-farm workers. Mandal’s family leases some bheris here, where they rear fish that locals love: several species of catfish, carp and tilapia. “The bheri gives us everything,” he says. “It takes the dirty water from the city and turns it into food and sustenance. … It’s like a snake eating...