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!”
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.
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.
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 its head!” And snakes there are aplenty — the wetland is extremely biodiverse, home to monitor lizards, many species of water birds and of course, the baghrol, local name for West Bengal’s state animal, the fishing cat.
Further down the Basanti highway, freshwater bheris give way to the brackish water estuaries of the Sundarbans delta where the Ganges and Brahmaputra rivers meet the sea. The bustling wholesale fish and vegetable markets in the nearby town of Malancha take the produce to the city. Chakraborty, who like most Bengalis is passionate about his fish, buys some. When I gingerly broach the question of whether fish grown in these sewage-fed bheris is safe to eat, he says an emphatic yes. A 2023 study found the levels of heavy metals in the EKW fish to be well within the safe limits defined by the World Health Organization. But such studies must be carried out more regularly, he admits. “There is a lot more industrial effluent going in now,” he says, as we pass the Kolkata Leather Complex, which houses about 500 leather tanneries and a widely criticized wastewater treatment system. Beyond, the water of the channel is flecked with foamy pollutants.
The Bidyadhari river glinting in the distance, Chakraborty points out all the geographical features that have helped maintain the East Kolkata Wetlands without much human investment thus far. “We’re now on a lower gradient than the city, so wastewater naturally flows here,” he says. The connection to the Sundarbans ensures that tidal flows regularly push wastewater from the western, highly populated parts of Kolkata, to the wetlands in the east. The marshy landscape soaks up the wastewater and rainwater runoff easily. “Such a system can only work in specific geographical conditions,” he says, but adds that several key parts of it — pollution-absorbing plants, plankton and fish; composting sewage sludge for agriculture; using the natural gradient in topography to move and aerate wastewater — are easily replicated.
In fact, the world over, cities are trying different types of nature-based solutions for wastewater treatment, as defined by the United Nations Environmental Program’s Global Wastewater Initiative. In northern California, the city of Arcata operates a wastewater treatment system that combines conventional treatment with constructed wetlands. China has also invested heavily in constructed wetland systems as part of its “sponge city” approach, using wetlands to absorb stormwater, improve water quality and reduce flood risks. And in Vietnam, several cities traditionally integrate aquaculture ponds into urban water management systems, using ecological processes to recycle nutrients and support food production.
EKW remains unusual because of its scale and the fact that it has survived for over a century. But as we drive back to Kolkata, it is clear that this landscape is under pressure. As we get closer to the city, a new feature appears in EKW’s marshy landscape: skyscrapers. The pressure from real estate developers, the creation of industrial zones and the drying up of the channels that vitally connect Kolkata’s sewers to the wetland are all slowly reducing the ecological functions that it provides.
There is another challenge: The bheris are getting fragmented among multiple heirs when their owners pass away. “Imagine, at one time, how this area would have looked when instead of these small bheris, there was one massive wetland,” Chakraborty says. “Over decades, they’ve grown smaller and smaller, and the connections between them are slowly getting silted or built over.”
The degradation of the wetland has had an “immediate impact” on Kolkata and its denizens, Chakraborty says. In 2025, when the city experienced the highest rainfall in 39 years, of the 10 people who died, nine were electrocuted by live wires in flooded areas. “In the past, excess rainwater would flow into the wetland, but now it has nowhere to go,” he says. “If the wetland is further degraded, the city will become less resilient to extreme weather and climate change.”
Dhrubajyoti Ghosh, an ecologist and engineer working for the water and sanitation department of West Bengal, coined the term “East Kolkata Wetlands.” After years of lobbying for international recognition, he managed to get the area designated as a Ramsar Site in 2002. The wetlands got legal protection years later, when the East Kolkata Wetlands (Conservation and Management) Act, 2006 was enacted.
Ghosh, who passed away in 2018, was the wetlands’ most passionate advocate, and argued on international and domestic platforms that they were not vacant lands awaiting urbanization but vital ecological infrastructure that cleaned much of Kolkata’s sewage, protected against floods and supported tens of thousands of livelihoods.
Looking at the high rises that have been constructed in the wetland, Chakraborty says, “Today, the problem of East Kolkata Wetlands is governance. The law exists, but of no use unless it is followed!”
Meanwhile, back in the malodorous fish market at Malancha, it is business as usual. Local fisher Rathin Das has sold a bumper catch of catfish and says there are plenty more in the wetlands and mangroves. “If only our market had a cold storage,” he says, putting a huge slab of ice through a noisy crushing machine, “we could catch more fish and make more money. Maybe that would convince more people living in the wetland to take care of it a little more!”