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.

Read the full article about the East Kolkata Wetlands treating the city's wastewater by Geetanjali Krishna at Reasons to Be Cheerful.