Kolkata’s Kidneys

Exploring The World's Largest Nature-Based Waste Management

Fishermen traverse the wetlands

Every dawn, on the eastern fringes of the bustling city of Kolkata, fishermen tread along paths lined with dew-laden grass towards an expansive mosaic of ponds shrouded in mist. The air fills with the soft rustle of reeds and the distant calls of early birds beginning their day. Amidst this tranquil setting, the fishermen prepare for another day's work in a place both vital and unassuming. For their ponds are fed by an intricate network of channels and pipes that weave through the city, collecting and moving sewage to its outskirts. 

While a small fraction of Kolkata’s sewage is treated in processing plants, the vast majority flows into these ponds and creates the world’s largest cradle-to-cradle process for human waste: the East Kolkata Wetlands. By lunchtime, a Kolkata resident may eat a fish caught in the wetlands that morning, yet most remain unaware that their own waste fed that fish.

The East Kolkata Wetlands are an extraordinary example of a nature-based solution—an approach that leverages natural processes to address environmental challenges. In the case of the East Kolkata Wetlands, nature not only purifies the city's waste, essentially serving as the city’s kidneys, but also supports the livelihoods of vibrant fishing and farming communities. As many regions worldwide grapple with water pollution and overburdened sewage systems, the East Kolkata Wetlands stand as a testament to how seamlessly human needs can align with environmental stewardship.  

The Hooghly River as it flows past the vibrant city of Kolkata

The journey of this natural marvel begins in the buzzing metropolis of Kolkata, a city with an estimated population of 15 million. Its lifeline, the Hooghly river, flows past a diverse cityscape of makeshift houses with rusting iron sheets, towering industrial sites, and timeworn temples. The river is accompanied by a cacophony of sounds: the steady hum of ferry boats, the melodic cries of street hawkers, the distant chimes of temple bells, and the endless chorus of honking horns. The air is thick with an aromatic medley—rich spices wafts from street food stalls and intermingle with the sweet perfume of flowers and incense from countless shrines. A salty tang of fish weaves its way into this vibrant tapestry of fragrances, and suddenly, an unmistakable odor cuts through: the putrid stench of sewage. Along its journey through Kolkata, the Hooghly River gradually transforms into an extension of the city’s sewage system as it meanders through the urban centre. With an increasing stench, it makes its way to the Eastern outskirts of the city. 

Kolkata’s sewage streams as they enter the East Kolkata Wetlands

As Kolkata's untreated sewage reaches the wetlands, a transformative process begins. Sunlight penetrates the shallow waters, igniting photosynthesis among a variety of well-adapted algae and aquatic plants. Acting as natural filters, plants absorb heavy metals and pollutants, while algae consume the organic matter suspended in the water. To enhance this natural purification process, the fishing communities deliberately plant species like water hyacinths and duckweed, known for their capacity to absorb contaminants. Meanwhile, bacteria break down harmful biological substances and further purify the water. This collaboration of flora and microorganisms not only cleanses the water but also enriches it, which in turn fosters a thriving ecosystem. In this nutrient-rich environment, microorganisms like phytoplankton and zooplankton flourish, forming the foundation of a complex food web that supports an array of aquatic life. 

Bamboo gates serve as a barrier for fish between the ponds

The wetland's network of ponds, locally known as bheries, embodies generations of careful planning by the fishing communities. The bheries form an intricate system where sewage flows through a series of interconnected ponds, each serving a specific purpose in the natural treatment process. The initial ponds receive the most polluted water and host fish species that thrive in nutrient-rich conditions. As water advances through the system, each subsequent pond offers progressively cleaner environments and allows for the cultivation of various fish species at different life stages. Juvenile fish are raised in cleaner nursery ponds before being transferred to adult ponds. Drawing upon traditional knowledge, the fishermen carefully manage this ecosystem by determining optimal pond depths, regulating water flow rates, and adjusting fish stocking densities. This sophisticated arrangement not only ensures efficient waste treatment but also maximizes fish production by matching different species and age groups to the most suitable water conditions.

With nets as wide as the bheries, the fishermen corner and catch the wetland’s fish

Harnessing this nutrient-rich resource, the fishermen transform waste into sustenance within their bheries. Each morning, this process starts by retrieving their carefully stored, expansive nets from the boathouses. In groups of eight, they set out onto the wetlands—some wade through waist-deep waters, others board simple wooden boats destined to carry the day's catch. Slowly and carefully, with practiced precision, they stretch the nets across the pond, and gradually draw the fish into an ever-smaller area until they can close the net and haul the catch into their boats.

Once the fish are collected in the net, the fishermen sort them and lift them aboard

A fisherman shows his catch

The fisherman sort through their nets, releasing smaller fish and water snakes back into the water. After repeating this process several times, they bring their catch to the shore. Community members await them to begin processing. They sort the catch by size and species, then weigh it in large tin pots and kettles. With makeshift transport bikes, small vendors bring the morning’s catch to one of over 50 local fish markets, ready to be sold and consumed by Kolkata’s residents.  

 A bucket of freshly caught fish being auctioned at the fish market. Small vendors buy these buckets here, to then sell the fish in the city

Although the fish farms are the first to benefit from the wetland system, the water flowing downstream from the bheries still teems with beneficial nutrients. This water is channelled from the bheries to nearby vegetable farms, where it enriches the soil and promotes crop growth. Through these integrated processes, the wetlands produce 20,000 tons of fish and 50,000 tons of vegetables a year, providing significant food supplies for the city, and a livelihood for 20,000 people.

 Chanpa Sarrdar is waiting in line, with other fishing community’s vendors, to get their payment of the day. 

Long before the local fishing communities developed this ingenious cycle of production and waste management, the population of East Kolkata had been sustained through fishing in the original freshwater environment. However, this environment and its fragile ecosystem suffered severe degradation under British colonial rule in the 19th century.

By the early 20th century, determined to restore their livelihoods, the local fishermen began harnessing the sewage streams from the rapidly expanding city of Kolkata. They tapped into these nutrient-rich waters for fish farming, marking the beginning of what would become the East Kolkata Wetlands. Today, this vast area spans 12,500 hectares, dotted with around 250 individual ponds, and processes an estimated 650-750 million litres of sewage daily. 

The East Kolkata Wetlands

While the fishermen’s role in utilising the city’s sewage and providing food for its residents is crucial for the success of this nature-based solution, the wider community has had to work hard to protect this system. For decades, the East Kolkata Wetlands remained largely unrecognized for their ecological significance, with their vital role overlooked by Kolkata’s citizens, politicians, and even scientists. This began to change in the 1980s, thanks to the pioneering research of Dr. Dhrubajyoti Ghosh, who first uncovered the wetlands’ remarkable capacity for natural waste management. His work popularised the term ‘kidneys of Kolkata,’ highlighting the wetlands’ essential function in cleansing the city’s waste. Dr. Ghosh’s research was instrumental in halting the encroachment of urban development into this critical area.

This research spurred scientific interest in understanding the wetland’s natural wastewater treatment processes. In 2002, the East Kolkata Wetlands were designated a ‘Wetland of International Importance’ under the Ramsar Convention, solidifying their protection and raising awareness of their importance. The joint effort of researchers and government were instrumental in halting the urban encroachment into this critical area and keeping it intact. Furthermore, researchers started supporting the fishing communities to further improve pollutant absorption and decomposition, jointly ensuring the safety of the fishing communities and consumers. 

Fishermen await another day in the wetlands. 

As urbanisation accelerates and the impacts of climate change intensify, there is growing global interest in solutions that leverage the innate properties of nature to address environmental challenges. The East Kolkata Wetlands stand as a powerful example of such a nature-based solution. Their success stems not only from a rich aquatic ecosystem but also from traditional knowledge cultivated through generations of community stewardship and intimate connection to place. The Wetlands show us the necessity of going beyond just simply consulting with local communities when addressing environmental challenges and instead following the lead of communities as stewards of their land. We must remember that nature-based solutions are a tool where the wielder of the instrument is just as important as the instrument itself.

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The West Bengal Water Project is comprised of Oxford researchers Raphaela Betz, Patrick Robichaud, Susana Higueras, William Wallock, and Victoria Taylor.

The research group is an official Oxford University project sponsored by the University’s Exploration Club with support by the Royal Geographic Society and other funders. Part of this journey included a visit to the East Kolkata Wetlands, guided by Professor Subhamita Chaudhuri. Learn more at waterwestbengal.com