The city of Kolkata, India sits on the Hooghli River, a major distributary of the Ganges, near where the river empties into the Bay of Bengal. According to the World Wildlife Fund, the projected effects of climate change on Kolkata—and eastern Kolkata in particular—make it one of the most vulnerable population centers in Asia. Increased flooding poses the primary threat, with flooding of the Hooghli already a daily reality during monsoon season (The World Bank, 2010). Rapid urbanization has compounded this issue, making the city hotter and less permeable by replacing open space with impervious construction. This has been accompanied by an influx of new climate migrants living in informal settlements on available open space at the city’s periphery who have been displaced from their homes along the Bay of Bengal due to increased flooding from storms and sea level rise (Sengupta, 2018).


Covering more than 240 hectares, the East Kolkata Wetlands are the world’s largest landscape-based sewage system, a collection of approximately three hundred man-made fish ponds, with additional agricultural plots, fed and fertilized by wastewater diverted from the city by a series of canals (New Agriculturist). Using flora, fauna, sunlight, and gravity, instead of the more-energy-intensive technologies seen in mechanical wastewater treatment facilities, the Wetlands filter toxins from human waste, remediate the watershed, and create aquaculture and farming opportunities in the region. This, in turn, sinks approximately 180Gg CO2 each year while reducing soil nitrogen fixation and thus the need for chemical fertilizers. The Wetlands also act as an absorption buffer for the city to mitigate flooding caused by increased runoff from Himalayan ice melt upland and from storm surges in the Bay of Bengal. The ponds, which have been constructed from the remains of a drying wetland over the past century, are independently owned and operated while sharing access to the municipal canals. They were first identified formally as a single entity by the ecologist Dhrubajyoti Ghosh, a vocal champion of the fisheries who gave them the collective name “East Kolkata Wetlands” as a part of his advocacy efforts to preserve them in the face of competing urban land development pressures.


While not disputing the ponds’ capacity for climate change mitigation, some who have studied the East Kolkata Wetlands have cautioned against exporting the model to other parts of the world, especially to the West, without making region-specific amendments. Ghosh has noted that the Wetlands’ ability to provide food, jobs, and free sanitation depends upon two conditions: sunlight and poverty. The former condition transforms waste into nutrient-rich fish food, the latter condition creates the imperative for the intense, unpleasant, and underpaid labor required to operate the fish ponds while maintaining their thin profit margins (Jadavpur University, 2011). There are also regulatory structures in many parts of the world that would make selling fish and crops grown with sewage legally challenging; even in Kolkata, residents who have a choice are unjustly skeptical of the quality of the fish that are raised in the Wetlands (Chakraborty and Gupta, 2019). 

Organizations & Governance

wetland - aquaculture - sewage - wastewater - effluent - peri-urban - sprawl - aeration - macrophyte - traditional ecological knowledge (TEK) - anaerobic digestion - monsoon

Kolkata

With waters rising in the northern Bay of Bengal at nearly twice the rate of most of the world (a positive trend of 6.26 ± 1.29 mm per year) and a rate of land subsidence as high as 13.78 mm per year, the city of Kolkata is extremely vulnerable to climate change-related flooding. This threat is compounded by the city’s expanding urbanization, concentration of poverty, the loss of farmland and food security, as well as open spaces that might otherwise absorb rainfall. The East Kolkata Wetlands attempt to address many of these issues, while illustrating a climate mitigation project tailored to a specific landscape and labor conditions.

Kolkata sits near the southern extreme of the state of West Bengal, which stretches from the Himalayan mountain range at the north to the Bay of Bengal at the south. West Bengal’s position between the mountains and the bay makes it particularly susceptible to extreme weather, particularly during monsoon season (July-September). Kolkata’s low-lying position and proximity to the Hooghli River places it further at flood risk.

Over the last fifty years, Kolkata’s footprint has expanded rapidly and unevenly. Its expanding periphery encroaches on the surrounding wetlands in the form of speculative development, informal villages, and expanding waste and infrastructural externalities from a growing population.

The East Kolkata Wetlands rely on a steady stream of wastewater from the city’s extensive system of urban canals to maintain functioning fish habitats. The Wetlands process nearly half the city’s effluent, while the other half is directed/channeled to Kolkata’s mechanical treatment facilities or into the Hooghli River untreated. Purifying human wastewater for agricultural and aquacultural use relies on a range of processes/factors including the anaerobic digestion (decomposition/composting) of the waste itself, the coordination and enforcement of land protection policies, the availability of local manual labor, and access to reliable electricity. 

The Wetlands system is complex and depends on underpaid labor to process the city’s garbage and sewage. Canals are controlled through a series of locks that manage levels and flow rates into the ponds. The operators of the fish ponds (bheri) use several techniques to prepare incoming wastewater and grow their fish populations. The result is both 10,000 tons of fish grown (Doshi, 2017) and wastewater cleared of nearly all pollutants by the time it leaves the system through eastern outlet canals en route to the Bay of Bengal (Jana, 1998). Many of the bheri workers live in villages surrounding the wetlands area.