East Kolkata Wetlands: Turning Sewage into Fish

East Kolkata Wetlands: Turning Sewage into Fish

Nov 13, 2023

Patrick Robichaud

After a 30-minute drive from our apartment in southern Kolkata, we found ourselves arriving at the East Kolkata Wetlands. The journey takes us from the dense cityscape of concrete buildings to a sudden change of open ponds and marshes. Minutes after leaving the city, we saw a large hill that was the garbage dump of Kolkata and smelled the smoke from its smouldering fires. We left the smoke behind us and were soon entering the wetlands, which cover a total area of 12,500 hectares. Upon arrival, ponds greeted us on both sides of the road. It felt like we were in the middle of the countryside, rather than ten minutes from the edge of seventh largest megacity in India.

We then drove across the Bantala Lock Gate, which controls the level of sewage and stormwater flow entering the wetlands from the Kolkata Municipal Corporation. There are two channels of water entering the wetlands that are controlled. The first is the Dry Weather Flow channel, which was currently in use when we visited, while the second is the Stormwater Flow channel, that is used during the monsoons. We were accompanied by Professor Subhamita Chaudhuri from West Bengal State University who has worked in the wetlands for several years. She informed us that the city generates around 650 to 750 million litres of sewage daily. From the lock gates, the sewage is diverted into the fishery feeding canal, which directs it towards the fishponds where it is treated naturally.

Wastewater inflow from Kolkata. Birds gather to eat on the water hyacinth plants.

Once passed the lock gates, we saw how the sewage flowed through the ponds in a series of three from receiving untreated sewage in the first pond to fishing grounds in the third where the water is already treated. Sewage is nutritious for the fish as it is high in nitrogen. This effective system that the fishermen have designed works as follows:
In the first pond where the sewage flows first, the fishermen grow small fingerlings from fish-eggs, as this pond contains the highest amount of nutrition.
The fingerlings are then transferred to second pond, where they grow for three to four months.
Finally, they are transferred to the third pond for another three-six months, where they grow until they are big enough to be caught and sold.
The fishermen and women we met showed us their fishing techniques. They explained how some of them are part of fishing cooperatives that work collectively to manage the fish in their ponds. By being part of the cooperative, they share all the profits.

Normally, fishermen start fishing very early in the morning, before sunrise. We watched as eight fishermen skillfully used their boat and bamboo poles, as well as working in the water themselves, to corral the fish into the net. The pond was approximately 300 meters in length and 200 meters in width, while the fishermen’s net was around 40 meters. As such, they steered the fish into one corner of the pond for capture. When they were pulling up the net, the catch was thoroughly sorted. Only the larger fishes were kept, while everything else was thrown back into the pond, including water snakes and a monitor lizard. The fishermen work in every pond seven days a week, with a goal to catch 60 to 70 kg of fish from each pond. On this occasion, they had to deploy the net four times to reach their goal, however, on a good day, they would only need three deployments.

Fishing in the pond

As the fishermen worked the ponds, a few other people gathered at the banks to receive the fish and carry it to the market. They came either on foot or by bike and each had a large aluminium vessel to carry the catch. As the fish were unloaded, they were weighed on a balance and sorted by species. They informed us that there are about 7 to 10 species of fish in the ponds.

Woman carrying vessel containing fish

The local market, where the fish are sold, is a 20-minute walk or 10-minute bike away. We visited just one of 50 markets in the wetlands area that sells fish to wholesalers and small sellers from the city. We watched as the fishermen from the cooperative sold the fish to the middlemen at the market and who then sold it to the people that will sell it in the city. The fish go through a chain of buying and selling before being sold to the final consumers. This process is quick, with the fish being caught in the early morning and on the plate for lunch or dinner.

Fish at market being sold

Leaving the market, we realized how people utilize the city’s sewage, the pond system, the fish, and the markets to collectively create a circular cycle where all parties benefit. Although there is the risk of contamination from pharmaceuticals and microplastics, the fish appeared healthy, and the fisherman say they do not get sick from working in the wastewater. The East Kolkata Wetlands are the only wetlands in the world that clean sewage on a large-scale. They are a truly unique place, that exists due to the combination of ample sunlight, low wages, and an appetite for fish. This system, which has long been implemented by fishermen, was not recognized until the early 1990s when Dr. Dhrubajyoti Ghosh demonstrated the significance of the fishermen's contribution to wastewater treatment in Kolkata. The East Kolkata Wetlands are a unique space of nature and humans collaborating with co-benefits.