India - Calcutta Oriental - Aprovechándose del Humedal: Aguas Residuales, Pecinas y Agricultura

  • Autor: Amanda Suutari
  • Aportaciones editoriales: Gerry Marten
  • Fotogalería de esta historia

Los humedales de Calcuta Oriental son un sistema complejo y poco conocido. Procesan los residuos de la ciudad (tanto sólidos como líquidos) – un servicio que sería muy costoso reemplazar. También son hogar de varias poblaciones humanas y animales. Este caso demuestra la importancia de "permitir a la naturaleza hacer el trabajo", un principio de los Puntos de Inflexión Ecológica.


Sometimes called the "kidneys of Kolkata (Calcutta)," the East Kolkata Wetlands (EKWs) are the largest of their kind in the world, covering an area of 12,500 hectares which were designated as a RAMSAR* site in 2002. This multiple-use wetland lies east of the city and includes a garbage dump known as Dhapa Square Mile, a mosaic of vegetable fields, a series of 300-odd fishponds connected by major and secondary canals, rice paddies, wholesale markets, a few roads, and 43 villages (with 60,000 people in all). The EKWs extend almost equally on both sides of a Dry Weather Flow Channel, which discharges into the Kulti Gong (the wastewater outfall of Kolkata 28 square kilometers to the east).

The area is divided into eleven zones and includes four major sub-regions: freshwater fishponds, brackish fishponds (both of which are known as bheris), garbage farms, and paddy lands. Some thirty kilometers eastward, the river Kulti-Bidyadhari drains into the Bay of Bengal. Underneath the city is a large supply of groundwater. The fishponds produce some 13,000 tons of fish annually, whose yield at 2-4 times higher than average fish ponds, is among the best of any freshwater pisciculture in the country. Some 150 tons of vegetables per day are harvested from small-scale plots irrigated with wastewater.


The EKWs are part of a region which is the mature delta of the Ganges River where its tributaries eventually drain into the Bay of Bengal. The climate is hot, humid and monsoonal, with the average yearly rainfall at about 1600 mm (mainly between mid-June to mid-October). January is the coolest month while May is the hottest. The tropical region is naturally suitable for using solar radiation to improve wastewater quality.

The EKWs are home to over 100 plant species, rare mammals including the marsh mongoose, the small Indian mongoose, the palm and Indian civets, and the threatened Indian mud turtle. There are also some 40 species of birds, both local and migratory, which include kingfishers, grebes, cormorants, egrets, terns, eagles and sandpipers. The wetlands act as a "sink" for the city of Kolkata, which has no conventional sewage treatment facility for its 12 million inhabitants. Some 3,500 tons of municipal waste, and 680 million liters of raw sewage enter the wetland system every day. Still, only 30% of the total wastewater is used for aquaculture or irrigation, while the remaining 70% flows directly into the Bay of Bengal (which pollutes the estuary, highlighting the need for improved efficiency of the system).

Land Use History

Throughout the ages, urban wetlands, either natural or manmade, have been a part of many Indian cities, which were preserved by people as the main source of water supply for drinking and irrigation. From the 15th century, the Ganges changed its main flow from the Bhagirathi to the Padma River, and this eastward change in direction profoundly altered the process of delta creation in south and central Bengal state. A number of tributaries were cut off from any upland flow and that signalled the end of these channels. But human interference in the region further cut off this spill area and channel beds rose, escalating the process of decay, which caused a river which fed into the wetland areas, the Bidyadhari, to dry up, stopping the silt deposition in spill areas.

Kolkata grew up along the Hooghly River, a tributary of the Ganges. The Hooghly River lies to the east and the now defunct Bidyahari River to the west. Since the mid-1800s, the area east of the city had been used as a garbage dump, where nearly 60% of its waste is delivered. When garbage was first brought to the area, the fertility of the soil began to improve (because the garbage had a much higher proportion of biodegradeable waste in contrast to the higher percentage of synthetic and nonbiodegradable content today). The soil then became ideal for farming. At the same time, the drying up of the Bidyadhari spill channel caused the delta to deteriorate into a derelict, brackish swamp. In the 1930s, the Bidyadhari only carried city sewage and in the process became choked further because of the high silt content of the sewage. When a Dry Weather Flow channel was constructed in 1943-44 to carry city sewage to the Kulti Gong (another river in the area), more wastewater was brought there, which increased its freshwater content. A large fish producer began growing fish in a large water area using city sewage. Local farmers stocked some of the ponds and dug new ones as well. Landlords, many of them absentee, leased most of the ponds to commercial managers, while others were managed by the government, and still others were given to fishermen's groups and co-operatives. These sewage-fed fisheries spread quickly, and were innovated, developed, and upgraded by local fish producers and farmers.

Part traditional knowledge, part enterprise and creativity, much of the folk technology is retained in the informal, oral tradition. The system reached peak productivity in the middle of the century but is now declining due to enocroachment and development fed by a real estate boom starting from the end of the 1980s.

Functioning of the System

In India, night soil and wastewater use in agriculture is a traditional practice which had been followed for centuries, but there are no other examples in India or the world of this scale and ingenuity.

The EKWs are a delicate, complex and poorly-understood - and so highly vulnerable–system in danger of being swallowed by development. From the 1980s, the system was the focus of study by various academics, but this has failed to yield concrete results in protecting the wetlands. The West Bengal Government's Institute of Wetland Management and Ecological Design has also initiated various projects to study and conserve the wetlands in a practical process that includes participation by all those who depend on the wetlands. It has worked with the UK's Department of International Development (the government's foreign aid agency) to create an action plan. Despite this, aspects of the system, such as the health issues of sewage-fed fisheries and contaminants in the garbage-raised vegetable plots, need further study. (See below).

At the eastern edge of the city lies the fields which grow vegetables on the waste from Dhapa, the garbage dump. The vegetable production is a household activity, with people renting small plots or subletting smaller plots for household sustenance and income. These are designed with alternate bands of garbage-filled lands and long trench-like ponds known as "jheels," where sewage is detained for some time, and then used to irrigate vegetable fields.

The wastewater enters the wetlands through a network of drainage channels which flow into the canals and feed the fish ponds. On the way, the sunlight acts as a purifying agent on the sewage, which triggers biochemical reactions. For example, BOD (biochemical oxygen demand) is reduced through a symbiosis between algae and bacteria, where energy is drawn from algal photosynthesis. Each hectare of a shallow water body can remove about 237 kg of BOD per day. This helps in the reduction of coliform bacteria prone to be pathogenic which even conventional mechanical sewage treatment plants may not be able to fully eliminate. The effluent from the fishponds is then made to drain further southeast where the paddy fields have been strategically located to benefit from the use of the effluent. Most of the fishponds range in size between two and ten hectares, but some individual ponds are over 70 hectares. Three types of ponds are needed according to the stage of production: the nursery pond, the rearing pond and the stocking pond. Each needs a proper inlet-outlet management of sewage. The main requirement for a productive fish pond is the proper supply and quality of wastewater. Poor quality sewage reduces quality of nutrients, a higher toxic load on fish, and requires external inputs of nutrients. Countering this situation is done by allowing fish to grow bigger, but then this leads to a conflict of interests with unions, because this means a reduced number of harvesting days (and therefore days of work). The flow of water is mostly directed by gravity but in some areas diesel-powered pumps are used.

Fish are raised in five major phases: Pond preparation (done in the coolest months when ponds are drained, and maintenance or repairs of dikes are carried out), primary fertilization (initial introductions of wastewater into the pond and allowed to undergo natural purification, and stirring of the pond in order to reduce anerobic conditions in the sediments), fish stocking (where farmers initially stock a small number of fish to test for water quality, subsequently stocking up to four times), secondary fertilization (periodic introductions of wastewater into the ponds throughout the growth cycle), and finally fish harvesting (taken at different times according to species).

About 12 different species of fish are raised which occupy different ecological niches of the pond ecosystem, including Indian major, minor and exotic carp, varieties of non-native tilapia, mourala, and the freshwater giant prawn. In general, fish yields in the wetlands are found to be 2-4 times higher than those from ordinary fish ponds. Fish is generally sold in the fish markets in wetland areas or in Kolkata.

"Silt traps," which are three meters wide and 30-40 centimeters deep, are pits at the edges of bheris, which trap some of the silt buildup in the ponds themselves. These are periodically dredged and used to strengthen pond dikes. Ponds themselves are periodically drained before the monsoon season (which will naturally help to refill them) for various reasons: Draining them helps to free up some of the nutrients in the sediments if they are exposed to air, and it also kills some of the parasites that affect fish production.

Those who work in the wetlands either live on-site in the villages, migrate to various parts of the site depending on seasonal needs, or migrate in for work from outside. Jobs include farming, garbage sorting, fish farming, trading, auctioneering, selling, raising fish seed, making nets, maintaining drainage, and reinforcing banks. Many people depend on the wetlands not only for livelihood but also for the fish and vegetables produced in the area. In "Wastewater-Fed Aquaculture in the Wetlands of Calcutta - An Overview," Dhrubajyoti Ghosh writes that there is an "outstanding demonstration of human skill in managing a productive system."

Water hyacinths also play a role in the system's function. By breaking up surface waves, erosion of the ponds' banks are reduced, replacing more expensive alternatives like stone or concrete to strengthen the banks. Also, they provide shade to fish during the summer months, and the roots, well known to absorb metal ions, help to leach heavy metals out of the water. Occasionally they must be harvested, and they are used as buffalo feed or decomposed and fed to carp. Other important species in the system include domesticated animals such as cows, buffalo, pigs and ducks. Pigs run semi-wild and feed on whatever they can find. Ducks eat aquatic snails harvested from the ponds. Algae also plays an important role, as it not only provides food to fish, but supports certain bacteria which act on organic matter in the sewage.

Land Ownership

There are various types of land ownership of fish ponds:

  1. Owner-managed (but this is disappearing because of uncertainties of the wetlands' future)
  2. Cooperatives: Some 13 Cooperatives with about 300 farmers, these function well without any external assistance
  3. State government corporation (only two ponds are under this control).

Much of the land is also leased out by absentee landowners, some of which are further sublet.

Income and Expenditure

This has not been studied extensively, and the reliability of existing data is poor, as private farmers may not disclose everything to researchers, and the State-managed fishponds are too few to fully represent the entire area.


Despite their potential to provide multiple natural and social services, the wetlands are an informal system. As such, they need further study, effective maintenance, monitoring, and upgrading.

Urban development: With encroaching eastward development of the city, there is speculation that certain areas will succumb to development. Whether or not this happens, this speculation alone is a threat to the future of the wetlands as it makes locals wary of investing their labor in long-term maintenance and upkeep of the ponds.

Pollution: Some unregulated industries, such as tanneries, have been releasing untreated effluent directly into the wetlands, further threatening the water quality.

Siltation of the ponds: Through natural processes, the buildup of silt has reduced the capacity of fish ponds over time. Some, originally six feet deep, have silted up to two feet from the surface, which has reduced fish production by two-thirds. The dredging and transport needed to clean out the larger ponds requires a large investment, even though the silt can be used for landfills planned for new townships north of the city.

Complexity: Because this is an urban area, there are more competing, conflicting interests and agencies than would be found in a village. Moreover, unlike rural development, the issues are less black-and-white, for example, income opportunities exist locally whether or not the wetlands are preserved (although the income and quality of life for those who depend on the wetlands might change dramatically if they were to disappear).

The presence of the Mafia: The presence of the Mafia in the control of fish ponds is a subject no one knew much about (or were not eager to talk about).

Safety of fish and farm produce:Although a study done by Ghosh et al. in 1980 found that the production of fish was not affected even by levels of ammoniac-nitrogen concentration of 5.13 milligrams/liter (the maximum limit is 0.1 mg/liter), there is a need to study the health issues of raising fish in sewage, and feeding water hyacinths, which may contain heavy metals, to water buffalo and fish.

The separation of garbage: While many Indian municipalities rely on the informal sector to supplement overburdened waste management services, there remain questions about the system's effectiveness and more importantly, the environmental and moral issues of relying on so-called "ragpickers" to sort municipal garbage. First, these are the poorest members of society, typically women and children. The work is filthy and dangerous as it puts workers - most of whom do not wear protective clothing or gloves - into contact with broken glass and medical and other hazardous wastes. Many suffer from injuries and chronic skin diseases. Secondly, only garbage which can be sold is removed, which means that some non-biodegradable wastes that have no commercial value, such as plastic and foil wrappings, remain in the soil where vegetables are grown. (On our tour we saw the fields where vegetables were grown and it was full of tiny bits of foil and plastic. We also saw piles of plastic garbage being burned, releasing foul-smelling black smoke into the air.) This brings into question the safety of the food and fish grown in the wetlands, with the possibility of contamination of soil or vegetables with heavy metals or other toxins. (See quotes from Nitai Kundu below). Third, there is the fact that a composting plant which has opened at Dhapa is taking biodegradeable garbage (which is sold to tea gardens) away from the vegetable farmers, so the soil quality is declining. As a result, vegetable farmers have begun using chemical fertilizers to replace the lost soil fertility.

Efforts to Preserve the Wetlands

Some NGOs have been lobbying for the preservation of the wetlands in fear that the "invisible" services this vast region provides - such as wastewater treatment, collection of rainwater run-off, preservation of biodiversity, food production, and income generation - will be lost, resulting in an increased financial burden to replace them. There have been some efforts to address some of the wetlands' problems by various government agencies under their jurisdiction, including the Department of Fisheries, Department of Agriculture, Irrigation and Waterways, the Kolkata Municipal Corporation, the Kolkata Metropolitan Development Authority, and the Panchayats (District Councils).

The Institute of Wetland Management and Ecological Design was set up by the Government of West Bengal, and among other projects, it has taken several wetlands initiatives. First, it has used GIS (geographic information systems) technology to create various maps of the wetlands in order to analyze the state of each zone, and better understand the hydrology, land use, topography, and changes to the system over time. The Institute has also created the East Kolkata Wetlands Management Committee and various sub-committees which are working on an action plan that brings together all "stakeholders" of the wetlands to share their knowledge and cooperate on their preservation. It is also intended to create better technical knowledge and systems for waste management and farming practices as well as addressing legal issues which would prevent conversion of water bodies, specify property ownership, and amend existing land use policies. The project gets funding from DFID (Department for International Development, the UK government's official overseas aid agency), the ADB (Asian Development Bank), and the central government of India.

A series of "multi-stakeholder workshops" (11 local and 3 major) was held to come up with specific proposals for the wetlands. These took place between 2000 and 2003 through interviews with fish, rice and vegetable farmers, community meetings and household interviews. What emerged were several insights which helped to create an EKW conservation plan. Water management was within all of the larger issues (silt buildup, pollution, decrease of nutrient content, sluice gate management). Solutions proposed during these sessions varied in focus and scale. For example, some were technical (desilting of canals), government-related (policies), environmental (pollution control), institutional (creation of new agencies), and social (capacity building and training). The work is still in its early stages, but according to Stuart Bunting, the action plan has had good feedback as it has finally brought together and coordinated groups that were working in chaos and cross-purposes, and set the stage for more clarity and organization in the conservation process.

According to a report by the World Bank and the International Development Research Center in Canada, the reuse of sewage in fish culture, algal and aquatic plant production and energy production have been identified as new, promising technologies that radically change the context of urban sanitation. The EKWs are an opportunity for research and study that may influence the development of low-cost sanitation technology, especially for developing countries.

In "Wastewater-Fed Aquaculture in the Wetlands of Calcutta - an Overview," Ghosh writes:

"This system shows the significance of reuse in waste disposal and sanitary engineering which has not been well appreciated. It's been shown that integrated resource recovery and institutionalization of urban solid and liquid waste management can reduce costs to municipalities by 30-90%."

If the wetlands fall prey to urban expansion, the results will be not only the loss of food and livelihood, but will require the construction of an expensive sewage treatment facility to replace the services it currently offers. This case demonstrates the value of "invisible services" such as waste management, food production and livelihood, and it highlights how nature (sun, bacteria, algae, water hyacinths, fish and various fauna) can "do the work" to provide these services on a relatively large scale and in an urban environment.

Therefore in this context, the EKWs, despite their problems, are an excellent example as one of the world's largest systems of resource recovery.


  • Bunting et al. "Workshop Proceedings: The East Kolkata Wetlands and Livelihoods." July 2001. Online link
  • Ghosh, Dhrubajyoti. "Wastewater-Fed Aquaculture in the Wetlands of Calcutta - an Overview." 1990. Online link
  • Ghosh, Dhrubajyoti. "Turning Bad Water into Good." Changemakers Journal, October 1998. Online link
  • Kundu, Nitai. Planning the Metropolis: a Public Policy Perspective. Kolkata: Minerva Publishers, 1994.

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Traducción: David Nuñez. Redacción: Gerry Marten

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