Wastewater-Fed Aquaculture in the Wetlands of Calcutta - an Overview

By:  Dhrubajyoti Ghosh / Institute of Wetland Management and Ecological Design

Tomado de: Wastewater-Fed Aquaculture / Proceedings of the International Seminar on Wastewater Reclamation and Reuse for Aquaculture, Calcutta, India, 6-9 December 1988


Little scientific and technical attention has hitherto been given to the wetlands to the east of Calcutta, a unique agro-ecosystem that has sustained the world's largest practice of integrated resource recovery for more than 50 years. Only recently has conservation of wetlands become a point of discussion amongst environmentalists, and we have to go a long way to recognize the full potential of the Calcutta waste disposal system for its range of environmental, economic and scientific benefits. An overview of the Calcutta wetlands system is presented and a case is made for further study to improve its operation. The system has potential as a case-study for the development of integrated resource recovery alternatives for low-cost sanitation technology.


The significance of reuse in waste disposal and sanitary engineering has not, in general, been appropriately recognized. It has been shown that integrated resource recovery and institutionalization of urban solid and liquid waste management can reduce their cost to municipalities by 30-90% (Gunnerson 1984). In a report published jointly by the World Bank and the International Development Research Centre, Canada, reuse of sewage in fish culture, algal and aquatic plant production and energy production have been identified as new and promising technologies that radically change the context of urban sanitation (Rybczynski et al. 1978). It is in this context that the traditional practice of using sewage in aquaculture and agriculture in the wetlands to the east of Calcutta assumes significance.

In the course of the last decade, both natural and artificial wetlands are increasingly being identified as efficient ecosystems for improving wastewater quality and recovering its nutrients (Maltby 1986; Mitsch & Gosselink 1986). To the east of Calcutta the natural wetland has been modified into a cluster of ponds that grow fish on sewage which is locally called the Bheris. Pond effluent is then used for irrigation (Ghosh and Sen 1987). The present study of Calcutta's wetlands assumes additional significance because the technology of modifying wetlands for efficient disposal of wastewater is indigenous. However, the practice is largely retained as an oral tradition of the local farmers.

This agro-ecosystem presents an opportunity for research and study that may influence the develpment of low-cost sanitation technology for developing countries using the new approach envisaged in the World Bank/IDRC report (Rybczynski et al. 1978). The present overview attempts to highlight this aspect using observations collected by this author in the course of the past eight years of association with this agro-ecosystem, and those of the persons who created it and manage it. The present enquiry has not provided well researched answers to all the questions that keep on arising out of such a complex system. However, its purpose is to stimulate scientific minds to initiate the organized research effort on this agro-ecosystem, which the Calcutta wetlands richly deserve.

The Calcutta Wetlands

The wetlands are located to the east of Calcuta between the levée of the River Hooghly to the west and that of the River Bidyadhari, presently a derelict channel, to the east. These extend almost equally on both sides of the Dry Weather Flow (DWF) Channel which discharges into the Kulti Gong, the wastewater outfall of the city of Calcutta, 28 km to the east. The Wetlands lie approximately between 22°25' to 22°40' latitude north and 88°20' to 88°35' longitude east. The tropical region is naturally suitable for using solar radiation to improve wastewater quality.

When low lying land is available at the edge of a city for the purpose of dumping solid wastes, the entire area is gradually filled up and simultaneously raised. Uniquely, for Calcutta this did not happen. The person who took lease of a piece of land measuring about one square mile (the Dhapa Square Mile) on the eastern edge of the city wanted to grow vegetables on a garbage substrate (Ghosh & Sen 1987) which needed a constant supply of water for irrigation. Therefore, he evolved a garbage disposal plan that left out intermediate gaps for water storage which emerged as elongated ponds. Fish might have been cultured in these ponds in the nineteenth century which means that the practice of sewage-fed fish culture in the wetlands of east Calcutta might be older than is generally supposed.

Since 1850 the wetlands of Calcutta were reclaimed for brackishwater aquaculture. The source of water at that time was the tidal River Bidyadhari. This river, however, lost its flow and was declared dead by the Irrigation Department of Bengal in 1928. With the death of the river and the elimination of the river and the elimination of the tidal water which fed fishponds, the entire area gradually became a vast derelict swamp. As the fishponds were deprived of a continuous supply of tidal water to run viably, one of the leading fish farmers experimented with sewage as a substrate for growing fish in 1929-30 (Ghosh & Sen 1987). The fish farm where sewage was first successfully used still exists although it is under serious threat from the advancing metropolis of Calcutta. However, wastewater-fed aquaculture did not spread over the entire wetland region until the completion of Mr.B.N.Dey's Outfall Drainage Scheme eatwards to the River Kulti. Provision was made in the scheme to raise an adequate water-head and to supply sewage to most of the fishponds by gravity. The wastewater-fed fishponds grew farther east to cover over 8,000 ha.

The region in which different practices of resource recovery take place covers about 12,000 ha (Table 1). Waste recycling includes four principal resource recovery practices: garbage vegetable farms; wastewater-fed fishponds; paddly fields using fishpond effluent; and sewage-fed brackinshwater aquaculture. The area of demarcation follows the administrative boundaries of the villages.

The effluent from the wastewater-fed fishponds is utilized to grow non-monsoon paddy. This downstream resource recovery practice in which fishpond effluent is used to irrigate paddy fields is a recent innovation in comparison to garbage vegetable gardens and wastewater-fed fishponds. Using sewage in agriculture is a well known tradition in India but in the Calcutta wetlands initial resource recovery and purification of sewage in fishponds have already taken place. The potential health risk from agricultural produce is much less by using effluent from wastewater-fed fishponds than using raw sewage.

Sewage-fed brackishwater aquaculture has flourished since 1960 in estuarine areas 30 km east of Calcutta (WBSLUB 1984). These systems use city sewage that drains into the River Kulti, the receptacle for Calcutta's wastewater.

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In fact, a major share of Calcutta's wastewater does not enter any of the wastewater-fed fishponds at all but flows direcly to the river. The requirements for sewage in brackishwater aquaculture, both quantitative and qualitative, are poorly undestood and a study will be needed before a comprehensive planning of resource recovery can be completed. A planning effort on integrated resource recovery in the Calcutta wetlands should consider the distribution of the city sewage to both wastewater-fed freshwater fishponds and the brackishwater aquacultural systems located beyond the Kulti outfall point.

The importance of drainage outfall systems can hardly be overstressed in wastewater-fed aquaculture. Domestic sewage and storm water from the city of Calcutta are mostly carried through combined sewers. However, the outfall drainage channels were separately designed for dry weather flow, termed the "Dry Weather Flow" (DWF) Channel, and for storm water flow, termed the "Storm Water Flow" (SWF) Channel.
The population covered by the outfall channel capacity is 4 million, over an area of about 94.5 km2.

The DWF Channel starting from Topsia Point A covers a lenth of 32 km to reach the River Kulti Gong at Ghushighata. At 6.4 km from Topsia Point A, the outfall channels are connected with the sedimentation tanks at Bantala. These tanks were constructed in 1943.
The design capacity of the DWF Channel is 387 cusec (cubic feet/second) (14.3 m3/sec) which accounts for 296 cusec of wastewater generated by an assumed population of 4 million, allowing for a water consumption of 50 gallons/caput/day (250 l/caput/day). The River Kulti Gong is a tidal stream and an additional provision of 69 cusec is necessary to account for the tidal lockage (on account of the tidal conditions of the river, the water level of the river is higher than that of the drainage canal for part of the day; for that period the gates at the mouth of the DWF channel are closed to stop the entry of river water into the drainge canal for part of the day; for that period the gates at the mouth of the DWF channel are closed to stop the entry of river water into the drainage canal, a period known as tidal lockage).

The SWF Channel was constructed to carry the storm water of the city and a part of the adjacent urban and rural areas. Covering an area of about 150 km2, the SWF Channel begins at the Ballygunj drainage pumping station and traverses about 34 km to reach the River Kulti Gong. The capacity of the SWF Channel at one stage was 2,011 cusec, in accordance with the recommendation of the Greater Calcutta Master Plan Technical Committee set up in the late 1950's by the Irrigation and Waterways Directorate of the Government of West Bengal.  Subsequently, with the rise of population and an increase of impervious areas within the command area of the city, the capacity of the SWF Channel was remodelled to accommodate a discharge of 4,966 cusec (Table 2). The channel section was also remodelled to account for the above discharge along with the normal tide lockage provision.

Table 2. Remodelling of the capacity of the Storm Water Flow Channel  Capacity (cusec).
DPS is drainage pumping station

        Section                                                                                                                    Capacity (cusec)
1. Ballygunge DPS through                                                                                                     1,300
    Suburban Head Cut
2. Palmer Bridge DPS through
    Town Head Cut                                                                                                                  2,650
3. Dhapa Lock DPS through
    Town Head Cut                                                                                                                     500
4. Chowbhaga DPS through
    Town Head Cut                                                                                                                  1,450
5. Pagladanga and Kulia Tangra
    DPS through Town Head Cut                                                                                                307
    Total                                                                                                                                   6,270
    Less 20% due to non-synchronisation
    of peak pumping of all the stations                                                                                       4,966

Combining Sewage Treatment with Aquaculture

People's Knowledge in Integrated Resource Recovery

The farmers in the wetlands grow vegetables and rear fish using municipal garbage and sewage, respectively. This is an outstanding demonstration of human skill in managing a productive system that specialists now desire to understand. The entire tack of water quality management is carried out with the help of natural indicators. One should not forget that the farmers have run the system for the last 50 years while conventional mechanical sewage treatment plants have performed dismally in India, with frequent breakdowns before they are finally abandoned as junk. The major causes contributing to this phenomenal wastage of public money for setting up such treatment plants are two-fold:lack of proper operation and maintenance; and shortage of municipal funds to meet the high operational cost of such plants.

Scientist should now step in amidst the experience of the farmers to find out the scientific basis for what the farmers believe and practice. This would lead to an understanding of how the system operates and to the standardization of the knowledge so that the system can be better managed and more widely disseminated.

Learning as a process is incomplete as long as it remains confined within books and laboratories and planning efforts will suffer from lack of grass-roots information. This inhibits the prospect for understanding how such a self-reliant technology developed. It is in this light that the knowledge of the farmers needs to be viewed to understand this technology to recycle municipal waste which has such potential for poorer cities.

The Sewage-Fed Fishponds

The hydraulic regime of the wastewater-fed fishponds of the wetlands has unique properties; it is neither lentic nor lotic. Wastewater is introduced into the fishponds in batches and is similarly released. When these ponds are large, more than 40 ha, the wastewater inflow is almost continuous and the water regime becomes lotic.

The lacustrine features resemble fairly closely those of facultative ponds designed for wastewater treatment, buy they are aerobic at all times because of a very low organic loading. Thus, they also have features of maturation ponds. The inlet arrangement in wastewater-fed fishponds uses an edge feeding system in which sewage in small doses is released into one side of the pond during the fish culture cycle.

The depth of the ponds varies between 50 to 150 cm. The pond bottom is fairly flat. The photosynthetic activity within these ponds is the basis of natural biological purification. Tropical regions are always more suitable than subtropical and temperature regions for treating sewage in ponds. However, data on organic loading rates of sewage-fed fish ponds in the Calcutta wetlands are lacking.

Calendar of Activities

The wastewater-fed fishpond system follows a systematic calendar of activities involving five major phases: pond preparation; primary fertilization; fish stocking; secondary fertilization; and fish harvesting (Table 3). Each phase comprises one or more activities.

Pond preparation is carried out during the coolest months of the year and involvs: complete draining of the pond; sundrying of the pond bottom; and dike repairing activities. In the middle of Febreruary primary fertilization takes place when wastewater is introduced into the pond and is allowed to undergo natural purification; before any fish are stocked, the pond is stirred intensely to reduce anaerobic conditions in the sediments and promote the development of benthic organisms for fish feed.

From the middle of March, fish stocking takes place. To ascertain the quality of water for fish growth, the farmers have introduced a process of stocking test fish in which a small number of fish are stocked as probe species to test water quality.

Table 3. Five major phases, each involving one or more activities, characterize
the Calcutta sewage-fed aquacultural system.

                           Phase                                                                                    Activity
  1. Pond preparation                                                                                        1. Pond draining
                                                                                                                        2. Sun drying
                                                                                                                        3. Desilting silt traps
                                                                                                                        4. Tilling
                                                                                                                        5. Repairing dikes

2. Primary fertilization                                                                                        1. Filling with sewage
                                                                                                                        2. Facultative stabilization
                                                                                                                        3. Stirring

3. Fish stocking                                                                                                 1. Test fish
                                                                                                                        2. Fish stocking proper

4. Secondary fertilization                                                                                    1. Filling with sewage

5. Fish harvest                                                                                                   1. Net selection
                                                                                                                        2. Team management
                                                                                                                        3. Haul disposal

After obtaining satisfactory water quality, fish stocking proper takes place. In general, fish are stocked four times. Indian major carps are stocked four times. Indian major carps are stocked twice. The size of fish stocked varies in the first and second stockings, being 50-60 fish/kg (16.7-20.0 g) and 10,000-40,000 fish/kg (0.025-0.10 g), respectively. Silver carp are stocked in July and common carp in December, each at between 400-600 fish/kg (1.7-2.5 g). Data are not available on the total weight of fish stocked. This pattern is the main trend but is not followed invariably.

Secondary fertilization consists of periodic introduction of wastewater into the ponds throughtout the growth cycle. However, the wastewater inflow may be continuous in ponds larger than 40 ha for a prolonged period of 15-21 days with the same volume of water being drained from the fishpond to maintain a constant water level. Sewage is added to the fishpond to stimulate sufficient plankton growth for fish feed but care is taken to ensure that DO concentrations do not fall to the extent that fishes die.

Indian major carps stocked first are harvested between May to July, while those stocked later are harvested during August to October. Silver carp are harvested in December.  The winter months are the safest months to dry and prepare the ponds but if fish culture continues, fish harvesting also takes place at this time.

The type of sein net selected depends on the size of fish to be harvested. A typical harvesting team comprises 10-20 fish farmers and a superviser to provide general directions to optimize the haul. The fish are sorted in a boat and the harvested fish are taken to the nearest auction market within two hours by another group of workers and sold to "bidders" who take the fish to retail markets withing another hour. There is no system of refrigeration or use of ice and the entire harvest is sold fresh.

Aquatic Weeds and Silt Traps

The farmers utilize water hyacinth (Eichhornia crassipes), the floating aquatic weed, around the periphery of the pond for a number of useful purposes. A 3-4 m wide band is grown around the pond margin and is restricted by a system of fencing. Water hyacinth breaks the surface waves that would otherwise have reached the bank and eroded it. They therefore replace more costly alternatives like pitching (stone, brick or concrete) to avoid bank erosion. Water hyacinth also provides shade to the fish during summer and the roots are well-known absorbers of metal ions present in wastewater. Periodically, there is a need to harvet the weed, which is either used as buffalo feed or is decomposed on-site as feed for carps.

Silt traps, which are 3 m wide, 30-40 cm deep borrow pits around the fishpond periphery, serve a number of purposes. The bottom of the ponds are essentially flat and during the process of netting fis the bottom deposits are dragged into these pits. This ensures a considerable reduction in the rate of raising of the pond bottom due to sedimentation of sewage solids. Farmers restrict themselves to periodic excavation of these silt traps instead of digging the entire pond bottom and the removed silt is used for streghtheing the pond dike.


There are various forms of ownership of sewage-fed fishponds in Calcutta. Owner-managed fishponds is the oldest practice of wastewater-fed pond management in the wetlands. This form of management is, however, dwindling fast on account of the uncertainties that prevail in this area (Ghosh and Sen 1987).

There is a small cluster of co-operative sewage-fed fishponds in the region. Unlike experience of co-operatives elsewhere, these ponds are run fairly capably. More significant is the performance of a co-operative, still non-formal in nature, created by about 300 farmers which is doing well without any external financial or sicientific assistance.

There are only two sewage-fed fishpond systems being run by a State Government established Corporation. One of them has, of late, increased its efficiency after sorting out a number of problems of cash flow. Book keeping in these two farms is better than on farms managed in other ways.

Income and Expenditure

A number of studies have been made on income and expenditure of various resource recovery syste,s (Ghosh 1985a, 1985b; IWMED 1986) but the reliability of data on income and expenditure of sewage-fed aquaculture and vegetable cultivation is less than desirable for two basic reasons: farmers owning private farms provide information no better than what they desire us to know; and State managed systems comprise only two farms which are not representative samples for this purpose.
However, more dependable data on income and expenditure are being collected in a year long study of a co-operative sewage-fed fish farm which ha allowed complete access to books of records.

Planning Need

Integrated resource recovery systems and waste recycling regions in peripheral wetlands are new planning concepts for cities. Urban waste can be used in sewage-fed aquaculture and agriculture for improved sanitation and to provide food and employment. The Calcutta wetlands demonstrate the viability of such a system. The system, however, is informal and needs effective maintenance, monitoring and upgrading.

Expansion of cities by draining and raising the level of wetlands is a common practive. Immediate planning is needed to preserve the heritage of the Calcutta wetlands which are gradually being lost to urban expansion. The call for the preservation of the Calcutta wetlands originates from a public interest frame of reference for a resource recovery agro-ecosystem which has grown in symbiosis with natural biological phenomena and the city's waste disposal. Losing the wetlands would be the extinction of this creative heritage of the farmers, the world's largest system of resource recovery practices, and the only example of its kind in India if not in the world.

It may be appropriate to plan for the preservation of Calcutta's wetlands in the ambit of the Biosphere Reserves Programme undertaken by the Government of India. Such programmes are undertaken on the basis of definite spatial planning of "biosphere reserve" areas. A special task force convened jointly by UNESCO and UNEP in 1974 in Paris recommended a zoning pattern comprising a "core area", an "inner buffer zone" and an "outer buffer zone", subsequently called "core area", "buffer zone" and "transition zone", respectively.  This zoning pattern would have particular significance in the case of areas like the Calcutta wetlands because of their close proximity to dense human settlement.

A unified plan would have to be drawn up on the basis of all four sub-systems of waste reuse in Calcutta (vegetable cultivation; wastewater-fed freshwater aquaculture; fishpond effluent utilization in rice cultivation; and sewage-fed brackishwater aquaculture), The diurnal and seanosal requirement of sewage for each would need to be established to ensure improved utilization and management practices.

A serious challenge is to co-ordinate the various activities that are now being taken up by different agencies like the Department of Fisheries, the Department of Agriculture, the Department of Irrigation and Waterways, the Calcutta Municipal Corporation, the Calcutta Metropolitan Development Authority, Zilla Parishad Panchayats (District Councils) and others. It may be appropriate to create a separate- co-ordinating agency to synchronize the required study.

The establishment of a planned intervention could lead to he development of a resorce efficiente approach in using wetlands for waste disposal. Calcutta can give a lead, particulrly to provide for such needs in developing countries, given the valuable experience of an age-old, time-tested practice of waste recovery in the city. Studies in Calcutta could lead to the introduction of new alternatives in low-cost sanitation technology and the wetlands could become an international demostration centre for training and research in this field.


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