Kolkata, India Aquaculture: A Case Study in Wastewater Reuse
Below is a case study of the wastewater reuse aquaculture system in Kolkata, India.
Kolkata, India Aquaculture: A Case Study in Wastewater Reuse
Providing sanitation is recognized as being of prime importance in improving the general health of populations. By providing sanitation, infant mortality caused by communicable diseases, such as cholera, typhoid and diarrhoea is greatly reduced, as is the incidence of severely malnourished individuals with associated physical and mental health problems. The World Bank has suggested that life expectancy in communities generally increases as a result of providing sanitation. Inadequate sanitation results in the degradation and contamination of groundwater and surface water, in such situations it is often recommended that contaminated water be boiled, a process that uses large amounts of fuelwood. The combustion of which results in atmospheric pollution and may lead to an increase incidence of respiratory disease.
Sanitation needs are crucial in informal settlements where most of the migration to urban centers is landing at a rapid pace. The illegality of these informal settlements means there is little to no government resources to build the needed infrastructure to deal with the rapid increase in raw sewage and other wastewater. A common feature of low income settlements is untreated sewage running freely through the little open space available in these typically dense settlements. Even in Sao Paulo, Brazil, where government programs exist to upgrade the infrastructure in informal settlements, untreated sewage runs in open concrete channels which ultimately flow into the city’s river system, contributing to the eutrophication of the city’s watershed. This problem is especially acute in the informal settlement of Canthino do Ceu, which is located on the edge of one of Sao Paulo’s drinking water reservoirs.
Christian Werthmann’s class, Green Infrastructure in the Non-Formal City, asks students to research green technologies in order to develop a tool box for infrastructure upgrades in informal settlements through the lens of two informal settlements in Sao Paulo, Brazil – Canthino do Ceu and Paraisopolis. While both are impacted by untreated sewage, finding a wastewater treatment solution for Canthino do Ceu has been the focus of my research because of its adjacency to a drinking reservoir and its relative abundance of land. My research trajectory has been to find technologies that not only treat wastewater, but also provide benefits culturally, economically and ecologically. In essence, to find ways to not waste wastewater, but to utilize it as the resource it is.
While there are different technologies that convert human waste (aka “resource”) into electricity, gas or heat, I am interested in its agricultural application for several reasons. Firstly, human “resource” contains the same nutrients in commercial synthetic fertilizers – nitrogen, phosphorous and potassium. Secondly, the residents of Canthindo do Ceu are already practicing urban agriculture as is seen in aerial photographs. Additionally, we learned from Elizabete Franca, from the Favela Bairro program in Sao Paulo, that the residents have a close connection to the culture of their hometowns in Northeast Brazil. Assuming that food is central to cultural expression, I figured that the encouragement of urban agriculture will be accepted here over Paraisopolis, where the population equates the farming culture of Northeast Brazil with low social status. By designing a way to combine wastewater treatment flows with agricultural flows, we could economically close the nutrient loop of wastewater as well as provide an efficiency of uses. Food production with wastewater effluent could 1) mitigate contamination of the drinking water reservoir, 2) provide sanitation for residents, 3) provide inexpensive, fresh food locally, 4) provide cost-savings on food expenditures, 5) provide cost-savings on fertilizer, 6) provide a cottage industry and 7) has the potential to build community and create safe public spaces.
My research found that the practice of fish farming (aquacultre or aquaponics) with wastewater effluent has the greatest potential for social acceptance and the least contamination issues. This method of closing the loop with regards to wastewater and food production is also known as integrated biosystems. One of the most famous and most cited examples of this type of managed reuse of waste resources are the aquaculture ponds in Kolkata, India. According to Mara and Cairncross wastewater reuse through aquaculture, which occurs predominantly in urban settings, could be an important component in the sanitation strategies of poor communities in developing countries. The focus of this paper will examine the Kolkata aquaculture as an in-depth case study. Before I begin, however, I would like to outline some of my previous research trajectories to show a brief comparison of the benefits and constraints of alternative reuses of wastewater.
First, I investigated the technologies associated with wastewater irrigation, but the lack of consensus regarding its public safety made it unfeasible. Most researchers agreed that wastewater irrigation could not be used on crops that would be eaten raw. Its use was limited to crops that would be processed, such as industrial crops like wheat, sugar, corn and so forth. The uncertainty over the pathogen presence ultimately gave wastewater irrigation an unshakable social stigma.
Secondly, I investigated the possibility of utilizing pure urine as a fertilizer, since urine doesn’t contain any pathogens and has high levels of nitrogen, which is a key component in accelerating plant growth and crop yields. The problem here is the oversupply of urine from Canthino do Ceu’s 65,000 residents. Since urine is applied as a fertilizer, about three times a growing cycle, at most, there would be a sizable surplus. It is probable that the surplus could be manufactured into a fertilizer for export to neighboring farms and retail garden centers. However, the cultural element of food production would not be fully realized in this scenario. The cultural stigma of urine fertilizer and the lack of precedents constrained the full potential for this research trajectory. Even though, it is quite experimental it may still be a feasible avenue to deal with part of the wastewater at Canthino do Ceu and to provide some residents with a cottage industry in organic fertilizer production.
Lastly, I found a body of research and case studies on integrated biosystems, which are based on natural systems of nutrient cycling and recycling. These systems offer any combination of sewage treatment, aquaponics (growing fish and plants in the same tank), aquaculture (fish farming), wastewater irrigation and biogas (producing gas from biomass) technologies in response to the size and goals of any particular site. Whereas conventional wastewater systems are linear (wastewater to treatment facility to dumping in natural water body), the integrated biosystems are closed loop systems that use and reuse the resources in waters of varying qualities. Ultimately, the system is able to provide low-cost water filtration by maximizing water resources and localizing nutrient outputs. This systemic thinking, is not only able to capitalize on nutrient resources in wastewater and effluent from various wastewater treatments, it also helps contributes to the health of the overall watershed. If designed in the public realm, integrated biosystems could help promote awareness on how human sewage and land use affect water quality.
The Kolkata aquaculture system is the most famous example of integrated biosystems and have served as a model for how to develop these systems in urban and peri-urban conditions. They are cited in numerous research paper written on the topic of combining wastewater treatment with fish farming. Moreover, the Kolkata aquaculture ponds have become so important to the ecosystem of the area that they are recognized by the United Nations honors and are protected by the local government.
The practice of Kolkata aquaculture began in 1850 when the River Bidyadhari contained tidally influenced, brackish water. The intensive use of the river as a source of irrigation caused the river to lose its flow in 1928. For the next couple of years, farmers began experimenting with the usage of sewage to cultivate fish.
From 1930 until now, farmers have orally passed down their methods of farming from one generation to the next. In 1988, researcher Dhrubajyoti Ghosh canonized the aquaculture design and schedule of activities in his paper, “Wastewater-Fed Aquaculture in the Wetlands of Calcutta – an Overview” Figure (1) is a diagrammatic section of the pond design he described.
The ponds are flat-bottomed and dug to the shallow depth of 50 – 150 centimeters. This allows the strong tropical sunlight to sanitize the pond water.
The banks of the pond are about 3 – 5 meters wide and are planted with water hyacinth. The water hyacinth provides bank stabilization, shade for the fish, and biofiltration of the wastewater by capturing phosphorous on its roots and taking up other nutrients into its biomass.
A silt trap ditch, measuring 3 meters wide, is dug around each pond. This area is where deposits of silt and sewage sedimentation are put during the course of the harvest season, in order to maintain the right depth of the pond. The silt build up also strengthens the pond’s edge.
At key moments during the year, human and industrial sewage is permitted over the pond edge. It should be noted that even though the sewage has not been treated in a conventional wastewater treatment plant before it is released into the fish ponds, it has been diluted and biofiltered as it has traveled from its source through the Kolkata wetlands. The nutrients from the sewage cause algae growth, which then serve as fish feed. This cuts out the expense of fish feed, which is one of the more expensive elements in a typical industrial aquaculture operation.
While fish consumption, sunlight and water hyacinth uptake biofilter the sewage water, the effluent from the fish ponds still contains nutrients from fish waste. The effluent from the Kolkata fish ponds then irrigates rice paddy fields.
Overall, the Kolkata aquaculture system has the characteristics of a lake, where the water is neither completely free-flowing nor static. It is completely aerobic at all times during the fish’s life. There is a constant, slow-paced flow, which means that the system essentially acts like a facultative pond in a conventional wastewater treatment system, but at a decreased cost and with added productive value.
The schedule of fish farming in Kolkata is a year-round process that can garner several yields of fish per year, depending on the fish species being raised. Fish farming employment in Kolkata offers profitable year-round full time jobs for many of the families in the area. A breakdown of the overall schedule can be found in Figure (2).
During the first phase, the fish ponds are prepared during the coldest time of the year because the fish prefer warm waters.
During the second phase in mid-February, diluted sewage water is introduced to initiate the fertilization of the ponds. The pond water then stabilizes for one month. And at the end of this phase the pond is stirred vigorously to reduce any anaerobic conditions and to promote benthic organisms for fish to eat.
The first step of fish stocking in the third phase is to introduce a few small fish into the pond to test the water quality. Once the fish display healthy characteristics, fish are stocked. Ghosh did not mention whether the ponds accommodated polycultures or monocultures of fish. He just continues to describe the different schedules of stocking and harvesting for popular fish species. For example, Indian Major Carp can be stocked twice a year. The first stock happens at the ratio of 50 – 60 fish per kilogram of water. The second stock happens at the ratio of 10,000 – 40,000 fish per kilogram of water. Silver Carp are stocked in July at the ratio of 400 – 600 fish per kilogram of water. The Common Carp is stocked in December at the same ratio of 400 – 600 fish per kilogram of water.
During the fourth phase, a second wave of fertilization from the diluted sewage water is introduced into the fish ponds. The amount and frequency of this fertilization depends on the fish species, however, the amount is much less than the initial fertilization. This time only enough sewage is allowed to encourage plankton growth.
The last phase is the fish harvest. The Indian Major Carp are harvested between May and July, while those stocked second are harvested between August and October. Silver Carp are harvested in December. While the winter months are the safest to dry and prepare the ponds, if fish culture continues, fish harvesting is also done at this time. Large teams of 10 – 20 fish farmers collaborate to harvest the fish using nets. Typically a supervisor is present to offer general directions and optimize the haul. The fish harvest phase is indicative of the social and land organization present in the Kolkata aquaculture system, wherein most of the farmers lease their land and 300 operate in farmer cooperatives. The fish are then sorted in a boat and the selected fish are taken to the nearest auction market where they are sold to “bidders” alive. The bidders then take the live fish to retail markets within the hour. The fish harvest is able to fulfill the high market demand for fresh fish in Kolkata to all social classes, especially the poor.
In 1998, 3,500 hectares of land containing 250 fish ponds utilized 15 million litres of sewage per day in Kolkata. This amount of sewage made up only a third of the total sewage outflow at that time. The system produced 8,000 tonnes of fish per year, employed 2 people per hectare, produced a fifth of the fish sold in Kolkata, provided a clean food source and supported 4,000 poor immigrant families from Bangladesh. The aquaculture practice also spawned peripheral economies in fish farming accessories and fish trade, which employed 20,000 people.
In summation, the Kolkata aquaculture provides three basic human securities: food, sanitation and livlihood. The list of benefits goes beyond these basic necessities to include the provision of a fresh source of protein to urban markets, reduced resources for transportation of food, stormwater drainage, low-cost wastewater treatment, reduction of eutrophication in the watershed and green “lungs” that improve the health and well-being of urban residents.
The Kolkata aquaculture is not without its constraints. Recent industrial development around the Kolkata fish ponds is comprising the system with increased flows of heavy metals. While this has presented some challenges, there isn’t any consensus on the levels of contamination in the fish. Additionally, studies in 1994 and 1996 found indicators (including land area, people employed and financial viability) that suggested that the Kolkata aquaculture was in decline. There are also problems of fish poaching, job loss to higher wage jobs and lack of wastewater fertilizer.
The applicability of the Kolkata aquaculture to Canthino do Ceu in Sao Paulo, Brazil needs more investigation into the specifics of the site. There seems to be potential in the fact that 83% of Sao Paulo’s sewage is untreated; the Brazilian aquaculture industry is growing (Figure (3)); and, Canthino do Ceu has a fair amount of unoccupied land. According to my estimated calculations, 43 tonnes of fish could be harvested per year from the 640 million litres of human sewage generated in Canthino do Ceu per year. This would result in low-cost sewage treatment, livlihood for many families, cost savings on food and the provision of nutrient rich irrigation water to other forms of urban agriculture.
The largest constraint for application is the amount of land required to first treat the sewage, then utilize in fish ponds and then to distribute its effluent. Using integrative biosystems as a guide, I will have to combine several technologies that are more scale appropriate. Urban agriculture groups in the United States and indoor aquaculture systems offer models of a more compact scale.
Growing Power in Milwaukee, Wisconsin uses an indoor, compact aquaponics system. In this system, fish and vegetables are grown in the same tank. The nutrients from the fish waste and fish feed work double duty to supply nutrients to the vegetables growing hydroponically in the tank.
The Rhizome Collective in Austin, Texas, as well as Greywater Guerillas in San Francisco, California have built low-cost vertical wetlands to biofilter greywater. I will have to investigate the capability of these systems to biofilter human sewage.
There are also many examples of indoor, high-tech aquaculture systems that focus on saving space and high productivity. While these systems may be more expensive and high tech, they could provide informal settlement dwellers with high tech training and skills.
Another site application strategy I could try is to work backwards. First, I would determine the amount of suitable and available land. Then figure out how much aquaculture sewage treatment could be accommodated on that size of land.
Ultimately, it has been interesting to learn about the model project that inspired a generation of integrative biosystems that will surely help us to better manage our resources in the future.
Melissa Guerrero
GSD 6445, Green Infrastructure in the Non-Formal City, Christian Werthmann
04 11 08
