Image: Fish ponds in the East Kolkata Wetlands – the largest sewage-fed aquaculture system in the world today. Source: Edwards, 2008. 1
After we eat and drink, we excrete into toilets, which use water to flush our effluent into municipal sewage systems. By and large, the resulting sewage is either untreated, or treated in different kinds of wastewater treatment plants, the most advanced of which are expensive to run and have high energy demands. 2
But even if sewage is treated, effluent is still high in levels of nitrogen, phosphorous, dissolved oxygen, and biological matter—essential nutrients for life on Earth. This causes eutrophication. The high levels of these nutrients lead to algal blooms, which in turn may produce toxins leading to mass fish deaths and biodiversity loss in rivers, lakes, and oceans. 3
Essentially, the core of the issue is that rather than nutrients being recycled, as occurs in most ecosystems, it’s a one-way flow. Fixing these problems by, for example, making water use more efficient, or using more energy-intensive sewage treatment plans, doesn’t solve the root of the problem: the nutrient cycle is leaky. And you can’t fix a leaking sink by changing the amount or kind of water you use.
Too much of a good thing
If we want to fix the leaking sink, we need to move away from the idea that human waste is inherently toxic, or that human activity is always bad for the environment. This way of thinking is grounded in the assumption that humans are somehow separated from nature. The logical conclusion of this assumption, then, is to separate us from natural cycles even more: building more refined, chemically and energy-intensive sewage treatment, building hard boundaries between food production and watersheds, and, failing that, using large-scale geoengineering experiments to clean our rivers.
But the main issue here is not that we are somehow toxic and so a burden to our environment. It’s that the nutrients we are releasing into the environment are too highly concentrated. This is especially the case when it comes to the “problem” of eutrophication. Caused by high-nutrient wastewater and agricultural run-off, it is generally thought as a bad thing. But consider the Greek root of the word: “becoming well fed.”
The main issue is not that we are somehow toxic and so a burden to our environment. It’s that the nutrients we are releasing into the environment are too highly concentrated.
Eutrophication is only bad because good nutrients like nitrogen, carbon, and phosphorous, necessary for the majority of biotic life, are too concentrated—causing rapid algal growth, leading to too little oxygen in the water, as well as too many toxins produced by algae, both of which are deadly to fish. However, fish eat algae, so if algae growth were to be slowed down a bit, fish populations would multiply instead. The problem is not that wastewater is polluted, but that there is too much of a good thing, too highly concentrated for the ecosystem to absorb.
How to fix a leaking sink
I first learned about the system of treating sewage through aquaculture when I lived in Hanoi. There, I found out that it’s actually very common, especially in poor agricultural communities, to reuse human waste for production.
Image: An overhung latrine on a fish pond in Vietnam. Source: UNEP International Environmental Technology Centre. (2002). Environmentally Sound Technologies for Wastewater and Stormwater Management: an International Source Book (Vol. 15). International Water Assn.
In countries like Vietnam and Indonesia, toilets are often placed above fish ponds. Human and livestock waste may also be collected manually and put in fish ponds. Why? Stimulated by the added nitrogen, phosphorous, and carbon, algae and phytoplankton grow rapidly and start breaking down the nutrients and bacteria and produce oxygen. As oxygen levels go up, fish are able to swim in the water and eat the algae and phytoplankton. Then the fish are caught and sold on the market. Finally, when the pond is drained, fish droppings and any remaining sediments can also be used to fertilize surrounding crops, like rice or fruit trees.
In China, the use of excreta in agriculture and aquaculture has a tradition of centuries. During the communist period, many fish farmers had limited access to fish feed and local state cooperatives started organizing human waste collection systems. Eventually, in many Chinese cities, up to the 1990s, trucks and boats collected human manure in cities—some run by the state and some clandestine, illegal operations—and transported them to aquaculture operations in peri-urban land. From 1952 to 1966, about a third of fertilizers (which includes fish feed) used in China came from nightsoils, and by 1966, 90% of excreta were recycled. 4 Incidentally, today, massive seaweed production off the coast of China has likely greatly reduced the likelihood of eutrophication—an accidental form of bio-remediation and nutrient recycling. 5
Image: Sewage water being pumped into a fish pond in the outskirts of Hanoi, Vietnam. Source: Edwards, 2005. 6
Image: Wastewater after treatment in fishponds, Hanoi. Source: Edwards, 1996. 7
One interesting large-scale example is the system that emerged in the outskirts of Hanoi in the 1960s. Hanoi, the capital of the newly independent communist nation, fighting a drawn-out war against Western occupying forces, had no municipal wastewater treatment. Sewage led out into two rivers, which flowed south and eventually merged with the Red River.
During the communist period of collectivization of farmland, Vietnamese farmer cooperatives were excluded from the international market and so often used whatever resources available to them to feed fish, such as slaughterhouse wastes or spoiled grains. Seeing the untreated wastewater in the canals—a resource out of place—farmers started pumping it into large ponds.
Seeing the untreated wastewater in the canals—a resource out of place—farmers started pumping it into large ponds.
After trial and error, and investing the little they had in infrastructural improvements, they determined the right sewage-and-freshwater ratio needed that would dilute the wastewater enough so the fish wouldn’t die.
Image: A local retail fish market in Yen So commune. Anders Dalsgaard. Source: Thi Phong Lan, Nguyen, et al. "Microbiological quality of fish grown in wastewater-fed and non-wastewater-fed fishponds in Hanoi, Vietnam: influence of hygiene practices in local retail markets." Journal of Water and Health 5.2 (2007): 209-218.
Farmers also grew plants such as water hyacinth to reduce the erosion of the banks, and which could then be fed to livestock. These also had the benefit of drawing out heavy metals from the water. They also practiced fish polyculture, where species like catfish, carps, and tilapia were farmed together, and thus were more effective in cleaning the water and protecting small fry from predators. Every year, the ponds were drained, and the sludge at the bottom was then applied to nearby fields, further reusing available nutrients.
Eventually, these farmers developed a system that, by 1995, provided 40-50% of Hanoi’s total fish supply every year. Scientific measurements showed that the water from the fish ponds, when pumped back into the river, was well below the World Health Organisation’s recommended level for biological oxygen demand—an indicator to determine the efficiency of water treatment systems. 7 Essentially, they had created a water treatment plant for a city of 1.5 million people, at almost no cost to the state.
A “low-cost folk technology” serving an entire city
You might be thinking: sure, this is one example of an interesting, but ultimately doomed, alternative to wastewater treatment. It is an aberration, and couldn’t possibly be maintained for long. Unfortunately for your internal cynic, it actually can be. The city of Kolkata (formerly Calcutta), India—population 14.8 million—has the largest sewage-fed aquaculture system in the world. Though farmers had been using sewage to feed fish in different ways since the 19th century, the system became more developed starting in the 1940s.
Image: Fish ponds in the East Kolkata Wetlands, the largest sewage-fed aquaculture system in the world today. Source: iStock.
During the British colonial period, administrators built a series of canals through the city that functioned as its sewers. These let out into the Bidyadhari River. However, this river quickly silted up and became unusable. As a result, an adjacent wetland area transformed from tidal salt marshes to primarily freshwater marshes. Two sewage canals were then built in 1940 to further extend the city’s effluent to the ocean. It was at this point that local farmers started rerouting the sewage water into fish ponds in the former salt marshes, growing vegetables on the banks of the sewage canals, and forming cooperatives to manage the wastewater.
Though the Kolkata system was developed over time, it is quite systematic. Every year, ponds are first drained and sludge is applied to fields. Sewage water is fed into the pond slowly at low depth and allowed to sit for two weeks. This basically mimics conventional sewage treatment systems, where sewage is first treated through stimulating algal and bacterial growth, harmful sediments are left to settle, and most parasites are killed because their eggs and worms die if they don’t find a host within two weeks. Then, fish are stocked in another pond, and slowly sewage water is introduced into the pond at a sewage-to-water ratio of 1:4. All of this requires skill and knowledge developed over generations, allowing farmers to know when oxygen levels are too low, which could kill the fish. 4 8 The resulting effluent can reach the water quality of conventional treatment. 9
Image: A sluice gate made of bamboo at the Eastern Kolkata Wetlands. Water hyacinth is grown to help purify the water and to feed livestock. Source: Mukherjee, 2020. 8
Image: Every year, the ponds are drained, and the sludge at the bottom is applied to nearby fields, further reusing available nutrients. Source: Take pride in the East Kolkata Wetlands (Facebook-page).
Through trial and error and good judgement, local farmers have developed a wastewater treatment system that is extremely efficient and adaptive to local conditions. They can distinguish the kind of effluent—industrial or domestic—through the hues it gives off, and will control or dilute it when necessary. For example, sewage from tanneries can be toxic to fish, so they will not use it. They vary water levels according to season, weather, and available quantities of effluent. They know the hue of greenish-black the water needs to be to have an optimal oxygen and ammonia level for fish. They can tell whether there is too little oxygen by paying attention to the degree by which fish come up to the surface to gulp air. Farmers harvest snails in the water to protect fish growth, which are then crushed and fed to ducks, whose droppings in turn fertilize fish ponds and nearby soils. They plant water hyacinths and duckweed to absorb heavy metals from the sewage water. 9 1
The Kolkata fish farms provide 40% of the region’s fish production and process 80% of the city’s sewage.
The Kolkata fish farms provide 8000 tons of fish per year to the city, or 40% of the region’s fish production. It processes 80% of the city’s sewage, and reduces the nutrient and organic loads of the city’s sewage water by 50-90%, while keeping bacterial loads to an acceptable level under WHO guidelines. It is calculated to save the city an equivalent of $64,400,000 per year in sewage treatment costs—making Kolkata an “ecologically subsidized city”. 10 The system provides farmers a return over investment of 28% and provides 200,000 people with a livelihood. 11
While profit shouldn’t itself be the goal of this system—it’s a public service, after all—it certainly helps to defray costs of wastewater treatment. In a small municipality in Karnal, northern India, one study showed that municipal sewage-fed fish ponds, installed in the 2010s, provided over $25,000 of net profit per year to the municipality, as well as indirect benefits such as improving nearby soils through the sale of treated wastewater to farmers. 12
Image: Waste-fed fish ponds provide steady sources of protein for small-holder farmers. Source: Fish Farming in the East Kolkata Wetlands, Ramble On, Priya Mallic.
Image: Fish harvested from the East Kolkata Wetlands. Source: Fish Farming in the East Kolkata Wetlands, Ramble On, Priya Mallic.
Still, when introduced into small rural communities, the benefits extend far beyond monetary profit, to the social, cultural, and ecological services provided by the fish ponds. This includes improving soil quality, adaptability of local communities to climate change, leisure (e.g. fishing with friends), and steady sources of protein for small-holder farmers. For example, even if they don’t sell the fish, a small sewage-fed fish pond can provide a family of six with 8kg of fish, per person, per year—a significant raise in protein intake for many rural communities. 13 In the case of the Eastern Kolkata Wetlands, the fish ponds also help to recharge the ground water—a serious issue in India where many aquifers are nearing depletion. 14
Kolkata’s wetlands are a “low-cost folk technology” 15 treating the majority of the sewage of a city with a population the size of New York. This is made possible through the development of a vast human-fish-plant ecosystem, a city-scale wastewater treatment plant that emerged through the creativity, ecological knowledge, and direction of local farming communities.
Over 90 systems in Germany in the early 20th century
By this point, your internal cynic might have come up with another counter-argument: sure, so it works at scale. But you would have to be pretty desperate, and poor, to stoop down to farming fish in sewage water. While it might work in India, and worked for a while in Vietnam and China, it would never work in developed countries, where there are higher sanitation standards, and where no one would want to eat the fish farmed in sewage anyway.
Image: A view of the former sewage-fed aquaculture system in Munich, Germany, today a bird sanctuary. Photo: Peter Schleypen, 2012. Source: Historisches Lexikon Bayerns
You may be surprised to learn that, in fact, over 90 such systems existed in Germany in the early 20th century. 16 Up until the 1990s, the city of Munich still processed most of its wastewater through fish farming. Indeed, Germany has pioneered some of the more detailed and rigorous scientific investigation into the large-scale viability of sewage-fed fish ponds, as early as the 1890s.
Up until the 1990s, the city of Munich in Germany still processed most of its wastewater through fish farming.
Like in China, wastewater-fed fish ponds have a long but unappreciated history in Europe. Castle moats, monasteries, and villages often had wastewater-fed fish ponds. As cities grew rapidly in the 19th century, untreated wastewater was simply flushed into rivers, leading to the collapse of fisheries across Europe as well as generally unsanitary conditions and the spread of disease. There was a growing recognition that sewage should be treated; one common indicator of adequate treatment methods was that trout are able to live in the treated water. As a result, civil engineers and scientists constructed small fish ponds to test the quality of municipal sewage treatment plants.
Gustav Oesten, a civil engineer charged with wastewater treatment in Berlin, began to experiment in the late 1880s with using fish to treat wastewater, and to harvest fish as a secondary product of sewage treatment. He was able to spend the good part of a decade conducting experiments with different fish species, designs for ponds, and various local and weather conditions. 16
Image: Feed channel for the fish ponds of the Munich sewage-fed aquaculture system. Image by Bjs (CC BY-SA 3.0), Wikimedia Commons.
Through these experiments, he showed conclusively that fish growth accelerates in sewage water, and that fish in turn help purify sewage water. Trout were not very good fish for this purpose, because they cannot tolerate water with high oxygen levels—common in wastewater systems, a byproduct of rapid algae growth. Carp, who can come up for air when oxygen levels are intolerable, grew very well—those fed with sewage far exceeding production of those in normal fish ponds. But, using trout, he proved that the water was of high enough quality to enter back into the water shed. His experiments suggested that fish ponds could be designed to help address Europe’s water crisis and, at the same time, provide an economic return through the sale of fish.
By the beginning of the 20th century scientists throughout Germany started conducting more small-scale experiments. Bruno Hofer, a fish scientist better known for pioneering the study of fish pathologies, started scaling up these experiments, showing in the early 1900s that wastewater of larger institutions like hospitals, breweries, and factories, as well as smaller municipalities could theoretically be treated with fish ponds. He even went further, and “dared” to propose such a system for a city as large as Munich—a notion that was perhaps considered outlandish at the time.
Image: A sprinkler introducing secondary treated wastewater diluted with river water into a wastewater-fed fishpond in Munich, Germany. Source: Edwards, 2005. 6
By 1929, however, after several successful implementations of Hofer’s design around Germany, the city of Munich built its own fish pond wastewater treatment system, which served the whole city until the 1990s. This was the largest such system implemented at the time in the world, initially designed to process the wastewater of 500,000 people. The system was so efficient that the water leaving the ponds, fully treated, was comparable to natural water in quality and nutrient level. 17
As these examples illustrate, sewage-fed aquaculture is a solution to many interlinked problems. It processes waste—from agriculture, livestock, and cities—and cycles those nutrients back into the system through food and agricultural production. It reduces nitrogen and phosphorous levels in the water, preventing eutrophication further downstream. It reuses available water, slowing down the water cycle and replenishing groundwater. It further reduces unnecessary inputs like chemical fertilizers, phosphates, and energy-intensive fish feed. Finally, it creates jobs and a source of income, especially necessary in poor countries.
If we were to calculate the fertilizer potential of sewage water alone, this would be reason enough to develop systems to reuse it. For example, one study estimated that, in the year 2000, all of India’s sewage was worth an equivalent of $2,000,000 per day in fertilizer costs. 18 In other words, on any given day, all of India is flushing several million dollars down the toilet. Waste-fed fish ponds would be of great help in capturing this wealth. Perhaps counter-intuitively, scientists have found that waste-fed fish ponds may actually be especially useful for arid countries, where water is scarce, by re-using wastewater for protein production. 19 Fish ponds don’t have to be for productive use alone. They can be integrated into wetlands and conservation areas, leisure fishing, tourism areas, or educational sites. They provide opportunities for improving biodiversity and making urban life more permeable for nature.
Fish ponds don’t have to be for productive use alone. They can be integrated into wetlands and conservation areas, leisure fishing, tourism areas, or educational sites.
Another reason waste-fed fish ponds continue to be relevant is that it is low-cost and low-tech, and therefore has little barriers for implementation. While high-tech, high-input systems like hydroponics, vertical gardening, and automated agriculture are getting a lot of press these days, the fact is that the majority of the world’s farmers have little to no access to capital and relies on small, but mostly sustainable, interventions to feed a stunning 70% of the global population. 20 Waste-fed fish ponds offer a source of subsistence at little financial risk to these small farmers. 1 Equally, when developed at the municipal level, they offer small towns, villages, and resource-poor communities opportunities to defray the costs of wastewater treatment, as well as generating local employment and improving sanitation. 12 13
Why don’t we do this more often?
Despite many advantages, most sewage-fed aquaculture systems have either been totally stopped or are in decline. So what happened? The first possible reason, and the one that most people might raise, is the “yuck factor”. Perhaps it’s just too gross for most people to eat fish grown from poop. By and large, this wasn’t the problem: consumers’ surprising acceptance of waste-fed fish is a constant in the research on urban fish ponds. 21 Furthermore, about 10% of the world’s population probably already consumes food irrigated with wastewater 22, and, even in the European Union, where agricultural regulations are famously strict, many farmers already apply sewage sludge to their fields—but European consumers don’t seem to care too much.
Image: Vietnamese catfish.
Image: Common carp.
The second possible reason for their decline is that it’s not safe. And, it’s true, here is where the most care needs to be taken in designing effective wastewater treatment. There is good evidence showing that sewage treatment in fish ponds can be as safe as conventional methods. Some of the strongest evidence to support this comes from a city-sized experiment conducted in the 1980s in Lima, Peru, sponsored by the World Bank and the United Nations Development Project. Aid agencies worked closely together with the city government to design a large-scale aquaponic sewage treatment site. 23
The site was basically a city-sized proof-of-concept. Endless measurements were taken over its two decades of operation, adjusting different variables throughout the project’s lifespan, and controlling for changes in volume of sewage and weather. It was found quite conclusively that fish-based sewage treatment was not only a viable and economical alternative for low-income countries, it also met stringent World Health Organization guidelines for water sanitation. The fish were also tested for human consumption. In all three trials, 100% of fish tested were rated at “very good” in safety levels. 24 This study wasn’t alone: numerous studies have investigated the safety of fish grown in sewage ponds. 25
More than just a leaking sink
If it’s not the “yuck factor” or safety, then what was it? In Hanoi, the waste-fed fishponds were not fully recognized for their potential, and peri-urban development in the 1990s began to encroach on the fish ponds. As the communist era came to an end, land near the city became increasingly valuable, and ponds were filled up for housing construction. Sewage became mixed with untreated industrial effluent, leading to large amounts of sewage being poisonous to fish, in turn leading farmers to switch to pelleted feed, by then increasingly available as Vietnam’s domestic market was opened to foreign trade.26 27 Today, Hanoi only treats 22% of its sewage, the rest flows directly into its river systems, and 180,000 cubic meters of waste water are discharged every day into the To Lich river, the same river that serviced the fish ponds. 28 29
The disappearance of fish ponds in Germany can also be largely attributed to urban growth. As cities grew, peri-urban areas—where fish ponds necessarily needed to be placed due to them having to be close to sewage lines and sources of fresh water—became more valuable. Pressured by booming real estate prices, less availability of land, high costs of labour, as well as diminishing returns on investment as domestic fish breeding had to compete with international markets, governments inevitably chose to close the fish ponds, or convert them into more conventional sewage treatment plants. Even in Munich, the largest system built in Germany, management was costly and became less and less appealing to the municipality. Munich’s fish ponds were eventually converted into a nature reserve, where migrating birds come to rest. Fish production is no longer its primary goal, and the estuary only absorbs a small percentage of Munich’s wastewater. 16
Image: The East Kolkata Wetlands in 2005. Source: Google Earth.
Image: The East Kolkata Wetlands in 2019. Source: Google Earth.
The system at Kolkata is still operational, but suffering from similar symptoms. At their peak, fish ponds in the East Kolkata Wetlands were as large as 12,000 hectares. This has shrunk to 4,000 hectares due to encroaching urban development. In Kolkata, too, workers struggle to deal with industrial effluent such as that from the sizeable leather tanning industry, which is poisonous to the fish and indiscriminately dumped into the municipal wastewater system. 8 11 30 31 Thankfully, unlike Hanoi’s government, the city of Kolkata and the Indian government recognized the importance of this system, and put in a series of regulations to protect it from further development. Still, informal and illegal development—where developers fill up ponds with debris overnight and then build on it as farmers are forced to abandon it—is slowly chipping away at the wetlands.
So the main driver of their disappearance is urban expansion into the peripheries. This is largely due to the global speculation on real estate—which constitutes 60% of all capital investments today. 32 When given a choice between selling peri-urban land to the highest bidder, and pairing sewage treatment with some fish production, most officials won’t think twice—the fish ponds have got to go! A second reason is the high prevalence of toxic chemicals in our water systems—which are too concentrated for ecosystems, and aquaculture systems, to absorb. We should ask ourselves if it’s really worth it to permit these products if they make it harder for us to mend the ecological rift between our settlements and their surroundings.
More messy, organic systems are often derided as backwards and primitive, when in fact they may be far more appropriate and sustainable than the energy-intensive, easily replicable “solutions” valued by planners and engineers.
A third reason is the relatively cheap cost of fossil fuels. In most industrialized countries, it is much more rational to choose for sewage treatment plans with a small land footprint but a large carbon footprint. In a world where energy is cheap, environmental costs can be pushed further and further downstream. But they will eventually circle back to us, and already are. Finally, a significant factor, and one which we shouldn’t ignore, is the bias of our leaders and of professional engineers against more messy, organic systems like that of wastewater-fed aquaculture. Such low-tech solutions are often derided in popular culture as backwards and primitive, when in fact they may be far more appropriate and sustainable than the energy-intensive, easily replicable “solutions” valued by planners and engineers. 33
Each reason points to a deeper problem: our economy’s inability to value the right things. Like so many sustainable solutions today, and many of those discussed on this website, sewage-fed fish ponds suffer from the “you can’t change this one thing without changing the whole system” problem. These systems are beset by global real estate speculation, toxic chemicals in our food and household products, contamination by industry, the cheap price of fuel, and the deep-seated idea that humans are separate from the ecosystems they are embedded in. At the root of it all is a system of value that is not in line with our ecological needs as a species, and as a member of Earth’s living community.
Fish ponds are a low-tech, low-cost, safe, and sustainable way to fix our society’s leaking sink. But when we get down there on our hands and knees, we might find a lot of other things that need fixing.
Thank you to Henning Fehr for doing research on the fish pond system in Germany, Michael DiGregorio for telling me about the Vietnamese system, Phuong Anh Nguyen for the extra research into it, and Geert Vansintjan for always keeping me inspired.