Paths for freedom and progress
Urban Fish Ponds: Low-tech Sewage Treatment for Towns and Cities

In countries like Vietnam, Indonesia, and China, toilets are often placed above fish ponds. Human and livestock waste may also be collected manually and put in fish ponds. 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 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.

After trial and error, and investing the little they had in infrastructural improvements, these Asian cultures determined the right sewage-and-freshwater ratio needed that would dilute the wastewater enough so the fish wouldn’t die. They also let the untreated sewage water sit in primary and secondary ponds before mixing it into fish ponds, effectively killing harmful pathogens and allowing large solids to sediment, further promoting algal growth.

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. The resulting effluent can reach the water quality of artificial treatment.

Asian farmers also grow plants such as duckweed and water hyacinth on the water and on its banks, which could then be fed to livestock—and had the dual benefit of drawing out heavy metals from the water. They practice fish polyculture, where species like catfish, carps, and tilapia are farmed together, and thus are more effective in cleaning the water and protecting small fry from predators. Every year, the ponds are drained, and the sludge at the bottom is then applied to nearby fields, further reusing available nutrients.

SOURCE: https://www.lowtechmagazine.com/2021/03/urban-fish-ponds-low-tech-sewage-treatment-for-towns-and-cities.html
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