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This Terrarium Filters Dirty Water By Mimicking The Amazon Biosphere

A design student borrowed from plant biology and the ecology of rainforests to build the system.

As pollution becomes more of a global problem, the issue of contaminated drinking water impacts communities in big cities and small villages alike. After studying a water crisis in the central forests of his native India, one design student from London’s Royal College of Art created a natural way to filter dirty water.

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Basing his research on a scientific theory called the biotic pump, recent RCA graduate Pratik Ghosh created a filtration system called Drop by Drop. Using a vacuum system based on water pressure, transpiration, and the evaporation of water through one square foot of herbs, Drop by Drop can clean about three-quarters of a cup of water in one day. While Ghosh acknowledges that isn’t very much, he’s more interested in Drop by Drop as a proof of concept that could be scaled up. By covering the average U.K. roof–which is about 818 square feet–with plants, the system could clean about 35 gallons of pure drinking water in 12 hours of daylight.

How do plants manage to naturally filter so much water? Plants act as a natural filter–a process that’s called phytoremediation. In essence, when you water plants, the roots suck up the water from the soil. In the roots, the water passes through plant xylems, or veins in the plant’s tissue that are so small that particulates in the water get trapped in the roots and can’t make their way to the rest of the plant.

Ghosh, who before getting his master’s degree at RCA worked in the life sciences division at General Electric, says that these xylems work just as well as the most advanced man-made nanofiltration systems. One 2014 study from MIT shows that about half a square inch of plant xylem from the sapwood of coniferous trees can effectively filter two-thirds of a gallon of water in a single day. Particles like heavy metals, nitrates, and other pollutants are too large to pass through the xylems, and so the water that passes through the plant is pure distilled water.

[Photo: courtesy Pratik Ghosh]
Drop by Drop combines this property of plants with a theory called the biotic pump, which occurs in tropical places like the Amazon. There, millions of trees drink millions of gallons of water per year, but only retain about 1% of the liquid. The rest transpires, or evaporates through their leaf systems, returning to the atmosphere in the form of water vapor. Then, the vapor forms clouds. The difference between the low pressure humid air and the high pressure within the plants creates a partial vacuum in the atmosphere, drawing even more water out from the plants. This cycle, combined with the massive surface area of plants’ leaves, dramatically speeds up the process of transpiration–contributing to the precipitation in the rainforest.

[Image: courtesy Pratik Ghosh]
So how does Drop by Drop work? In a small terrarium, Ghosh recreates the four factors necessary for transpiration–humidity, heat, light, and airflow–using an air pump that maintains the appropriate temperature for water vapor to turn into water. To use it, water the plants using a glass of dirty water–whether that’s water from the tap, gray water that’s already been used, or even lake water–and let the plants do their thing. The water passes through the plants and transpires into the air. When it condenses, Drop by Drop captures it in a jar. It’s similar to desalination, where salt water is evaporated and condensed to make fresh water, but with plants acting as the filter.

Ghosh has tested Drop by Drop using both tap water, which often has chlorine in it, and water from Hyde Park lake in London (he chose water from the part of the lake where there were lots of ducks, so the water was full of duck poop). Afterward, he tested the water for nitrates, bacteria, and chlorine. After doing some basic chemical tests, Ghosh found that Drop by Drop effectively filtered nitrates, heavy metals, chlorine, and bacteria out of the water.

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Still, Ghosh says he hasn’t actually drunk any water the system has filtered yet since he’s a designer, not a scientist. Though he’s tested it, he wants to be safe and test the filtered water in a lab approved by the U.K.’s health and safety authority.

[Photo: courtesy Pratik Ghosh]
Ghosh hopes to move the project forward by spreading the word about his system and reaching out to people who might be interested in working with him. However, he doesn’t want to turn his idea into a business because he believes water shouldn’t be a commodity. “It’s about changing our value systems, looking at the way we procure water, and changing the way we perceive water,” he says.

About the author

Katharine Schwab is a contributing writer at Co.Design based in New York who covers technology, design, and culture.

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