Tropical tree seeds provide sustainable water filtration

Researchers have designed a simple drinking water filtration method using sand combined with the extract of seeds from a tree commonly found in equatorial regions. A small prototype filter completely removed bacteria from water in which the concentration of Escherichia coli was more than 100,000 times as great as that of wastewater (Environ. Sci. Technol. Lett. 2017, DOI: 10.1021/acs.estlett.7b00490). The researchers hope to develop the design into an easy, inexpensive, and sustainable way for households and communities in the developing world to clean their water.

The seeds of the tree Moringa oleifera have positively charged, water-soluble proteins that attract particles and kill bacteria; researchers suspect the proteins protect the seeds from microbes. For generations, people have used pulverized seeds as a coagulant to precipitate particulates out of water, says Stephanie Butler Velegol of Pennsylvania State University. But this doesn’t work well for purifying drinking water because the seeds release unwanted organic matter into the water. Now Velegol, Manish Kumar, and their colleagues have designed a simple filtration approach using the seeds to clean water more effectively.

A few years ago, the team developed a way to avoid the release of organic matter by mixing ground-up Moringa seeds in water, and then combining the resulting liquid with sand (Langmuir 2012, DOI: 10.1021/la2038262). The seed proteins stuck to the sand, while the organic matter could be washed away. The resulting “sticky sand” clarified water and killed bacteria similarly well to the seed proteins alone.

In the new study, the team, including a group of Penn State undergraduates led by graduate student Boya Xiong, packed the sticky sand into filter columns about 1 cm in diameter and 5-10 cm high, and testing the columns with extract from different amounts of seed to optimize performance. In experiments with 1-µm-diameter polystyrene particles, which have about the same size and charge as bacteria, they found that the sticky sand caught 99.99% of particles, compared with 13.55% for sand alone. The sticky sand also removed 10⁸ viable E. coli cells per milliliter. They estimate that a household-scale filter 1 meter tall and 5 cm in diameter that provides 10 L of water per day would require 0.2 kg of seeds per year, whereas a Moringa tree produces about 480 kg of seeds per year.

The team still has to scale up the filter to sizes that could be used by households and communities. But they estimate that a 1-meter-tall, 70-cm-diameter filter could provide clean water for 1,000 people per day. They also plan to check whether sticky sand can remove viruses and nanoparticles. Once they have a proven design, they plan to work with contacts in Rwanda to test it in an actual community water supply. They envision that eventually families and communities could make simple filters with local resources alone. Xiong says friends from Trinidad, India, Sri Lanka, and Tanzania have all mentioned having a Moringa tree in their backyard.

Arup K. Sengupta, an environmental engineer at Lehigh University who works on sustainable drinking water solutions for the developing world, says that the team’s approach shows a considerable enhancement of performance compared with sand alone, and takes advantage of two natural materials. However, he says, more work needs to be done to see how pH and other water components may affect the performance of the filter, since the ionization of the proteins in the seeds could depend on pH.