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Seed Could Bring Water to Millions

engineering / research

The United Nations says 2.1 billion people lack access to safe drinking water. Carnegie Mellon University Biomedical Engineering and Chemical Engineering professors Bob Tilton and Todd Przybycien co-authored a paper with Ph.D. students Brittany Nordmark and Toni Bechtel, and alumnus John Riley, further refining a process that could soon help provide clean water to many in need in developing nations. The process, created by Tilton's former student Stephanie Velegol, also a co-author of the paper, uses sand and plant materials readily available in many developing nations to create a cheap and effective water filtration medium, called "f-sand."

"F-sand" uses proteins from the Moringa oleifera plant, a tree native to India that grows well in tropical and subtropical climates. The tree is cultivated for food and natural oils, and the seeds are used for a type of rudimentary water purification that leaves behind high amounts of dissolved organic carbon from the seeds, allowing bacteria to regrow after just 24 hours. This leaves only a short window in which the water is drinkable.

Velegol, now a professor of chemical engineering at Penn State University, had the idea to combine this method of water purification with sand filtration methods common in developing areas. By extracting the seed proteins and adsorbing them to the surface of silica particles, the principal component of sand, she created f-sand. F-sand kills microorganisms and reduces turbidity, adhering to particulate and organic matter. These undesirable contaminants and DOC can then be washed out, leaving the water clean for a longer period of time and the f-sand ready for reuse.

While the basic process was proven and effective, there were still many questions surrounding f-sand's creation and use — questions Tilton and Przybycien resolved to answer, which could have big implications for the future of f-sand.

Fatty acids
One of the major reasons M. oleifera is cultivated is for the fatty acids and oils found in the seeds. These are extracted and sold commercially. Tilton and Przybycien were interested to know if these fatty acids had an effect on the protein adsorption process as well.

They found that removing the fatty acids had little effect on the ability of the proteins to adsorb. This finding also has beneficial implications for those wishing to implement this process in developing regions. Since the presence or absence of fatty acids in the seeds has little effect on the creation or function of f-sand, people in the region can remove and sell the commercially valuable oil, and still be able to extract the proteins from the remaining seeds for water filtration.

Concentration
Another parameter of the f-sand manufacturing process that Tilton and Przybycien tested was the concentration of seed proteins needed to create an effective product. The key to achieving the proper concentration is ensuring there are enough positively charged proteins to overcome the negative charge of the silica particles to which they are attached, creating a net positive charge. This positive charge is crucial to attract the negatively charged organic matter, particulates, and microbes contaminating the water.

This relates to another potential improvement to drinking water treatment investigated by Tilton, Przybycien and Nordmark in a separate publication. In this project, they used seed proteins to coagulate contaminants in the water prior to f-sand filtration. This also relies on controlling the charge of the contaminants, which coagulate when they are neutralized. Applying too much protein can over-charge the contaminants and inhibit coagulation.

"There's kind of a sweet spot in the middle," Tilton said. "It lies in the details of how the different proteins in these seed protein mixtures compete with each other for adsorption to the surface, which tended to broaden that sweet spot."

This broad range of concentrations means that not only can water treatment processes be created at relatively low concentrations, thereby conserving materials, but there is little risk of accidentally causing water contamination by overshooting the concentration.

Water hardness
Water hardness refers to the amount of dissolved minerals in the water. Although labs often use deionized water in a process meant to be applied across a range of real world environments, researchers have to prepare for both soft and hard water conditions.

Tilton and Przybycien found proteins were able to adsorb well to the silica particles, and to coagulate suspended contaminants, in both soft and hard water conditions. This means the process could potentially be viable across a wide array of regions.

Overall, the conclusions that Tilton, Przybycien and their fellow authors were able to reach have major benefits for those in developing countries looking for a cheap and easily accessible form of water purification. Their work puts this novel innovation one step closer to the field, helping to forge the path that may one day see f-sand deployed in communities across the developing world. They've shown that the f-sand manufacturing process displays a high degree of flexibility, as it is able to work at a range of water conditions and protein concentrations without requiring the presence of fatty acids or a need for fractionation.

"It's an area where complexity could lead to failure — the more complex it is, the more ways something could go wrong," Tilton said. "I think the bottom line is that this supports the idea that the simpler technology might be the better one."

Tilton and Przybycien recently published a paper on this research, "Moringa oleifera Seed Protein Adsorption to Silica: Effects of Water Hardness, Fractionation, and Fatty Acid Extraction," in ACS Langmuir.