Growing safe drinking water: from the ground up

The Privilege of clean drinking water

It’s easy to take for granted the availability of clean, safe water at the turn of a tap. Access to safe drinking water greatly decreases the risk of developing certain diseases, and generally improves standards of living. However, this access is not a reality for millions around the globe. Great strides have been made: “only” 785 million people globally were without clean water in 2017, compared to 1.2 billion in 2000.This increase in access to clean waters has largely been accomplished along the urban-rural divide. The number of rural poor without access to clean water only decreased by 4% (from 88% to 84%) in the same time period. The big difference in urban vs. rural communities is largely due to differences in population density. In urban communities, where people live much closer together, a single drinking water disinfection system can reach many more families. This significantly decreases the cost per-capita and makes it easier to attract attention and investment. 

In order to develop systems for rural communities, where people are much more dispersed and there is less of an economy of scale,  smaller-scale technologies need to be developed at a much lower cost point. Furthermore, distribution of necessary supplies is also a real concern. These technologies, employed largely at the household level, are broadly referred to as “point of source” technologies and can include: boiling, chlorination, membrane, ceramic pot filtration, and solar disinfection. They’ve been shown to decrease the chances of developing gastroenteritis by 61%. Despite these health benefits, only a third of rural households in low and lower-middle income countries (LMICs) perform some sort of water treatment, largely due to the cost. This means that low-cost options have been the most successful, namely solar disinfection (SODIS).

How does solar disinfection work?

Solar disinfection takes advantage of the ability of UV light to kill living pathogens in water. Drinking water is transferred into transparent containers (largely PET) and exposed to sunlight for 1-2 days . This works well because many LMICs receive high levels of solar irradiation (sunlight) and the process is not expensive. Even though SODIS is a big improvement over untreated water, this technology does not make water completely safe for drinking. While SODIS kills most bacteria, it doesn’t remove man-made pollutants and heavy metals. The process is also very slow at destroying viruses, and it doesn’t achieve the 99.99% destruction level recommended by the EPA.

Researchers at Yale University sought to improve the efficiency, effectiveness, and economy of SODIS at deactivating viruses through the addition of photosensitizers in a proposed “Farm-to-Tap” approach. Photosensitizers are chemicals that absorb light and can use it in reactions that destroy pathogens (like bacteria and viruses). Adding photosensitizers to water essentially multiplies and speeds up the light reactions used to disinfect water in SODIS.

Finding the perfect ingredients

The addition of photosensitizers to water in SODIS is not a new idea, but a lot of commercial photosensitizers are expensive or not easily available for rural users in LMICs. To address this challenge, the team of researchers sought out photosensitizers that could be extracted from natural materials and processed locally. They also wanted the photosensitizers to be edible, so they didn’t have to be removed from the water after disinfection (this can be a difficult and costly process). The team sorted through many options to make sure that the photosensitizers had strong light absorption, were sourced from common plants that can grow in the regions of interest, were easy to extract from those plants, and were edible. 

They ultimately  arrived at three chemicals to test that satisfied these criteria: Chlorophyll (which can be extracted from any green plant), curcumin (which can be extracted from turmeric, a native plant to south and southeast asia), and hypericin from St. John’s wort.

Figure 1. The different steps in the “Farm-to-Tap” approach put forward by researchers at Yale University.  Available for use from Ryberg et al.  under an ACS Free to Read License.

The research team also needed to repeat the same process looking at dispersants/surfactants, because natural photosensitizers are hydrophobic and will clump up in water without additional aid. These dispersants ensure that the photosensitizers are well distributed in the water to be disinfected and can therefore best function to disinfect water evenly and thoroughly. 

Testing the contenders

The goal is to deploy this technology widely in the field:  LMICs with varying SODIS setups, chemicals that have been extracted by residents from locally grown plants, and different amounts of sunlight. However, understanding the fundamental science at play is much easier in the laboratory, or at “bench-scale”, which is where the Yale University researchers conducted this initial work. The scientists were thoughtful about accounting for differences between well-controlled lab and the “messy” field. In their experiments, they used low-purity chemicals that were extracted from plants (more reflective of the chemicals that would be available for use in the field). They also interpreted their results accounting for differences in light spectra (specifically UV content) between their solar simulator and real sunlight.

Figure 2. A map of the world indicating projected time for chlorophyll-aided SODIS systems to destroy 99.99% of targeted virus in drinking water. Available for use from Ryberg et al.  under an ACS Free to Read License.

After testing all the combinations of photosensitizers and dispersants, they found that while most photosensitizer-dispersant pairings could increase virus destruction compared to no photosensitizers, chlorophyll was the most successful photosensitizer overall. Figure 2 shows a map of the time it would take to remove 99.99% of the model virus (desired level according to EPA Safe Drinking water standards), using solar irradiation data from around the world. Each map shows chlorophyll paired with a different dispersant. While SDS is the fastest, lecithin and saponin are particularly desirable because they can also be sourced from plants.

The next step

The results of this research team’s efforts indicate that the addition of chlorophyll could be a cost-effective way to greatly improve the efficiency and effectiveness of SODIS at eliminating viruses from drinking water. However, there are still a lot of supplementary experiments (like testing different kinds of viruses, using real field conditions to extract chlorophyll and treat water) that should be conducted before this technology starts being implemented on a large scale. In the meantime, we can do our part to keep water clean and accessible by preventing water pollution, conserving water, and donating to organizations that promote global safe drinking water.

Source Article: E. C. Ryberg, J. Knight, and J. Kim  “Farm-to-Tap Water Treatment: Naturally-Sourced Photosensitizers for Enhanced Solar Disinfection of Drinking Water” ACS EST Engg. 2020.

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Mary Davis

I earned my PhD in Chemical Engineering from Princeton University in 2018, where my research focused on nanoscale polymer systems and how their properties change with geometry. I am now applying my background in polymers to environmental systems. This involves studying the breakdown of plastics and plastic byproducts in the environment, as well as their interactions with other pollutants. When I’m not working in the lab, I enjoy crafting, cooking, and being outside.

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