Researchers from UConn and Yale have developed a new solar-powered water disinfection system that combines several existing methods
Photo by Liana S on Unsplash
For many people living in developed nations, towns and cities take care of ensuring residents' water is clean and safe.
Municipalities have advanced filters and UV light disinfection technologies at their disposal. Some households have additional filters as well.
In many places in the Global South, however, these technologies are not widely available. These areas, such as parts of Africa and South America, do have one advantage when it comes to water filtration - sunlight.
Eric Ryberg, assistant professor of allied health sciences in the College of Agriculture, Health and Natural Resources (CAHNR), and his colleagues at Yale University have developed a new solar-powered water disinfection system that combines several existing methods. They published their findings in npj Clean Water, a Nature journal.
The researchers developed a compact device that effectively utilizes a combination of techniques that all use solar energy.
"Many hands make light work in drinking water disinfection," Ryberg says. "[It] really allows us to check a lot of boxes and provide water quality that allows people to feel dignified, no matter what resources they might have available to them."
The oldest and best-known method for making water safe to drink is simply boiling it. But this energy-intensive method isn't the only way to clean water.
There are physical filtration methods that use ceramic pots or sediment to physically filter out pathogens.
Another method is known as solar pasteurization which uses sunlight to heat water to a temperature that inactivates pathogens. This method requires less than half the energy of boiling. However, it is not effective in winter or in cloudy conditions.
Solar disinfection - simply leaving a bottle of water in the sun to kill pathogens - is another approach. On a sunny day, this process kills 99.9% of bacteria in about six hours.
"The UVA radiation will interact with compounds in the water or in the organisms themselves to generate oxidative stress to inactivate the organism," Ryberg says. "Or UVB radiation, the one that's responsible for our sunburns, will actually induce DNA damage in the organisms themselves."
Viruses, however, can take closer to 30 hours of direct sunlight to inactivate. This method also lacks a visual indication of when the water is safe to drink.
Ryberg is an expert in yet another disinfection method that uses photosensitizers, compounds that react to sunlight. Photosensitizers transfer solar energy to the oxygen molecules in the water. This causes the oxygen to enter an "excited state" making it a reactive molecule. This means it can interact with viruses and bacteria in the water and inactive them.
One key advantage of photosensitizers is that they are effective at killing viruses, which are harder to kill with other methods due to their small size.
"Having multiple ways of disinfecting and treating the water is always better than having one, because while that prefiltration step is really effective for removing large organisms, like protozoa or worms, some smaller bacteria will slip through the filter," Ryberg says. "Having technologies like pasteurization or solar disinfection are quite effective against bacteria. But those pesky viruses that don't get inactivated quickly by those technologies, that's where the photosensitization can really come in."
For the system described in the paper, Ryberg used a photosensitizer called erythrosine, a common food dye. Because Ryberg is using a dye, the water changes color as the photosensitizer breaks down, providing a clear indication of when the water is safe to drink.
At peak sunlight, measured as 1100 watts per meter squared, the researchers' model predicted that water would be disinfected to a safe standard in under an hour and subsequent batches would take only 28 minutes. A field test in Guatemala, with a sunlight intensity of 1050 watts per meter squared, confirmed the model's prediction.
The team used models of Cape Town, South Africa and Sololá, Guatemala, which have wide swings in sunlight availability between their dry and wet seasons, and Phoenix, Arizona which is sunny year-round. In each scenario, the models indicated that the system would be able to provide people with 50 liters of water per person per day, the recommendation set by the United Nations, all but 20 days a year.
This system could easily be scaled up to clean more water for a community.
"It might make sense for a community to have a system like this be built up into a larger scale, and have it serve the entire community," Ryberg says. "[Or] It might make sense to have it be on the relatively small scale and serving individual households."
Ryberg is working to develop a natural photosensitizer in place of synthetics like erythrosine. He has investigated the feasibility of chlorophyll, the compound that gives plants their green color, and hypericin, a compound found in St. John's Wort.
"The ultimate goal is that we can transition to natural things that have a much lower toxicological concerns," Ryberg says.
This work relates to CAHNR's Strategic Vision area focused on Enhancing Health and Well-Being Locally, Nationally, and Globally.
