The Sun As A Water Purifier

Image courtesy of M. Kowalski.

Water insecurity is one of the most pressing problems in developing countries. In places where waterborne diseases are prevalent, it is essential to find sustainable, cost-effective solutions to disinfect water. Even though pathogens in water can be inactivated or removed by adding chlorine, boiling, or filtering, these techniques are often expensive or resource-intensive. Using the sun to purify water, by comparison, has the potential to be an affordable and energy-efficient solution. However, at the moment, there are no clear and widespread techniques to use sunlight as a disinfection mechanism.

Motivated by a desire to develop viable techniques to combat water insecurity in developing countries, Professor Jaehong Kim, a Henry P. Becton Sr. Professor of Engineering in the Yale Department of Chemical and Environmental Engineering, co-led a study with graduate student Inhyeong Jeon to systematically compare various methods to disinfect water using the sun. “We wanted to explore how different technologies capture sunlight, transform photon energy to disinfecting power, and affect microbial inactivation via different mechanisms,” Kim said.

The study examined the feasibility of five methods of disinfection. The first method involves using semiconductors to generate reactive oxygen ions, thereby disinfecting the water. The second method uses dyes that absorb photons to generate singlet oxygen, another oxygen ion that can effectively inactivate viruses. The third method involves using a solar cell to power UV-producing LEDs to disinfect the water. The fourth method utilizes distillation, evaporating the water over time through light-absorbing materials and collecting it in a receptacle. The fifth method involves pasteurization, keeping the water above pasteurization temperature for a short period of time to inactivate pathogens.

Each method was tested extensively by measuring the parameters of disinfection capacity over different trials. A quantitative score was assigned to each technique, which was then used to compare the efficacy of each one, taking into consideration various uncertainties associated with geographical variations and the availability of existing data. The comprehensive analysis revealed that pasteurization is the most effective method to disinfect the water. “It seems to be much more reliable than others when all the decision points are considered as a whole, which is somewhat surprising,” Kim said. Regardless, he was optimistic that future work on the topic could extend the study. Kim’s research group is currently developing various prototypes to disinfect water using the sun to realize their goal of combating water insecurity.