Low-cost water filter uses sunlight to produce clean drinking water

Scientists at Princeton University have developed a device that uses sunlight to drive water purification. The new invention could allow for low-cost off-the-grid water purification, providing clean drinking water.

The new device greatly resembles a large sponge, though it soaks up water from a lake or pond while leaving contaminants – like lead, oil, and pathogens – behind. To collect the purified water from the sponge, one simply places it in sunlight.

The inexpensive flat sponge-like device is inspired by the pufferfish, a species that takes in water to swell its body when threatened and then releases water when danger passes. “To me, the most exciting thing about this work is it can operate completely off-grid, at both large and small scales,” said the device’s co-inventor Rodney Priestley. “It could also work in the developed world at sites where low-cost, non-powered water purification is needed.

Low-cost water filter uses sunlight to produce clean drinking water
The inspiration for the device came from the pufferfish. Credit: Xiaohui Xu

Sunlight is free,” said Xiaohui Xu, a co-inventor, who helped develop the gel material at the heart of the new water filter, “and the materials to make this device are low-cost and non-toxic, so this is a cost-effective and environmentally friendly way to generate pure water.”

The gel consists of a honeycomb-like structure that is highly porous and sits inside two other layers that stop contaminants from reaching the inner gel. The middle layer is a dark-colored material called polydopamine that transforms sunlight into heat and also keeps out heavy metals and organic molecules. The final external layer is a filtering layer of alginate, which blocks pathogens and other materials from entering the gel.

At room temperature, the gel can act as a sponge, soaking up water. When heated to 33 degrees Celsius (91 degrees Fahrenheit), the gel does the opposite – it pushes the water out of its pores. The alginate’s pores are small enough that they don’t allow pollutants or pathogens to pass through.

Xu said one of the challenges to making the device was to formulate the inner gel to have the correct water absorption properties. Initially, the gel was brittle, so she altered the composition until it was flexible. Xu synthesized the materials and conducted studies to assess the device’s ability to purify water.

Now, the team is exploring ways to make the technology widely available with the help of Princeton Innovation, which supports University researchers in the translation of discoveries into technologies and services for the benefit of society.