The system involves injecting CO2 gas into a stream of water to filter out particles. The researchers report that the gas mixes with the water in a system of channels, temporarily changing the water's chemistry and causing the contaminated particles to move to one side of the channel depending on their electrical charge. By taking advantage of this migration, they are able to split the water stream and filter out suspended particles.
"You could potentially use this to clean water from a pond or river that has bacteria and dirt particles," says Sangwoo Shin, an assistant professor of mechanical engineering at the University of Hawaii at Manoa.
Shin, the lead author of a paper describing the process, performed the research as a post-doctoral researcher in the laboratory of Howard Stone, the Donald R. Dixon '69 and Elizabeth W. Dixon Professor of Mechanical and Aerospace Engineering at Princeton.
Much more efficient
In a paper published recently in the journal Nature Communications, the research team describes a laboratory-scale system that removes particles three orders of magnitude (1000-fold) more efficiently than conventional microfiltration processes. The system is low energy. Besides the pump responsible for the flow, bottled carbon dioxide is the only moving part, and there is no physical filter or membrane that can clog or require replacement.
Carbon dioxide alters water's chemistry by making it slightly more acidic. The acidity means that when CO2 dissolves in water it creates charged particles (ions). One of the ions, a positively charged hydrogen atom, moves very quickly through the water solution. Another, a negatively charged bicarbonate molecule, moves more slowly. The ions' movement through the water creates a subtle electric field. This draws particles in the water (which have either negative or positive charges of their own) toward one side of the water stream.
Because most contaminants have some amount of surface charge, the electric field is an effective way to strain them from water.
The researchers' device takes advantage of this electric sorting effect by drawing the contaminants to one side of the flow and then splitting the water into two channels. One path carries water containing the contaminating particles and the other holds clean water. Because the system does not have a membrane or a mechanical filter, clogging is not a problem.
When finished, the dissolved CO2 can be easily removed by exposing the water to the air so that the water contains only normal levels of carbon dioxide and is not carbonated.
The researchers say using a soluble gas as a method of controlling particles in a solution could lead to other industrial or scientific applications beyond water filtration.
Shin believes the CO2 system could be particularly useful in the developing world because it does not require the installation and replacement of filters. The idea may be useful for portable systems. It is also relatively low cost, only requiring a canned source of carbon dioxide to use.
"It is definitely able to scale up to a hundred litres per hour, which meets a practical household standard," Shin reports.
Another use would be as a supplement to a desalinisation plant. Many types of desalinisation use membranes to strain out salt molecules, but biological particles including viruses and bacteria are able to pass through the membranes. Shin says the carbon dioxide system could strain out the particles before or after the water passes through the desalinisation membrane, reducing the need to treat the water with chemicals such as chlorine.
Shin is now working on methods to scale the system for possible use in water treatment plants to supply larger communities. He says the basic science works, but further engineering is needed to create a large-scale carbon dioxide filter.
Besides Shin and Stone, the researchers include Orest Shardt, who worked on the project as a co-lead author while at Princeton and is now a lecturer at the University of Limerick, Ireland, and Patrick Warren of Unilever. The project was supported by Unilever Research, the Natural Sciences and Engineering Research Council of Canada, and the Princeton Environmental Institute.