The Earth is covered by over 326 million cubic miles of water, according to the U.S. Geological Survey. Ninety-seven percent of this water is unsuitable for drinking because it contains salt. High volume desalination methods such as reverse osmosis (RO) are the most effective at removing high volumes of salt. They are also energy intensive. Sea water desalination costs five times as much to harvest, according to Food and Water Watch. Therefore, the majority of potable water comes from fresh and recycled water. The United Nations predicts that 14 percent of the world's population could face water shortages by 2025, making alternative purification methods essential.

This diagram of the new process shows how a shockwave (red line) is generated in salty water flowing through a porous medium with a voltage applied to membranes (green) at each side of the vessel. The shockwave pushed the salt ions off to one side of the flow, leaving fresh water at the other side where it can be separated out. Photo: MIT.

The Shocking Alternative
Researchers at MIT have developed a novel alternative that may increase the amount of potable water available. The filter-free system uses an electrically-driven shockwave in a stream of flowing water that separates salt and kills microbes. When a large electric current is applied across a charged porous medium, salt water separates into regions where salt concentrations are depleted or enriched. The electrified shockwave pushes salt water to one side of the flow. Fresh and salt water are then isolated and the two streams collected.

The shockwave method is suitable for converting low salt, brackish, or contaminated water into potable water. It shows potential for water contaminated by heavy metals and hydraulic fracturing. The separation is less likely to become clogged like RO or electro dialysis which use filters. The process is most efficient for lower salinity applications. MIT researchers say it is fundamentally better because water flows across porous material made of sintered glass particles called a frit, rather than through a membrane. These particles are not subject to fouling or degradation.

Heavy Metals Are No Match
Head researcher and MIT professor of chemical engineering and mathematics Martin Bazant says their method could be ideal for ultra-purification or removing heavy metals from toxic water filled with boron, argon, and lead. Unlike capacitive deionization, it is more efficient at removing all ions, including toxic impurities.

"The system can be very effective in wastewater recycling," explained Bazant. "We did a demonstrated production of ultra-pure water. We went down to 5 micrograms per liter where you could not detect salt."

Bazant says traditional electrochemical methods cannot lower salinity as effectively and would be highly compatible with remote monitoring systems like cellular SCADA.

"Being an electrochemical method, you can easily monitor how it is doing," explained Bazant. "It can be a relatively low voltage technology which would lend itself well with remote monitoring. RO is not so easily scaled down."

The MIT system can be designed as a small-scale compact process for remote applications and run efficiently in the 10-volt range without requiring huge pumps or pressures. The system holds promise as a practical scalable system, especially for the removal of toxic ions and simultaneous disinfection. Bazant and his colleagues hope to commercialize their system in the future. Project updates and technical information can be found at www.mit.edu/bazant.