Monday, May 12, 2008

Low-Energy Water Filtration

A new membrane-free water-purification system uses small amounts of energy.

By Lee Bruno

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Spin cycle: The water purifier shown above separates out contaminants using centrifugal force.
Credit: Palo Alto Research Center Inc. (PARC)

Most water-filtration technologies require a lot of energy to push water through membranes that eventually become fouled and need to be replaced. Both factors make water filtration costly for most applications.

Now researchers at Palo Alto Research Center (PARC) have been able to overcome those challenges by incorporating scientific insights from the physics of toner particle movements into a low-energy water-filtration device that doesn't use membranes.

That's all good news for the looming specter of filtering brackish drinking water that threatens much of the developing world and even some water-stressed areas in developed countries. In the past, however, the economics have been the stumbling block for creating affordable water-treatment systems. The United Nations estimates that over the next eight years, some 900 million people will need a safe supply of drinking water.

PARC researchers call their device the spiral concentrator. It is a spiral-shaped, 50-centimeter-long piece of plastic tubing that's one millimeter in diameter. As water is pumped through one end of the device, particles in the water are pressed up against the walls of the tubing. Particles as small as one micron in size are separated out by centrifugal force and shunted away from the clean water via diverging forks in the spiral concentrator.

The advantage of this approach is that it doesn't require as much energy as it would to push contaminated water through a membrane. Such membranes are typically built from resin and have many tiny holes perforated in them, ranging in size from a few micrometers to a few nanometers.

The PARC innovation sprang from an earlier contract research project with the U.S. Army. The aim was to design a device to concentrate biohazards like anthrax by concentrating few parts per liter of contaminants so that a sensor could detect their presence.

The PARC researchers have lots of experience with studying the physics of particles. Toner in copy machines is made up of miniature, electron-charged particles. Understanding the physics of how these charged particles move in both air and liquid has been a key area of PARC research. The lessons the researchers learned about particle toner were used for PARC's biological agent detection system and for the water purifier.

The purifier requires a constant flow rate of water so that the movements of the particles conform to predicted patterns. That flow of water can be achieved with a low power pump that can be driven by a panel of solar cells.

However, because the spin concentrator can separate particles no smaller than one micron in size, it can't remove bacteria. Scott Elrod, manager of the hardware systems laboratory at PARC, says that smaller particles could be separated out by adding alum to the water being filtered. Alum is used in water treatment plants to chemically bind small particles to larger ones, which can then be separated out using gravity. In the case of the spin concentrator, centrifugal force will supply the horsepower to remove those congealed particles.

Elrod says that in the next two months, the researchers expect to scale down the device into a parallel stack of spin concentrators that are small enough to be sold commercially. They also plan to test the system with larger volumes of water, to reach the maximum volume of 100 liters per minute filtration rate. Researchers have already done the calculations on paper indicating that the parallel schema and water volume should be able to be handled.

Low-Energy Water Filtration

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