Mixing moves osmosis technology forward

Capt. Dave Eaton commanded a six-man Air Force unit during the invasion of Iraq. A few weeks after the invasion was launched, his unit took part in a joint operation with Army Rangers to capture an airfield in western Iraq. The operation was successful, and when the Rangers departed, Eaton&rsquos Air Force team remained behind to secure the desert airfield and wait for more supplies.

When the supply plane failed to appear, the team realized that it would not be receiving a delivery any time soon. The soldiers would have to hold the airfield and fend for themselves. Among other challenges and threats, Eaton knew his men would soon face a critical shortage of drinking water in the blistering desert heat. His solution was to supplement the small amount of water they had with their own urine.

He explained the plan to his team. &ldquoThe guys looked at me sideways,&rdquo Eaton says. They didn&rsquot know that he had brought along a new filtration device developed by Hydration Technologies (HTI), Albany, Ore.

The soldiers urinated into a common container into which Eaton tossed a sealed plastic sack that resembles a medical IV bag. As the soldiers watched, the bag spontaneously filled as water migrated through the walls -- leaving impurities and poisons behind. They repeated this process for two weeks and held the airfield until follow-on forces arrived. The HTI filter was crucial to the unit&rsquos survival, Eaton says. &ldquoAnd [the water] tasted great!&rdquo

Membrane makes it possible
Many filtration and purification technologies are available today, but virtually all of them suffer from a familiar set of disadvantages. Most require energy and mechanical parts to force liquid through a filter -- and energy is often in short supply during a crisis. Many such systems are complex and demand special training. Others require components that are delicate, not designed for rugged use, or too heavy to be considered portable.

Virtually all mechanical filters capture impurities and eventually clog. Ultrafine filters -- those that are designed to reject pathogens and extremely small impurities -- clog quickly when filtering turbid water.

One alternative to mechanical filtration is chemical purification. But chemical treatments require precise dosages and exposure times, and can leave an unpleasant taste. They are also ineffective against threats such as chemical poisons, heavy metals and even some microorganisms. Another critical disadvantage is that many chemical treatments are ineffective at purifying water that is extremely cloudy or turbid.

The one filtration concept that is free of all of these disadvantages is forward osmosis (FO). (See sidebar: &ldquoForward Osmosis: How it Works.&rdquo)

&ldquoThe forward-osmosis concept has been around for years,&rdquo says Robert Salter, CEO of HTI. &ldquoIn theory, it looked like the perfect alternative to energy-reliant technologies -- mainly because it is so simple.&rdquo

In Hydration Technology&rsquos FO filtration system, water diffuses into the bag when a generic sport-drink powder is in contact with the inner, clean side of the membrane. The process is spontaneous. It requires no energy, no moving parts and no special expertise. It won&rsquot clog in even the most turbid water (up to 1,000 Nephelometric Turbidity Units), because dirty water is not being driven by hydraulic pressure into the filter material. Instead, clean water is being drawn through the filter osmotically.

Keith Lampi, cofounder and COO for HTI, says, &ldquoThe FO concept was elegant, but the key to making an FO filter work was the membrane. FO filtration was a terrific idea, but it just wasn&rsquot practical until we discovered how to create a hydrophilic membrane that works efficiently. With previous membrane materials developed by our researchers and many others, water diffused too slowly into the bag.&rdquo

Lampi adds, &ldquoAnother critical issue is the consistency of the membrane and the size of the pores that reject impurities. Filtering out dirt particles and other large impurities is easy. Our goal was to develop a membrane that would reliably reject the impurities that present the most serious threats, including bacteria, viruses and common microorganisms.&rdquo

Triple-shaft mixer to the rescue
After a long development effort, the HTI team was close to success, but needed a mixer that could create the membrane material and meet consistency and cost-efficiency goals. The multistep process first requires aggressive cellulose grinding, followed by continued dispersion and dissolution as numerous other ingredients are added to a casting solution.

&ldquoWe knew that a single-shaft mixer -- even one equipped with multiple disperser blades -- could not handle the job. So, my first impulse was to test a dual-shaft mixer,&rdquo says Steve Peterson, P.E., and senior project engineer for HTI. &ldquoIn my experience, I have found that dual-shaft mixers are generally fast, versatile and cost-efficient.&rdquo

But in this case, tests using a conventional dual-shaft mixer in an open vessel produced unsatisfactory results. The process took six hours to produce an inadequate final dispersion. Looking for an alternative, the HTI engineering team scheduled a test at the Ross Test & Development Center in Hauppauge, N.Y.

After shipping raw materials to the lab, the HTI team simulated their process on a Ross VersaMix, a triple-shaft mixer that combines a slow-speed anchor agitator with a high-speed disperser and a rotor/stator high-shear mixer.

Forward Osmosis: How it works

When dirty or contaminated water contacts one side of HTI&rsquos semipermeable membrane while a concentrated sports-drink powder or syrup contacts the other side, clean water immediately begins to migrate through the membrane. This is forward osmosis (FO) at work (Figure 2). Acting as a molecular sieve, the HTI membrane allows water to pass into the interior of the bag, while rejecting a variety of organic and inorganic impurities. The HTI membrane is a rugged composite. An ultrathin (10 µ) FO layer is sandwiched with a microfilter and a fabric backing, which provides strength. With pores measuring 5 Å(0.0005 µ) in diameter, the FO layer successfully filters E. coli bacteria, anthrax spores, viruses, heavy metals, suspended particles and other health threats. Forward osmosis is normally a very slow process since it is not driven by hydraulic pressure. The HTI membrane allows water to pass at a much higher rate (as high as 1 L/hr in some of the personal hydration systems) because the FO layer is so thin. Also, since it is made from cotton-derived, cellulose-ester plastics, the FO layer is extremely hydrophilic. This allows water to pass through easily, accelerating the filtration process. Hydration Technologies&rsquo X-Pack is the first FO filter to be commercialized. The success of the membrane is due to the combination of ultra-small and highly consistent porosity with a high pass-through rate.

Figure 1: Forward osmosis allows clean water to pass through the membrane, while viruses, bacteria and other contaminants are left out.