Water purification: Ultrafiltration for portable water purification

The US Army have deployed a new generation of water purification technology during their military operations: an ultrafiltration pretreatment for portable water purification unit. In this article, we take a look at the technology and how it works.

The United States Army is utilising state-of-the-art membrane water purification technology to support highly mobile military operations and humanitarian missions. The Mechanical Equipment Company Inc. (MECO), of Sugar Land, Texas, has designed a Lightweight Water Purifier (LWP) that can be easily transported to remote locations to produce safe drinking water from almost any available raw water, including highly-turbid surface water, brackish water and seawater. Additionally, the LWP is also capable of purifying water contaminated with nuclear, biological and chemical warfare agents.

The new LWP is light enough to be carried by four soldiers and simple enough for two operators to assemble and begin producing water in just 45 minutes from a water source such as a river, lake, pond, or ocean. The entire system can be transported in the cargo space of a HMMWV and by a single haul of a medium-lift utility or assault helicopter such as the UH-60 Black Hawk. In the past, desalination systems had been designed for conventional seawater salt concentrations of 35,000 ppm, however in the Middle East, the salinity is much higher. It is 45,000 ppm in the Arabian Gulf, and in "waterholes" in the desert it can reach 60,000 ppm. MECO specifically tests each LWP to verify that it meets this 60,000 ppm requirement, enabling the systems to treat any water, anywhere in the world.

The compact LWP unit will produce 125 gallons per hour (gph) from fresh or brackish water and 75 gph from seawater, sufficient production levels to support company/battalion-sized units in the field. The exact number of people that the LWP unit can sustain in the field is proportional to the water consumption scenario. For example, if the unit is used solely for drinking water, it will be able to support many more soldiers than if it is used for general purposes such as cooking, cleaning, showers, or laundry.

Evolution of portable water treatment

Water filtration technology has evolved significantly since MECO began designing and manufacturing transportable water treatment systems for the US military more than sixty years ago. In fact, the original thermal desalination systems used by the Marine Corps in the invasion of Iwo Jima employed MECO's patented vapor compression technology.

The new LWP combines two types of membrane filtration: ultrafiltration (UF) membranes pretreat the water prior to processing by reverse osmosis (RO) membranes. This design replaces the Army's previous generation of portable water processing equipment, which pretreated the RO feed water with multi-media filters (MMF) and disposable cartridge filters.

The MMF and cartridge filters posed several problems. Fundamentally, they were only capable of removing suspended solids between approximately 1-5 microns, allowing some particulate breakthrough and causing quick fouling of the RO membranes.

The other problem was that the cartridge filters required frequent replacement, as often as every half hour in some cases. This problem extended beyond the labor involved in replacing the filters. More importantly, the continual re-supply of consumable items can be a logistical challenge – and quite dangerous – in remote locations and under combat conditions.

UF pretreatment of RO feed water

The UF membrane process in the new LWPs eliminates the need to replace and resupply disposable filters. Each system employs three ROMICON® ROMIPURE® ultrafiltration cartridges from Koch Membrane Systems, Inc. (KMS), of Wilmington, Massachusetts, USA.

The 5-inch diameter cartridges contain hollow fibre membranes with an internal diameter of 35-mil. The membranes and the cartridge housing are both composed of polysulfone, a high strength polymer thermoplastic noted for its excellent chemical stability under a wide range of temperatures.

The ROMIPURE UF membranes have a 100,000 Dalton nominal molecular weight cut-off (MWCO) that consistently produces filtrate water with turbidity of less than 0.1 NTU, a more than ten-fold improvement compared to MMF and cartridge filters. The UF membranes remove turbidity, suspended solids, bacteria and other microorganisms from the feed water that can foul the downstream RO membranes. The higher quality filtrate water prolongs RO membrane life and extends the time between RO cleanings, regardless of the feed water conditions.

The UF pretreatment not only serves to limit fouling of the RO membranes, but the ROMIPURE cartridges also have important features that limit and counteract their own fouling. The hollow fibre cartridges operate from the inside to the outside during filtration (see Figure 1). The feed water (retentate) flows through the center of the hollow fibre, and the filtered water (permeate) passes through the fibre wall to the outside of the membrane fibre. The tangential flow of the retentate sweeps across the membrane surface and continually acts to limit membrane fouling.

In addition, the structural integrity and construction of the ROMIPURE hollow fibre cartridges enable them to withstand permeate back flushing and the reversing of retentate flow. These cleaning processes are highly effective in restoring flux rates.

The ROMIPURE hollow fibre geometry allows for a large amount of membrane surface area in a compact module. This high packing density means large volumes of water can be filtered while utilising minimal space and power, both of which are critical advantages for the LWP application.

A high-pressure pump driven by a diesel engine draws water from a 40-gallon filtrate tank on the UF module and feeds the RO module under pressures of up to 1,200 psi. The purpose of this filtrate tank is to provide backwash and fast flush capabilities for the UF membrane, while allowing continued operation of the high-pressure pump.

RO produces pure water

The RO module consists of seven FLUID SYSTEMS® TFC®spiral RO elements manufactured by KMS. The membrane elements feature a proprietary thin film composite polyamide designed to increase efficiency and reduce costs. The pressure vessels are constructed of titanium.

General Atomics (GA) developed and patented the spiral wound reverse osmosis element in 1968. GA sold its Fluid Systems division in 1974 and it ultimately became part of KMS. KMS continues to be a leading worldwide supplier of RO seawater desalination elements, with large-scale installations on the Red Sea, the Arabian Gulf, the Caribbean, the Mediterranean, and every ocean of the globe. Compact FLUID SYSTEMS TFC RO elements are also widely used aboard ships and on drilling rigs.

The KMS RO membranes remove dissolved and suspended materials including organics and salts. Only molecules in the range of 5 Angstroms (0.0005 micron) or 100 MWCO will pass through the membrane. Approximately 30 percent of the RO feed water is recovered for use as drinking water, and the rest of the concentrate (or brine) is discharged as reject. The permeate from the RO module is passed through a chemical module, where it is metered and given a chlorine injection for residual disinfection.

Reliable filtration without chemical cleaning

Prior to field deployment, MECO and KMS built seven prototype systems that were subjected to rigorous and exhaustive testing by the US Army, including 5,000 hours of operating time and 1,600 miles of cross-country transportation testing in the back of a HMMWV. The design and testing process led to important system refinements and improvements. For example, MECO worked with KMS to address the challenge of designing a system that can sustain several freeze-thaw cycles, conditions that go well beyond the environmental requirements of most water purification systems.

"The KMS membranes have been remarkably resistant to fouling," said John Klie, Government Business Development Manager for MECO. "We worked closely with KMS engineers to develop and optimise an automated backflushing method that has practically eliminated the need for chemical cleaning operations. Of the various vendors that we researched and tested, KMS provided the only solutions that did not require the use of ancillary air scouring equipment to clean the membranes."

On top of simplifying the process, fewer chemicals make for better tasting water. "The water produced by the LWP is very palatable," said Klie. "We have found that the challenge with older conventional systems has been over-chlorination, which ultimately gives water a ‘swimming pool’ taste. The LWP's automated chlorine dosing system prevents this from occurring, and hence produces a drinking water taste that is very similar to commercially available bottled water."

An economical solution

The LWP has proven to be a highly economical solution in Iraq, where the cost of supplying bottled water to the troops can be as high as $5 per gallon. Water produced by conventional first-generation purification systems costs approximately $1 per gallon, including the cost of disposable filters, chemicals and fuel. The LWP can produce water at 7 cents per gallon, which is primarily the cost of diesel fuel to operate the system and chemicals for minimal, periodic cleaning.

The MECO LWP, with UF and RO membranes from KMS, provides a self-supporting water purification method for Special Forces and other remote military and humanitarian missions that eliminate the need for difficult, expensive and dangerous water resupply.

Contact:

John McArdle, Director Business Development, Koch Membrane Systems Telephone: 1-978-694-7000 Fax: +1-978-657-5208 E-mail: [email protected]