Powering Potable Water Systems

Research underway at the Air Force Institute of Technology using ultraviolet light emitting diode technology for water treatment shows a potential to replace existing contingency potable water systems with ones that are lighter, more water and electrically efficient, and require less maintenance.

  

 By Capt. Michael J. Spencer, M.SAME, USAF

 


ROWPU unit

Reverse osmosis water purification units filter any source of water into drinking water. While the systems are essential to provide potable water in contingency environments, they use a significant amount of energy. Research underway at the Air Force Institute of Technology aims to lessen the energy burden by utilizing UV LED technology to treat water. U.S. AIR FORCE PHOTO BY AIRMAN 1ST CLASS CHRISTOPHER REEL 


 

Give Air Force Civil Engineers a water source and a long, flat piece of land and they can build an air base. But as we have found in Afghanistan and other emerg­ing contingencies, it is the sufficient, safe water supply that is often the more difficult to acquire. Potable water is essential to military operations. Bottled water can be purchased, though it is not cost effective, and expired water adds to the waste stream.

Producing potable water with the Basic Expeditionary Airfield Resources (BEAR) Reverse Osmosis Water Purification Unit (ROWPU) water treatment system is typi­cally the choice in a bare base environment. However, the system requires significant energy to run and produces a wastewater stream. New Ultraviolet Light Emitting Diode (UV LED) technology may help to overcome these issues in water treatment within the coming decade.

Treating water with UV wavelength energy is common for water disinfection, as UV energy inactivates the DNA in organ­isms and, with an added oxidizer, can break down chemical weapons and other organic compounds. Municipal-sized UV water systems are now operating across the globe, with one of the largest facilities provid­ing up to 9-billion-l of treated water daily to New York City. These systems utilize fluorescent UV bulbs, which are fragile, contain mercury, have a short life span, require high voltage, and are not electri­cally efficient. Each of these limitations inhibits the application of UV energy in contingency potable water systems.

 

TECHNOLOGY ADVANTAGES

Lamps utilizing LEDs are now replacing fluorescent and other light producing tech­nologies in general lighting applications. Improvement in the electrical efficiency of LEDs is increasing much more rapidly than any competing technology.

Simultaneously, larger scale production and improvements in manufacturing is driving the cost of these devices downward exponentially. LEDs today are exceeding Haitz’s Law, which states that the light output from an LED will increase by a factor of 20 and the cost per lumen will decrease by a factor of 10 each decade.

The technology advancements are expected to significantly reduce electrical usage for lighting. In fact, the Department of Energy predicts that the cumulative energy savings produced by LEDs across the United States from 2013 to 2030 will be on the order of 2,200-TWh—a savings of $220 billion in electricity costs.

The primary energy advantage of LEDs is that they create photons with a much greater electrical efficiency than fluorescent bulbs. UV LEDs are on track to outperform fluorescent bulbs in this respect, but have only been manufactured for a few years and can only produce a small amount of UV light at low efficiency. If you compare the development to white light LEDs using Haitz’s Law, large-scale UV LEDs should be viable in a handful of years.

Another energy advantage is that LEDs can be instantly turned on and off. This can be seen with visible light LEDs, since they can be “dimmed” by pulsing on and off at short duty cycles. The practical use for water treatment is with a control loop that enables the system to adjust the light output or water flow rate to match a fluency level measured with a down-flow feedback sensor. LEDs have even more advantages over fluorescent lamps for contingency environments. They are mechanically robust. They have long life spans. And they can run on low voltage, making them more compatible with solar or battery power.

 

WARFIGHTER APPLICATION

The BEAR kit’s ROWPU unit would be a bear to replace in terms of capabil­ity. Reverse osmosis diverts every particle larger than 0.2-μ, including salt in saltwater. This gives a warfighter the confidence that the water is potable.

Unfortunately, the process produces 33 percent to 45 percent brine water from the raw water and uses an enormous amount of energy to create the internal pressures for osmosis. The 600-gal/hour ROWPU requires a dedicated 30-kW generator. The newer 1,500-gal/hour ROWPU requires a 60-kW generator.

As UV LED technology progresses to higher power outputs, a ROWPU-replacing UV LED system may be possible in as soon as a decade. It would require a basic multimedia filter, similar to the filter use on the ROWPU, but would run a pump at a fraction of the power required for reverse osmosis. No longer would a brine pump or storage be required—saving weight and space. The deleted brine-waste stream would decrease the upstream pump requirements. Also, the weight of the unit would be significantly less without large electric pumps. The long lifespan of the LEDs would reduce major maintenance. Daily maintenance would likely be related to the filter. The system would even decom­pose organic chemical warfare agents with oxidizer injectors.

Unfortunately, the current technology UV LED system cannot desalinate and the UV energy output is limited. So replace­ment of the ROWPU system in coastal environments is not immediately possible. Initial development should begin at the point-of-use. It is in this application that the small, lightweight, low-power attributes of these LEDs are most suited.

 

ROWPU comparison chart

Comparison of the 1,500-GPH ROWPU and potential UV LED systems. The 1,500-GPH ROWPU requires 37-kW for motor start-up and 23-kW for steady state operation. *The fresh water rate is used for direct comparison with a UV LED system. SOURCE: AFIT RESEARCH




RESEARCH UNDERWAY AT THE AIR FORCE INSTITUTE OF TECHNOLOGY

The UV LED technology piqued the interest of the Department of Defense and the U.S. Environmental Protection Agency, which have funded several research theses at the Air Force Institute of Technology, Graduate School of Engineering and Management.

Initially, five students worked simultaneously to create infrastructure and evalu­ate the technology. Capt. Michael Spencer, USAF, created an initial reactor, and characterized different vessel properties, such as shape, material and size as potential variables to increase the effectiveness of the unique LED output. Maj. Tho Tran, USAF, studied the efficiency of the UV reaction on bacterial spores while pulse driving the LED output. Capt. Kelsey Duckworth, USMC, also pulsed the output but created hydroxyl radicals with an Advanced Oxidation Process, which oxidizes organic chemical compounds, representing chemical warfare agents. Capt. John Richwine, USAF, composed a computer model for predicting the radiation dose at any 3D point in the reactor. Capt. Christopher Bates, USAF, investigated the magnitude of the UV LED power output and UV reflectivity of materials in air and water.

There are several ongoing research theses. Maj. Robert Scott, USAF, and Capt. Patrick Mudimbi, USAF, are continuing the Advanced Oxidation Process inves­tigation with different experimental conditions. Chelsea Marcum, an Air Force civilian, created bacterial inactivation curves, work that 2nd Lt. Cameron Harris, USAF, is continuing by measuring the efficacy of methionine as a radioprotectant from UV energy and ionizing radiation.

Future research by 1st Lt. Brandon Stewart, USAF, is aimed at understanding the effect of chlorine in the UV LED treatment process. Since UV lamps increase chlorine activation, implementing UV LED reactors in the BEAR water treat­ment assets would decrease the amount of chlorine used while maintaining an adequate level of disinfection. 1st Lt. Drew Gallucci, USAF, will be investigating a man-portable UV LED water disinfection system. Practical application will be potentially battery and/or solar powered and small enough to incorporate onto a water buffalo or backpack water system. And 2nd Lt. Dylan Klawuhn, USAF, will continue the investigation of radioprotectants.

  



As UV LED technology progresses to higher power outputs, a ROWPU-replacing UV LED system may be possible in as soon as a decade.


  

REALIZING THE POTENTIAL

A point-of-use UV LED system is viable today. The flow rate would likely be limited to something less than 1-gal/min because the UV energy output is relatively low. A practical man-portable device might be adapted onto a backpack water system, and battery and/or solar powered. No pump is required. The water would be disinfected as the user drinks. Another light-weight contingency option is a gravity-fed system with solar power. The system could be filled in the morning and run autonomously throughout the day.

Also in the near term, we might imag­ine the capability to reduce the chlorine concentration in water storage systems, such as water buffalos or storage bladders because UV energy increases the effective­ness of chlorine in water. By attaching a UV treatment system to the output of these storage systems, water could be treated as it leaves the storage system. This would disrupt the DNA of any microorganisms in the water before it is consumed. Such a system could reduce the logistical footprint of providing chlorine to the battlefield and significantly improve the taste of the water.

The near future of LEDs for lighting applications is bright—so too is the pros­pect of UV LED water systems. If success­ful, the development of these systems could improve safe drinking water access for U.S. servicemen and women along with many others around the world.

   


 

Capt. Michael J. Spencer, M.SAME, USAF, is Instructor, The Civil Engineer School, Air Force Institute of Technology, Wright-Patterson AFB, Ohio; 937-255-5654, Ext. 3525, or This email address is being protected from spambots. You need JavaScript enabled to view it." target="_blank">This email address is being protected from spambots. You need JavaScript enabled to view it..