Volume 15 | Number 2 | Summer 2003

Automation Issue

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Researchers Complete Bench-Scale Studies of Novel UV Disinfection System

Georgia Tech researchers John Pierson, left, and Larry Forney are developing a cost-effective, more efficient UV-based disinfection system for the treatment of food processing wastewaters.

Ultraviolet (UV) disinfection is a proven technology for supplemental treatment of process waters containing harmful pathogens. However, treatment efficiencies in conventional systems are impacted by such factors as nonuniform radiation levels across a flow stream and concentration boundary layer effects at the treatment surface. The more turbid or darker the water, the more problems there are in these areas, impacting the effectiveness of many treatment application systems.

In a groundbreaking research initiative, Georgia Tech’s School of Chemical and Biomolecular Engineering and the Georgia Tech Research Institute (GTRI) have teamed to develop a new high-efficiency treatment system based on the concept of a Taylor vortex.

The team originally focused on UV disinfection for low-turbidity water reuse applications; however, laboratory successes indicated that the system has the potential to treat other more opaque or cloudy liquid streams. Engineers and scientists have recently conducted numerous laboratory experiments as they develop a larger prototype of the existing system for on-site testing. Much of the work to date has centered on bench-scale reactor testing to define flow rates, lamp location, and cylinder rotation rates for the inactivation of E. coli.

Recent bench-scale tests demonstrated that the Taylor vortex system when operated with rotation achieved more than a 3-log reduction in the inactivation of E. coli when compared to a conventional UV channel with similar radiation dosages.

Although UV is a powerful, cost-effective tool for disinfecting potable water, its usefulness quickly decreases as the water becomes more cloudy or turbid. The Georgia Tech bench-scale system boosts UV performance by exposing E. coli bacteria or colonies to the equivalent of 100 lamps using only four lamps as it passes through the Taylor column. The design also ensures every particle passing through the device is exposed to an equal amount of UV, thus allowing the system to better disinfect liquid streams containing turbidity or solids. However, researchers note that the Taylor column design is not a stand-alone, one-stop-shop for water treatment.

For instance, many regulations require a reduction in turbidity or solids before use or reuse simply because bacteria can be attached to or hide in crevices, thus avoiding UV treatment.

“We believe the system inactivates bacteria that may be on solids, but we have no real test to confirm that. Most reuse applications require the final turbidity to be less than 2 to 3 NTU [nephelometric turbidity units] with total suspended solids levels of less than 5 mg/L [milligrams per liter]. Plating or other microbiological testing techniques indicate we have some success, but as turbidity or solids increase, the amount of UV light needed to ensure we achieve adequate treatment increases,” explains John Pierson, GTRI research engineer.

Researchers are testing enhancements to the prototype designed to improve disinfection of liquid streams that are more turbid. Furthermore, they estimate the prototype Taylor column being designed should enhance UV or other disinfection applications that include some level of filtration.

“Our work has made significant progress with understanding exactly where and how UV disinfection takes place in a liquid column. Because inactivation occurs only in a limited area, unless you have significant energy input, cost-effectiveness results from properly engineering the disinfection zone,” notes Dr. Larry Forney, a professor in Georgia Tech’s School of Chemical and Biomolecular Engineering. Forney concludes the real benefit may reside in treating marinations or juices.

For the Taylor column, an important consideration, explains Pierson, is that the design maximizes the time that bacteria reside within the germicidal UV light penetration depth. Addressing this critical aspect requires extensive laboratory and computer modeling. The recent bench-scale tests demonstrated that the system can achieve more than a 3-log reduction in the inactivation of E. coli when compared to a conventional UV channel with similar radiation dosages.

In addition, Pierson has evaluated competing technologies. “We are cost-effective at 12.7 cents per 1,000 gallons, although our current throughput capacity is more in line with chillers or bird washers and not a full-facility effluent. But when you compare disinfection systems, keep a close eye on the energy because that is where the real savings or expense will be found.”