Lighting the Way to Better Disinfection

By improving flow measurement, computer simulation optimizes the design of ultraviolet reactors used to destroy disease pathogens in water and wastewater

Computer simulation can substantially improve the design of ultraviolet (UV) light technology used to disinfect water and wastewater. In UV disinfection design, bacteria and viruses must flow in close proximity to a UV lamp for a sufficient period of time to dimerize their DNA. Without a method to determine the effectiveness of a proposed UV system design, engineers have been forced in many cases to over-design systems to ensure their ability to meet regulatory requirements, substantially increasing their cost. Computational fluid dynamics (CFD) coupled with irradiance modeling can track the trajectories of thousands of individual microorganisms and calculate their detailed motions and resultant UV dose or fluence. CFD provides more detailed information than can be obtained from physical tests, such as the flow patterns through the disinfection system and the UV dose received by various sections of the population of microorganisms. A key advantage of CFD is that engineers can evaluate alternative designs in much less time and at a lower cost than the traditional approach without having to build a physical prototype. This makes it possible to optimize the design of the UV system, and in many cases, eliminate short-circuiting and dead zones that can result in inefficient use of power and reduced contact time. Using CFD in this manner, the overall cost of a system can usually be substantially reduced while still meeting all regulatory requirements.

To prevent transmission of waterborne diseases, disinfection of water is controlled by stringent regulations. These regulations typically specify water treatment processes, nutrient removals, final effluent quality, and disinfection criteria. Chlorine, the traditional disinfection method, provides reliable results but presents environmental concerns and safety issues. One major health concern is the formation of chlorinated hydrocarbons such as trihalomethanes, which are suspected carcinogens, and organic halides. In addition, new safety regulations are driving up the cost of handling, transporting, and storing chlorine. Regulations designed to address aquatic toxicity also require dechlorination of water before discharge, which further drives up costs.

Rising Popularity of UV Disinfection
UV disinfection can potentially address all of these concerns. Recent findings have proven that UV is effective in eliminating disease-causing pathogens found in drinking water that are resistant to conventional chemical disinfection. UV does not alter the taste, color, or odor of water, and it does not produce any harmful byproducts. Over the past two decades, UV radiation has become an established disinfection technology. An extensive pilot and full-scale study sponsored by the state of California established that UV treatment generated no residual effluent toxicity and no significant byproduct formation. It also showed that UV treatment was more effective than chlorination in the treatment of viruses.

Presently there are more than 2,000 installations in the United States where UV radiation is used to disinfect primary, second, and filtered tertiary effluents. Designing these systems has become a major challenge due to the difficulty of ensuring efficient flows through the disinfection tank that will provide a relatively even dose to each microorganism. The traditional approach is to simply build a prototype disinfection system and perform tests designed to measure its efficiency with real microorganisms. But with an infinite number of possible designs and the high cost and time involved in the testing of each one, there is a strong tendency to substantially over-design the disinfection unit to ensure that one of the first tests provides positive results. The problem with this approach is that the unit will probably cost far more than necessary to build and operate. For example, it is difficult to accurately determine flow through a prototype, so it is often impossible to detect short-circuiting. This phenomenon causes bacteria to move quickly through the unit and requires a high level of UV radiation to meet regulatory requirements, which in turn drives up equipment and operating costs.

Simulation Improves Design Process
A new approach to designing UV disinfection systems has the potential to substantially improve the design process and reduce the cost of building and operating UV disinfection systems. The basic idea is to use CFD to simulate the movement of microorganisms through the disinfection system and calculate their exposure time to UV light. This approach makes it possible to quickly determine the effectiveness of any proposed design and to scale-up the existing technology to large-scale systems, which are typically greater than 50 million gallons per day (mgd). A CFD simulation provides fluid velocity, pressure, temperature, and other variables, as appropriate, throughout the solution domain for problems with complex geometries and boundary conditions. As part of the analysis, an engineer may change the geometry of the system or the boundary conditions, and observe the effect of the changes on fluid flow patterns or distributions of other variables. The path that an organism takes in the reactor determines the amount of UV radiation that it will be exposed to. The reactor can then be designed to eliminate short-circuiting and dead zones that can result in inefficient use of power and reduced contact time.

The position of microorganisms in the fluid can be tracked in one of two ways. Using the particle tracking method, organisms are treated as discrete, micron-sized particles. The flow and radiation fields are calculated to convergence. Particle tracking is performed in the post-processing mode. A user-defined function is used to calculate the UV dose along each particle track. The amount of UV dose accumulated by each particle is analyzed using statistical methods to assess the performance of the UV system. Another approach uses transport equations to model the bacteria as if they were a chemical species or compound. The software tracks the concentration of the species through the domain, and a user-defined function is used to model the destruction of microorganisms based on the chemical composition of the organism and the radiation field intensity.

Developing the Radiation Model
The radiation model also plays a critical role in the simulation. The optimum wavelength to effectively inactivate microorganisms is in the range of 250 nanometers (nm) to 270 nm. The intensity of the radiation emitted by the lamp is reduced as the distance from the lamp increases. The disinfection efficiency depends on the lamp power and residence time of the bacteria in the water around the lamps. Ideally, a disinfection system should have uniform flow past the lamps with enough radial mixing to maximize exposure to UV radiation. The UV-light emitting tubes typically span the reactor zone. Baffles placed in the UV reactor can be used to provide a more directed flow at the lamps. Fluent software has a number of radiation models that can be used coupled with the flow simulation to model a UV system. One of the most often used is the discrete ordinates (DO) radiation model. This model offers the ability to incorporate reflection and shading effects, and a banded modeling option to differentiate the effects of different wavelengths of UV light. It is also possible to use other UV codes to calculate the radiation field and then import the radiation field onto the CFD grid using a proprietary user-defined function.

Recently, there have been numerous studies comparing Fluent's DO model results with leading UV-specific software. In one such study, engineers at Metropolitan Water District of Southern California compared Fluent's DO model with a UV-specific code, UVCalc. Fluent's results were within 4 percent for the fluence calculation. After this validation study, the Metropolitan Water District of Southern California (MWD) compared their actual UV-reactor measured data with Fluent's simulation, and results were in very good agreement.

Conclusion
In conclusion, CFD makes it possible to compare multiple reactors with different geometries, hydraulic properties, and levels of energy consumption using different levels of water quality and compare the disinfection efficiencies. By giving engineers detailed information on the performance of as many designs as they wish without the cost of prototyping and testing, CFD can help engineers optimize the design well before the prototype stage. This approach makes it possible to evaluate a much larger number of designs than in the past. As a result, engineers can optimize the design to a much higher level, eliminating short-circuiting, minimizing head losses, reducing energy consumption, and generally building a far more efficient disinfection system than is possible using experimental methods. The CFD approach has also been validated by other major water companies such as Veolia Water, Maisons-Laffitte, Cedex, France, where CFD simulations have been compared to test results and found to provide excellent correlation with physical testing.

Note: The use of examples from the Metropolitan Water District of Southern California does not constitute a product endorsement.

This article originally appeared in the 09/01/2004 issue of Environmental Protection.

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