By George Diefenthal and J. R. Cooper, PhD

Introduction
Ultraviolet light has been used to treat water for decades. Initially, UV was used in drinking water applications, but in the past few decades its use has expanded to other municipal applications including wastewater, water reuse and ground water remediation. In addition, UV has become a standard part of the treatment train for ultrapure water applications and is expanding rapidly into many new uses, including cooling towers and onsite water reuse. The mechanism by which UV disinfects is when the UV photons cause the DNA in the cell nucleus to crosslink. This prevents the cell from reproducing and causing infections. DNA absorbs UV in the 240 to 280 nm range to produce this crosslinking, with the absorption peak being at 264 nm.

UV technologies
The term pressure used to describe the UV systems below refers to the style of mercury lamp used in the UV system and the pressure of the gas inside the lamp, not the pressure of the water being treated. Medium-pressure lamps have a continuous output spectrum ranging from 200 to 400 nm, with the spectrum being unique to each manufacturer. Low-pressure lamps produce two narrow UV output lines, one at 185nm and one at 254 nm. The 185 nm line produces ozone in air and is filtered out for many applications by selecting the proper quartz used to make the lamp.

Medium pressure. Medium-pressure systems have been used primarily for water disinfection for many years. These systems typically have a stainless steel chamber that fits in-line with the plumbing and has the lamp located perpendicular to the flow. This makes for a compact system which can be retrofitted to existing plumbing. The controls and ballast are typically housed in a cabinet nearby. The technology does, however, have some drawbacks when compared with low-pressure units. Medium-pressure systems use more energy, have a shorter lamp life and operate at a much higher lamp surface temperature (up to 1,600°C/2,912°F ) than a comparable low-pressure system.

Low pressure – conventional. The most recognizable UV systems in use today are systems that utilize low-pressure mercury lamps. These units are typically constructed in a stainless steel pressure vessel with the lamps installed parallel to the water flow. Chamber diameter, the number of lamps and the lamp length determine the capacity of the equipment. With minor improvements, this design has been in place for over 50 years. A major drawback to this design (as well as the medium pressure systems discussed above) is the fact that stainless steel absorbs about 80 percent of the UV light that impinges on its surface. This greatly increases the number of lamps and energy consumption required to achieve the desired level of UV treatment, which has added to the market perception that UV treatment carries a high operating cost.

Earlier attempts to reduce operating costs. There have been a number of attempts to improve the performance of UV systems by replacing the stainless steel-walled chambers with designs that overcome the inherent lossy (dissipation of electrical energy) nature of conventional chambers. A number of designs use an external reflector made of aluminum. Aluminum has a much higher level of reflectivity of UV (generally 80 to 90 percent or more) than stainless steel. In one such design, the flow tube is in the center, with lamps and parabolic reflectors surrounding the flow. This design provides an improved reflection of the UV, however, most of the UV is outside the flow of water, limiting the overall efficiency. Another drawback is that the systems can get quite large for higher flows.

Other systems use the property that light incident on a surface at a very shallow angle is almost completely reflected. These systems have the lamps at one or both ends of a long flow tube, so that most of the UV light that does reach the flow tube surface is reflected back into the water. Introducing the light efficiently into the long flow tube from its end is one of the challenges limiting the efficiency of this chamber design .

Optimized reflective chamber design
A design has been developed which overcomes the issues mentioned with the earlier reflective chamber designs discussed above. This design uses a double quartz configuration. The inner quartz lamp sleeve isolates the lamp from the water, as is done in most UV systems. A larger outer quartz flow tube is then utilized to create the outer containment of the flow. The reflective material is then positioned outside of the quartz flow tube. Figure 1 shows a cutaway of the design to help visualize the concept.

In this design, the lamp introduces essentially all of its UV directly into the flow tube and the reflector returns the photons to the water flow with only a small fraction of the UV escaping or residing outside the water flow volume at any given time. The UV stays in the water volume until it is absorbed by the target DNA or TOC molecule, rather than get lost (absorbed) by the chamber wall, or residing in a volume outside the water flow. This ensures that the light coming from the same low-pressure lamps is used more efficiently in this chamber than in the conventional or other reflective chambers.

There are two key requirements that are necessary for this design to achieve the significant improvement in performance that it can provide. These requirements must be met simultaneously – satisfying only one or the other will not provide the improved performance. The first requirement is that the reflective material needs to be at least 80-percent reflective to the UV light. The second critical requirement is that the reflective material must enclose at least 80 percent of the treatment zone (80-percent coverage). Tests and simulations have shown that chambers which don’t meet these two requirements do not provide a significant improvement in performance.

The chart in Figure 2 shows the strong increase in UV intensity relative to that in a conventional stainless steel chamber when both requirements are met. There are modest increases in performance over the baseline stainless steel chamber when the reflectance is above 80 percent and the coverage is below 80 percent, and also when the coverage is above 80 percent and the reflectance is below 80 percent, but the really significant increase only occurs when both the reflectance and the coverage are above 80 percent.

Independent research published by the University of Arizona1 has confirmed this effect both theoretically and experimentally. An earlier paper from the Chinese Academy of Sciences and the University of Alberta2 also indicates improvement with higher reflectivity, although this paper does not have enough information to determine the percent coverage.

Benefits
The reflective technology provides three main benefits to the end user: smaller size, fewer bulbs and lower energy consumption. These advantages in turn lead to additional, practical benefits to the operator.

Smaller size. As a general rule of thumb, a reflective UV chamber will be a third or less the size of a conventional UV chamber. The figure below compares a 50-gpm (189.2 L/m) conventional chamber to a 500-gpm (1,892.7 L/m reflective chamber.

Since the units are smaller, when designing skids that incorporate UV, the overall skid design is often smaller. In addition, the units can be mounted vertically to further reduce the footprint. Finally, due to the shorter nature of the equipment, the amount of free space necessary for the replacement of lamps and sleeves is less.

Fewer bulbs. A reflective chamber typically operates with 80-percent fewer bulbs, although this will vary depending on the application. This benefit directly addresses the operating cost issue associated with market expectations. Not only are the direct costs for bulbs, lamps, O-rings and ballasts reduced, but the soft expenses related to service labor are also reduced.

Lower energy. A typical reflective chamber will utilize 75-percent less energy than a conventional UV system. Fewer bulbs does not necessarily mean lower energy. Bulb count can be reduced by making the system longer, but the amount of energy needed to achieve the dose will not be reduced. Since the reflective system is using the light produced by the bulbs more efficiently, it takes less energy to achieve the required dose, which has been validated to US EPA’s ETV program by NSF and to California Title 22 requirements by independent contractors.

Conclusion
Designers and operators now have a new option to consider when incorporating UV systems into their treatment trains. Reflective UV systems have proven to be effective and highly efficient in providing the required treatment dose.

References

  1. [Zhang, Tianqi, et al., “Modeling the UV/ H2O2 oxidation of phenolic compounds in a continuous-flow reactor with reflective walls,” Journal of Environmental Chemical Engineering 7 (2019)]
  2. [Li, Megkai, et al., Measurement of the Fluence rate distribution in a UV reactor using a microfluorescent detector: Influence of different reactor walls, IOA IUVA World Congress & Exhibition, Paris, France – May 23-27, 2011]

About the authors
George Diefenthal is CEO of NeoTech Aqua Solutions and has been involved in the UV water treatment industry for over 20 years. He has an BS in electrical engineering (1981) and an MBA (1991) from San Diego State University. During his career he has held positions and managed groups in engineering, manufacturing, operations and sales.

J. R. Cooper, PhD, founded Ultraviolet Sciences, now NeoTech Aqua Solutions, in 2002. During his career, he has held the positions of VP of Engineering and Development at PurePulse Technologies and VP of Engineering at TEAL Electronics. Cooper received his PhD in electrical engineering from Texas Tech University in 1986. Since then, he has engineered and produced numerous innovative products, including the NeoTech Aqua Solutions line of UV equipment.

About the company
NeoTech Aqua Solutions manufactures UV systems with a patented reflective technology. These systems have been in the market for over 10 years and proven themselves in a number of industries, including life sciences, microelectronics, food and beverage, recreational water and cooling towers.

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