Robert C. Kappel
Revised March 29, 2022
Ultraviolet light has many uses. Some people tan their bodies under ultraviolet light. Dentists use ultraviolet light to cure certain types of polymers used in dental procedures. Parasites, bacteria, and viruses can be inactivated to make water safe to drink and recreate in and organic contaminates can be oxidized. For the purposes of this article, the focus will be on the disinfection of swimming pool water using ultraviolet light.
What is Ultraviolet Light?
Ultraviolet light (UV), as early as 1845, was known to influence microorganisms. Since then, disinfection of drinking water using UV has become a staple process around the world.
Ultraviolet light occupies a very small portion of the larger light spectrum of which itself is a portion of the full electromagnetic spectrum. The range of our normal experience of light in order of wavelength from longest to shortest is: near Infrared (heat lamps), red, orange, yellow, green, blue, indigo, violet and ultraviolet (tanning lamps). Each “color” of light can be organized by wavelength.
The measurement unit of wavelength used to describe light waves is called a nanometer (nm). A nanometer is 1/1,000,000,000 of a meter. Visible light wavelengths fall in the range of 400 – 700 nm. Infrared heat lamps produce relatively long wavelengths in the range of 700-1000 nm. UV light can be broken down into UVA, UVB, UVC and Vacuum UV. UVA, which are the tanning rays of the sun, fall in the range of 315 – 400 nm. UVB, which are the burning rays of the sun, fall in the range of 280 – 315 nm. UVC, which are the cancer causing rays of the sun, fall in the range of 200 – 280 nm. Finally, there is vacuum UV which falls within the range of 100 – 200 nm.
Waterborne Infectious Diseases and Inactivation
Our bodies’ immune systems are amazing and can handle virtually any pathogen encountered but when an ingested organism propagates millions or billions of copies of its self, our immune systems become overwhelmed and we get sick. Waterborne infectious disease can be broken down into four broad organism categories: Virus, bacteria, parasite and protozoa. Some common waterborne illnesses caused by these organisms are in order: Hepatitis A; Dysentery; Schistosomiasis and Cryptosporidiosis.
Every infectious organism’s cell(s) contain specific instructions for replication. These sets of instructions are called DNA and RNA. DNA tells cells what to do. These instructions are written in a language called the genetic code. Like any set of instructions, the instructions must be in the right order. Exposure to UV light can affect the instructions by removing pieces or tearing out big chunks. It is fascinating to note that that DNA and RNA absorb UVC light very well at around 254 nm. This absorption characteristic combined with UV light of the proper energy (brightness) can render infectious organisms harmless to us. This process is called inactivation. It is called inactivation because in most cases, the organism is not killed. It is simply rendered incapable of replication.
The heart of any UV system is its lamp. There are many types of UV lamps with varying characteristics but in the interest of keeping things simple, this discussion will be limited to medium pressure (MPUV) lamp technology. A typical MPUV lamp is similar in construction to fluorescent lights we see in offices and homes every day. The MPUV lamp tube is made of quartz-based glass and contains a small amount of mercury. At either end, there is a tungsten electrode. When electricity is applied to the electrodes the mercury vaporizes and the lamp emits heat and light. A MPUV lamp has a relatively high internal pressure when compared to atmospheric pressure and it is considered a polychromatic lamp because it radiates UV light at multiple wavelengths including 254 nm. Note that this just happens to be the same wavelength that is readily absorbed by DNA and RNA as mentioned in the previous section.
The MPUV lamp is protected from contact with the water by a device called a sleeve or thimble depending on the design of the UV chamber. A sleeve is open at both ends and a thimble is closed at one end much like a large test tube. Like the lamp, the sleeve or thimble is constructed out of glass made from quartz instead of silica because quartz is more transparent to UV light.
The sleeve/thimble and lamp assembly are then housed in a stainless-steel chamber. This chamber, sometimes called the reaction chamber, is where the water is routed to become disinfected by the UV light. Often, during the design stages, this chamber undergoes computational fluid dynamics (CFD) modeling as a first step to ensure proper UV light distribution throughout before certifying of the system to meet the appropriate USEPA UVDGM criteria (validation) and NSF/ANSI standards (certification). Certifications/validation programs in other parts of the world fall under the Austrian ONORM or German DVGW standards.
The quartz sleeve/thimble assembly can be prone to fouling. Fouling is an accumulation of contaminants found in the water. The products that can accumulate are phosphate, calcium, iron, and manganese that have photocatalytically reacted with the UV light and attach to the quartz. Much like clouds in front of the sun, this fouling will impede the ability of UV light to reach the disinfection target, and therefore it is important to install a UV system that has an automatic wiping system that helps to keep the quartz clean. The wiper system consists of a carriage that houses PTFE (Teflon®) rings that fit tightly around the quartz sleeve and act as a “windshield wiper” of sorts that moves back and forth along the length of the sleeve periodically or on an as-needed basis. If the sleeves foul often, regular disassembly and manual cleaning may be required.
Certification/validation standards typically require an electronic means to monitor the UV lamp performance to ensure adequate disinfection is occurring. This monitor called a UV sensor, is calibrated to a known value and then it watches the output of the lamp(s). Depending on the sophistication of the UV system, if the monitor detects inadequate amount of light reaching the water inside the chamber, the system can trigger one or any combination of the following events: a wiper cleaning cycle; an alarm- either audible or visual; a shutdown of the UV lamps; a shut off of water flow through the system; an automatic ramp-up of power applied to the lamp and an email or text message indicating the presence of the alarm.
UV light can inactivate waterborne infectious organisms but there is an important detail to understand: That is the concept of UV dose. UV dose is a calculation that consists of three variables:
- The speed of the fluid moving past the lamps. This speaks to “contact time”
- The transparency of the fluid to UV light. This is called UVT or T10
- The power of the UV light emitting from the lamp expressed in Wm2
The calculated dose is expressed in mJ/cm2 (J/m2). It is critical to expose the waterborne infectious organism to the proper UV dose because different organisms require different doses to become inactivated. If the proper UV dose is not applied, there is a very strong possibility that the organism could remain viable and cause disease. Therefore, choosing a UV system to match filtration system flow rates is a critical component of proper UV application.
There are many certification/validation standards around the world that accredited testing laboratories use to conduct 3rd-party testing. This testing ensures a customer buying a UV system for disinfection purposes is getting a system that performs per manufacturer claims.
Interferences of the UV Disinfection Process
As mentioned previously, a certain dose of UV light must reach the organism in order for it to be inactivated, however, it is important to understand that there are things that can interfere with the UV light and prevent it from reaching the target organism. First on the list is turbidity. Measured in NTUs, turbidity is caused by small particles suspended in the water. These particles block, shadow and absorb the UV light. Keep in mind that the largest viruses are about 200nm in size and a typical grain of sand is around 1mm. Using simple math, we can calculate that approximately 5,000 viruses can hide behind a grain of sand! Next on the list are chemicals and compounds in solution in the water that can adsorb UV light. Common chemicals and compounds that can be present in the water are: humic and fluvic acids, metals, nitrates, sulfites and sequestering agents like poly-orthophosphates. Another interference can be fouling of the quartz sleeve. Metals in solution in the water can photo-react with the UV light and deposit a thin coating on the sleeve housing the UV lamp and lower its apparent output. Water that looks crystal clear to the naked eye can actually have very low UV transmission characteristics and this is why it is imperative that the water is tested for UVT % before selecting a system for any given application. Finally, another common interference is simply the UV lamp ageing. Typical useful lamp life for a medium pressure UV lamp is approximately 9000 hours. As the lamp ages, its performance diminishes and its ability to output the proper UV energy reaches an end. Finally a process called solarization of the quartz sleeve can cause interference. Over time, the UV light can cause the quartz sleeve to become less transparent. Although the frequency of replacement varies by manufacturer, typically the sleeve is replaced proactively every 3-5 years.
Because UV is applied as a disinfection step in aquatics applications, it is critical that the health and well-being of the customer is foremost in the mind of the person selecting and installing the system. If a mistake is made, a contracted waterborne disease or worse could be the result. The first step is to have a clear understanding of the application by asking important questions and having a full water test done.
- Is the source water from a municipal or private source?
- What is the infectious organism of concern?
- Is the water source ground or surface water?
- What is the highest GPM flowrate the UV unit will see at any given time?
- What is the hardness of the water?
- Are there any metals present in the water?
- What is the UVT of the water?
Once these questions are answered, the proper size of the UV system selected. It only through these steps can a water professional be confident that their customer will be protected from waterborne infectious disease because the UV system is working as it should.
The environment is of the utmost concern of many people today and with this in mind, it is important to follow all local, state and federal regulations when disposing of used UV lamps. In many communities there is a program for the disposal of florescent lamps and this would be an appropriate method for the disposal of UV lamps. In the case of a lack of such a program, it is highly recommended that the UV system manufacturer is contacted for advice for the proper disposal of the lamp.
As always, all manufacturer recommendations for application criteria, installation, safety precautions and maintenance should be followed to ensure the warranty remains in effect, the system performance is maximized and the safety of the user and installer is preserved. Also, all applicable local, state and national codes should be followed to ensure the system is properly sized and installed.