UV Disinfection - The Basics
THE HISTORY OF UV
Many of us have heard the term “Ultraviolet” and in most cases it is in reference to our exposure to the sun rather than to its disinfection capabilities in water and other fluid. So before we move on, allow me to interject a little bit of history. It was as early as 1877 when two researchers, Downes and Blunt, discovered the dramatic ability of sunlight to destroy bacteria and provide an effective means of treating bacterial infections. Years later, in 1906, the first fused silica quartz arc tube was developed. This special glass tube allowed more light output than a standard glass tube. It was discovered the quartz glass tube could also withstand the higher output operating temperature associated with the higher tube pressure required to generate more light output. It was also discovered that the silica quartz itself (rather than glass) was the better material for this application since it was less chemically reactive to the hot chemicals and gases encountered within the lit arc tube. By 1910 these discoveries had led to the first full-scale UV disinfection apparatus.
So here we are today, a time when UV technology is used in virtually every country throughout the world and considered, by many, to be the best available method for treating waterborne microbiological contamination.
PERTINENT 'WATER TREATMENT' BACKGROUND
The term disinfection means the treatment of water to inactivate, destroy and/or remove pathogenic (disease producing) bacteria, virus, cysts and other microorganisms for the purpose of making the water microbiologically safe for use or human consumption. In effect, this means the elimination of disease carrying microorganisms. There are two primary types of disinfection; chemical and physical. Chemical disinfection employs the addition of substances such as chlorine, iodine or bromine to the substance being disinfected. Physical disinfection employs no chemicals but rather physically removes, neutralizes or destroys the microorganism through some physical means such as filtration, UV light treatment, electro-adhesion, etc.
One of the most commonly used chemical disinfection substances is chlorine. Many drinking water facilities and municipalities use chlorine as the method of choice in controlling waterborne microorganisms. It is applied by adding a known concentration of the chemical to a volume of water and ensuring that it is thoroughly mixed within the water supply. The chlorine immediately targets the microorganisms, killing them and making the water microbiologically safe. However, there is what is referred to as "free chlorine" still left in the water - the chlorine that was not needed to disinfect the water supply. This free chlorine bonds with other compounds, such as organic matter, causing changes in smell and taste as well as chemical changes. The modification of pH levels and the creation of various harmful chemical compounds can negatively impact the 'product water'. In physical disinfection the microorganisms are the only target. There are a variety of methods of physical disinfection, including distillation, boiling, filtration, electro adhesion and Ultraviolet (UV) disinfection. None these methods cause changes to the water while effectively deactivating or removing microorganisms. The water will not be altered in taste or smell and no new chemical by-products will be formed. The water is simply free of microbiological contamination. OutbackWater offers only physical (chemical free) methods of disinfection.
When it comes to water treatment the power of the sun artificially lies inside a mercury vapor lamp. Similar to a fluorescent light tube, a mercury vapor lamp (UV) emits its spectral output at 253.7 nm, a wave length that is very close to the 265 nm wavelength considered optimal for microbiological inactivation. To understand the functionality of a UV lamp, one must first understand the electromagnetic spectrum.
The electromagnetic spectrum (ES) is a range of all possible frequencies of electromagnetic radiation that extend from low frequencies used for modern radio, to gamma radiation at the short-wavelength end, covering wavelengths from thousands of kilometers down to a fraction of the size of an atom. The long-wavelength limit is essentially the size of the universe.
Ultraviolet light is electromagnetic radiation that lies between visible light and x-rays and is comprised of four basic segments. The shorter the wavelength, the higher the energy and as a result, vacuum UV (UVV) lies in the 100-200 nm wavelength. On the opposite end of the UV scale lies long wave UV (UVA) which has the lowest energy output and lies between 315-400 nm. Almost 99% of the sun’s output is in the form of UVA energy. Middle wave UV (UVB) have wavelengths between 280-315 nm and have more energy than long wave UV. For disinfection purposes, it is the short wave UV (UVC) that we are the most interested in as its wavelength covers the 100-280 nm range - which includes UV wavelength output optimal for microbiological deactivation.
As we all know, DNA is a form of genetic coding of all living organisms. At the 254 nm wavelength, UV light alters a microorganisms DNA. Within each DNA strand there is different sequence coding for different characteristics. The DNA strand of a microorganism is very simple, with the major coding addressing replication. UV light is absorbed quite readily at this spot in the DNA strand, causing a break in the bond of the strand. By breaking the bond the microorganism becomes sterile and is no longer able to replicate or reproduce.
Within a properly designed UV system, the process of disinfection occurs very rapidly. As water runs through the UV reactor it is exposed to the UV light given off by the UV lamp. This causes a genetic change in the microorganisms present in the water. This genetic change eliminates the microorganisms ability to replicate and re-produce (make colonies). Keep in mind that the microorganisms only activity and sole purpose in life is to replicate and produce colonies. Their efficiency in the attainment of this goal is the cause for such a high percentage of waterborne illnesses in our world. So in simple terms, microorganisms that cannot re-produce are the ones we do not have to worry about. They enter our system and will pass right through without causing any sickness or ailment.
UV LAMP TECHNOLOGY
Earlier we mentioned the vast majority of current UV systems use mercury vapor lamps in order to create UV energy. These lamps can be divided into two sub-categories, the low-pressure (LP) lamps and the medium-pressure (MP) lamps.
Low-pressure lamps are monochromatic in nature, emitting their spectral output at a single wavelength. They can be further subdivided in to three subgroups:
- Standard Output (LP) or Low Pressure ultraviolet lamps have been the “industry standard” for many years for good reason. Their warm up time is approximately 30-60 seconds and they offer the best electrical efficiency (up to 40% of their electrical power is converted to UV). Low Pressure lamps should not be used in an environment where the ambient temperature exceeds 40o C/104o F. They operate at 425 mA.
- High Output (LPHO) UV lamps are typically used in commercial applications. They operate at between 600 mA and 800 mA and their output is approximately two times that of Standard Output (LP) germicidal lamps. Their UVC output is related to ambient temperature.
- Amalgam (LPAM) UV lamps offer three to four times higher power density than other lamp types. They are designed for stable operation over a wide ambient temperature range (4–40o C/39-104o F). They are the 'lamp of choice' for long term applications with low or no cycles and they are usable in universal orientation applications. Due to these characteristics, amalgam lamps are frequently used in municipal applications.
Although not a part of our on-line product line, Medium-Pressure UV (MPUV) Lamps require a short discussion. These lamps offer the highest power density currently available. Unfortunately, the creation of this high density power results in the worst electrical efficiency of the group with only approximately 12% of the electrical power converting to UV. The operating temperatures of a typical MPUV system range from 600o - 750o C (1112o -1382o F). Consequently, use of these lamps is better suited to UV curing or to water applications requiring high/constant flows and/or extremely compact footprints.
OutbackWater offers a variety of UV systems using all three types of low pressure lamp technologies. All of our lamps are manufactured with a proprietary Long Life+TM coating which ensures a consistent UV output over the life of the lamp. In addition, all of our UV lamps are literally the most environmentally friendly UltraViolet lamps on the market with each lamp containing less than 10mg of mercury - up to 30% less than the mercury contained in UV lamps sold by our competitors. (The rules governing the disposal of UV lamps are included in the Toxicity Leaching Procedure (TCLP) requirements, which falls under the Resource Conservation and Recovery Act (RCRA) established under U. S. Federal laws for the disposal wastes. Our lamps meet the US TCLP requirements.)
The term UV Dose, or more simply 'Dose', indicates the total amount of radiant energy produced by a UV light source inside a system. It is the product of the UV light's Intensity, “I”, (expressed as energy per unit surface area) by “T”, the residence time (or exposure time). Dose can be expressed in a variety of unit measurements. For our purpose we'll use the most common expression of mJ/cm2 (millijoulle per centimeter squared) or W/m2 (Watt per meter squared). As an example, if a given UV light has an intensity (I) of xxxx, and the water flow produces a residence or exposure time (T) of xxxx the calculation of xxxxx times xxxxx tells us the "Dose" is xxxx. So, since dose is a product of "I" times "T", one can easily see how a change in either variable will change the corresponding dose. Since the buyer may not have any idea of the required dose level, they can easily ascertain their flow rate. Consequently many UV systems are rated for use at different flow rates - doing the work for you to assure the appropriate dose levels.
Required dose levels are typically represented at three distinct levels.
The earliest dose level requirement was based on an old US Public Health document outlining a UV dosage of 16,000 µWsec/cm2 (or 16 mJ/cm2) under the newer units where 1000µWsec/cm2 equals 1 mJ/cm2).
Over the years, a UV dose of 30mJ/cm2 has become the industry standard and the UV standard we implement in evaluating our products performance.
More recently, a UV dose of 40 mJ/cm2 was adapted by NSF and consequently by many US States as the new “standard” for UV dose levels. The simple reality is you can control the dose level for your system simply by using an optional flow restrictor to control the flow of the water through the system.
Many different kinds of microorganisms can be found in drinking water, each requiring a specific level of UV energy (dose) to inactivate or neutralize them.
The good news is that this all becomes simple because the biological contaminants typically found in water are all easily inactivated (or deactivated) using UV light. For example, E. coli is inactivated at a dose of 6.6 mJ/cm2 and both Giardia lamblia and Cryptosporidium are eradicated at dose levels of less than 10 mj/cm2. Although UV is effective against all forms of bacteriological contaminants, the deactivation of virus is difficult and typically requires the highest dose level for complete destruction. In some cases, virus such as Adenovirus, requires UV dose of 165 mj/cm2 for deactivation. Since this dose is much higher than the dose rating of traditional UV systems, a multi barrier approach is typically used to address virus inactivation.
UV SYSTEM DESIGN
The basic design of a UV system consists of four basic components: UV Reactor Chamber, a UV lamp, a UV Controller and a UV Quartz Sleeve.
The UV Reactor or Reactor Chamber is the component that physically houses the lamp and sleeve and through which the water flows as it is irradiated by the UV lamp. The chamber is a pressure vessel that should be manufactured in accordance with ASME pressure vessel manufacturing guidelines. (www.asme.org). In order to cut costs, some UV manufacturers use UV reactor chambers that are highly polished, and constructed of ornamental tube, creating the misleading appearance of quality and value. Don't be fooled. Higher quality reactor chambers are constructed of A249 pressure rated tube and are manufactured according to specific ASME standards. Both the material quality and the manufacturing standards are critical to the efficacy, structural integrity and longevity of the product. (If these are in question, ask the supplier for the pressure vessel rated calculations). Some manufacturers also elect to produce their pressure vessels from plastic which, over time, inevitably break down from the changes in temperature and varied water pressure. We strongly believe that stainless steel is the only correct material to use in UV reactor chamber construction. Based on the nature of the application either 304 or 316L stainless is appropriate. Although different manufactures elect to use different reactor designs, regardless of the design (shape) of the reactor, it is important to ensure that the manufacturer has done their due diligence by testing the efficacy of the UV rector in bioassay testing and /or calculations.
Here is an exploded view of an Ultra Violet Disinfection System, showing flow and typical components
The UV Lamp provides the necessary Ultraviolet energy (DOSE) to facilitate the disinfection process. These low pressure mercury vapor lamps can vary in length, diameter and type (LP, LPHO, LPAM), each having their own power density. During installation the lamps are insulated from direct water immersion and contact by a quartz sleeve. Due to the unique operating characteristics of each individual lamp type and power rating, each lamp must be matched to a specific lamp driver or ballast. A single ballast design may be able to accommodate a range of lamps. The effective rated life span of a UV lamp varies between 9000-12000 hours of continual use, based on the type of lamp.
A quick note: Over time the UV light will diminish in intensity (UV dose output will diminish). This usually takes place at about the rated life-span - 9000-12000 hours of continuous use or approximately one year. This is important to note and track because after this time, although the light will still emit energy (shine), the diminished UV dosage will reduce the lamp's effectiveness in its ability to deactivate microbiological contaminants. And remember to use caution when working with UV Lamps, the energy given off by a UV lamp is extremely harmful to the naked eye and one should never look directly at an illuminated UV lamp.
As previously discussed, all Outback Water UV lamps are manufactured with a proprietary Long Life+TMcoating to provides a consistent UV output over the entire life of the lamp. All of our UV lamps are the most environmentally friendly of any UV lamp on the market as each lamp contains less than 10mg of mercury, which is up to 30% less than those sold by our competitors.
The Controller generally houses both the ballast - which controls the electrical current for the lamp, and the electronics - which control the functions of the UV system such as visual displays, service reminders, and audible alarms. The controller should be 'intuitive' in nature making it easy to operate and understand, regardless of the location of the installation. The controller should have enough cable to conveniently attach to the reactor chamber and a method for an easy wall mounted installation. The controller should not be installed outdoors or be exposed to the elements yet it is still important that the controller be water-tight to protect the electronics. It should not be mounted in a location where exposed to water or moisture that could come in contact with the internal electronics. When shopping, it is important to look for controllers that have CSA or CE electrical certification.
The Quartz Sleeve provides a thermal barrier between the UV lamp and the water. UV lamps have optimal operating temperatures and the quartz sleeve protects the lamp from performance-effecting variations in water temperature. The sleeve should be manufactured from pure quartz (100% silica) due to the optimal transmittance characteristics of the quartz (its ability to transmit UV light). The open end of the quartz sleeve should be fire polished to reduce stress cracking and aid in handling. Based on the design of the UV reactor, the sleeve should be either domed on one end or open on both ends.
Sensor (optional) - Some regulated as well as some non-regulated applications choose to incorporate a UV intensity monitor or sensor to help measure UV system performance. The sensor is an optical instrument which incorporates a photodiode (a device used to detect light) which reads the UV light inside the reactor. The diode should be specific in its function to reading only the 254 nm wavelength of UV (not read the visible light spectrum). As diodes will all have some minor variations when manufactured, each must be individually calibrated to ensure they are within specification. The UV sensor should not be (or able to be) calibrated in the field. It must be factory calibrated with a NIST traceable reference sensor.
UV is one of the fastest growing segments in the water treatment industry. It is a universally accepted method to treat microbiologically contaminated water and is proving to be one of the most cost effective options as well. OutbackWater is committed to working closely with others in the industry to make available the technology innovations such as those in the LED industry. The future is definitely looking bright. ... pun intended.
Dose - Flow Rates and UV Fluence (Non NSF/ANSI Certified Products Only)
The understanding of fluence (dose) is critical in understanding how UV systems work. UV fluence is the product of the UV intensity and time. The UV intensity is represented by the actual energy produced by the lamp while the time is represented by the residence or length of time the water resides in the UV reactor chamber.
Unit equivalent: 1 mJ/cm2 = 1000 µWsec/cm2 = 10 J/m2
An important part of the effectiveness of a UV system is the quality of the untreated water supply. Specifically, UV transmittance (UVT) - a percentage indicating the UV light's ability to penetrate through the water - is a factor that will greatly affect the delivered fluence or dose.
For example: a UV system that is rated at 10 gallons per minute (10 gpm) at a dose of 40 mJ/cm2 at 98% UVT may only deliver a dose of 20 mJ/cm2 if the UVT is only 75%.
This is a significant factor and must be taken into consideration if the UV system is going to be sized correctly. UV systems cannot be compared by flow rates only unless both the fluence and UVT are equivalent. A water supply with a low UVT will require pre-filtration and/or a Low UVT system.
ADVANTAGES OF USING ULTRAVIOLET
- The use of ultraviolet light for disinfection purposes has many advantages, some of which are:
- A physical process, with no addition of potentially harmful chemicals
- Virtually instantaneous disinfection - no holding tanks are required
- No change in color, taste, odor, pH or conductivity of water
- No need to handle toxic chemicals
- No disinfection (chemical) by-products such as THM’s
- Very low power consumption
- Does not removal beneficial minerals (as does distillation)
- Very low capital and operating cost
- Environmentally friendly
- Universally accepted treatment method
- Automatic operation
- Easy maintenance
- Safe to use
- Effective against cysts
- UV transmission: 75% or greater
- Iron: less than 0.3 ppm (mg/l)
- Manganese: less than 0.05 ppm (mg/l)
- Tannins: less than 0.1 ppm (mg/l)
- Hardness: less than 7 gpg (120 mg/l)
- Turbidity: less than 1 NTU
TIPS, GOOD IDEAS AND ADDITIONAL INFORMATION
- When possible, install the UV reactor in the vertical position with the inlet at the bottom of the reactor.
- Disinfect the distribution system with household bleach (5.5% unscented) for a full 30 minutes to destroy any microbiological contaminants in the system and flush before use of water.
- Always install a 5 micron pre-filter ahead of any UV system.
- Install the UV system as your last piece of treatment equipment - the last device in-line, after other filters
- To ensure complete disinfection within the home make sure the UV system is installed in-line BEFORE the hot water heater and BEFORE any branch lines.
- When selecting a mounting location, always leave enough space to accommodate removal the UV lamp, quartz sleeve and any pre-filler(s).
- NEVER undersize a UV system. If in doubt, always move up to the next larger size.
- Install UV on a separate ground fault interrupter (GFCI) circuit.
- Use unions in the plumbing, before and after the system, in case it needs to be removed for some reason.
- Install by-pass loops in the event that the UV systems has to be taken out of service.
- Do not ignore and alarms or warnings being transmitted from the system.
- Change the UV lamp annually, without fail.
- Change the pre-filter annually or as needed.
- Clean the quartz sleeve as needed.
- Test your water regularly for traces of microbiological contaminants.