UltraViolet (UV) light has long been used by hospitals for sterilization and has become increasingly popular for removing airborne viruses, bacteria and other contaminants from breathing air as it passes through circulation systems. But, did you know this clean, effective technology is also available for removing microscopic traces of disease-causing bacteria, viruses, algae, fungi and protozoa commonly found in water? The UV process disinfects water, preventing living organisms from reproducing, where other types of filtration simply remove undesirable non-living contaminants. Ask us about it.
Although seemingly new to the average homeowner, the use of ultraviolet light as a means of water disinfection has been a proven process for many years. This method is now widely recognized and accepted by the homeowner as the concern of chemical contamination has increased. Ultraviolet disinfection equipment is a very attractive choice as more becomes known about chlorine's harnful side effects and the by-products produced, such as trihalomethanes (THM's).
Ultraviolet light is a point of contact disinfectant that does not require the addition of any chemicals or alteration of the water's taste or quality. It is virtually impossible to overtreat the water with ultraviolet. Proper equipment design and application insures that the required water treatment is achieved with ultraviolet light. Other major benefits of ultaviolet treatment include low equipment maintenance, service costs, and the ease of installation. Ultraviolet disinfection equipment can be applied to any new or existing water system and at a very economical price.
Ultraviolet Light And Its Germicidal Effects
Ultraviolet energy is germicidal, its effect is a result of the light's ability to penetrate the organisms cytoplasmic membrane (protective layer) and attack the DNA structure. The DNA is photochemically damaged, disabling its self-reproducing ability and rendering the cell lifeless. Ultraviolet light which is commonly referred to as UV is one region of energy on the elctromagnetic spectrum. This spectrum is the distribution of electromagnetic waves according to the frequency (cycles per second), or wavelength (meters, nanometers). UV is the spectrum located between visible light and x-rays. The UV spectrum ranges from 100-nm to 400-nm with the effective microorganism destruction range between 200-nm to 300-nm. The peak wavelength for germicidal action is at 265-nm. The ultraviolet lamp of choice for today's ultraviolet equipment design is the low pressure mercury vapor lamp. Approximately 89% of the relative spectral energy is produced at 254-nm, with the remaining energy peaking at 290-nm and with a low of 218-nm. There are two lamps available with differences being in the range of ultraviolet wavelengths that are produced. The 254-nm lamp produces the majority of its spectral energy at this wavelength. The 185-nm produces wavelengths in the germicidal range, yet also extends to the 185-nm range photochemically producing Hydoxyl free radicals which can be used for other processes such as TOC reduction. The effectiveness of the bacterial destruction is most pronounced at the 254-nm wavelength.
The effectiveness of the disinfection process is related in terms of ultraviolet system dosage. The total system dose is calculated by using three major parameters. First, the intensity of the ultraviolet lamp energy; second, the exposure time or flow rate through the contact chamber; and third, the ultraviolet transmission quality of the water. The total intensity the ultraviolet lamp emits is measured by microwatts at the specific ultraviolet wavelength. By combining the total lamp intensity and the time that the water is exposed to the light produces a measurement unit known as micro watt seconds per squared centimeter (uw-sec/cm2). It is important that when referring to dosage that it is at EOL (end of lamp life). The recommended minimum EOL dosage for ultraviolet equipment for use in the disinfection process of potable water is 16,000 uw-sec/cm2, as published by the Department Of Health, Education and Welfare. Today reputable ultraviolet manufacturers design equipment to produce a EOL dosage exceeding a minimum 30,000 uw-sec/cm2. Most bacteria, viruses and other microorganisms are readily destroyed with systems of this dosage design. Refer to chart #1 for specific requirements of destruction.
When sizing an application for the proper UV equipment, it is necessary to find out what the peak water flow rate will be. Since UV dosage is calculated with contact time being one of the factors, it is necessary that the unit does not flow more water that the equipment is rated for. There are charts available listing the average flow rate of fixtures for a household application. The other option is to see how long it takes to fill one gallon container and divide by 60-seconds. Next, determine possible combinations of multiple uses. For example, someone in the household gets a glass of water while the washing machine if filling and someone else is showering. Adding up these combinations of total water will give you the peak flow rate. It would also be advisable if the equipment selected has a flow restrictor to ensure the household does not exceed the equipment's rated capacity.
Ultraviolet Combination Systems
The application of a household water treatment system usually starts with a water quality concern from the homeowner. Once the consumer has become aware of a water quality situation, they want to produce the purest water possible with economic limits. If the quality problem is bacterial, applicable pretreatment equipment should be installed in front of the ultraviolet unit to improve the ultraviolet transmission characteristics of the water. Ultraviolet equipment should not be a stand alone system unless the overall water quality permits. It is necessary to reduce any Iron, Manganese, Sulfur, and turbidity to minimum levels. (Refer to Chart #2) These minimum requirements are neccessary as the transmission directly affects the dosage by regulating the intensity of the ultraviolet light as it passes through the water. If there is excessive turbidity beyond acceptable limits, it could possibly hide or shadow organisms that pass through the ultraviolet system, effecting overall system performance.
In applications where bacteria is not readily present, ultraviolet water disinfection equipment is applied to water filtration treatment systems initially designed for purposes other than disinfection. This application is designed to compliment the system and to make it complete with a disinfection safeguard. It is recommended that the ultraviolet equipment be installed as the last stage of treatment as a nesessary precaution, since there is the possibility of bacteria growing and breeding within other types of water treatment equipment. The ultraviolet equipment can be installed prior to the downstream distribution of piping to eliminate bacteria going downstream to the point of use through the house. The other option is to install a smaller ultraviolet unit at a single point of use, such as the kitchen sink for potable water. Ultraviolet disinfection equipment can be installed as a safeguard when bacteria is not presently detected at the time of the water test. Due to seasonal changes and the source of the water supply, it is possible for bacteria to be present only at a specific time of the year. The advantage to such an ultraviolet application is that the consumer will have the satisfaction of knowing they have a complete water system design to meet seasonal water quality fluctuations.
One of the most common water problems is hardness, which produces white-gray scale deposits in piping, water heaters, and whatever equipment the water comes in continual contact with. Water softeners are used to remove hardness (calcium and magnesium) and other minerals from the water supply. It is important this process be performed prior to ultraviolet influent levels. The water softener is applied as pretreatment to the ultraviolet equipment, see diagram #3 for typical water softener, filter and ultraviolet equipment combination. Without their removal, it is possible for the calcium and magnesium to plate out and coat the quartz sleeve. This build up on the quartz will limit the ultraviolet light's ability to transmit full intensity throughout the water. Where hardness is a problem, the use of a water softener will benefit the homeowner with higher quality water and will enhance the ultraviolet equipment's performance by allowing high ultraviolet light transmission, along with minimizing maintenance requirements for cleaning the quartz sleeve on a more frequent basis.
Point of use reverse osmosis (RO) equipment is capable of producing very high quality water at a slow rate. Because of the low water output, the water produced is stored in a holding tank. The combination of RO with ultraviolet equipment provides a complete system design where the equipment compliments each other while increasing performance. Ultraviolet equipment applied as pretreatment to a RO system assists in preventing premature bacteriological fouling and/or failure of the RO membrane. This pretreatment combination will result in a long term cost savings to the customer by extending the life of the RO membrane. Another application for ultraviolet is post treatment after the RO system prior to point of use for disinfection. Bacteria can breed in the RO membrane as well as grow through to contaminate the holding tank and potable water supply. The high quality water produced by the RO system will have excellent transmission characteristics and enhance the ultraviolet equipment performace. There are many designs of ultraviolet water disinfection equipment available. The optimum design to be combined with a RO system is the two pass ultraviolet equipment. One stream can be used for pretreatment to the RO system and the second stream after the RO and holding tank for point of use disinfection. (See diagram #4)
When filtration is primarily used to correct any water quality characteristics, the addition of an ultraviolet disinfection system is in order. Filters, as with other water treatment equipment, may become breeding grounds for microorganisms. The ultraviolet is applied to insure no organisms pass through the downstream piping. The use of a five micron sediment filter as pretreatment to an ultraviolet system is a minimum recommendation by the ultraviolet manufacturers. The use of such filtration is necessary to ensure proper performance of the ultraviolet equipment. Multiple filters may also be required depending on each particular case or water source. If any color is present in the water, the transmission quality will decrease, therefore lowering the dosage and system performance. In this situation, a carbon filter could be applied between the sediment filter and the ultraviolet equipment. Carbon filters will remove any residual chlorine which the homeowner may prefer. At this point the chlorine disinfection is lost and there is no disinfection downstream. Bacterial organisms are free to breed both in the carbon filter and downstream. Installing an ultraviolet disinfection unit after a carbon filter would insure proper disinfection to the point of use.
UV System Design, Features and Options
The diversity of models available and the quality ranges of equipment in today's marketplace may sometimes be confusing. Ultraviolet equipment for potable water applications vary from point of entry treating the entire household to point of use treating one despensing point for potable water. Ultraviolet system flow rates range from one gallon per minute to 20 gallons per minute. A variety of designs, models and options are available to meet performance design requirements; therefore, the ultraviolet equipment dealer must be aware of the water system application requirements specific to the point of installation. There is a wide range of designs, features, and options available that relate to system performance, maintenance and monitoring.
On potable water applications, the ultraviolet contact vessels are fabricated from 304 stainless steel. Plasma fusion welds with full penetration, purged with argon gas should be utilized on all wetted parts. This process ensures a food-grade quality, contaminant free weld consistent with the quality of steel used on the contact chamber. Some equipment manufacturers utilize PVC or plastic for the UV chamber. Ultraviolet light degrades plastics and will cause these chambers to weaken and crack over a period of time. This degradation may even cause taste problems in the water. Teflon is another material sometime used for the water chamber in close proximity to the lamp. Teflon absorbs UV energy and will limit the amount of energy transmitted into the water. Teflon will also degrade with time and end up in the water stream.
The lamps utilized shoud be of the low pressure type with a hard quartz envelope. This type of lamp has a rated life of approximately one year, this is known as end of lamp life. There is equipment available which utilizes low output "soft glass" lamps, which the lamp manufacturers only rate the lamp life at 3,000 hours EOL. The lamp life is not how long the lamp will burn for, but rather how fast the glass will solarize. Solarization is the reaction of the UV light with the quartz or glass envelope lowering the actual UV output with time.
A quartz sleeve should be incorporated in the design of the equipment to isolate the lamp from the water. This isolation insures that the UV lamp can be operated at its optimum temperature of 105 degrees. Some designs put the lamp in direct contact with the water causing the lamp to operate at a lower temperature. This design could have as little as 25% the output of the same lamp at its proper operating temperature. It is also important that the quartz sleeve be of a high grade material to allow proper transmission of the UV energy into the water.
Monitoring devices range from indicating that the lamp is on to monitoring UV intensity insuring proper dosage and disinfection. Many units have a LED or viewport which shows that the lamp is operating. This requires the homeowner to periodically check the unit to visually insure the unit is up and running. Some models have audible alarms indicating lamp failure. These types of systems are adequate on high quality water systems providing the quartz sleeve is clean then a narrowband photo-discrete 254-nm sensor with an intensity monitor would be an appropriate option. The monitor will sound an alarm and trigger a solenoid valve in the event of inadequate disinfection. The failure could be the result of a surge of poor incoming water, fouled quartz sleeve or lamp failure.
Manual Quartz Sleeve Wiper Systems
Models are available with manual quartz sleeve wiper systems which are designed to clean the quartz sleeve without disassembly of the unit. If used properly this type of system could be useful and functional. The drawback to such a cleaning method is if the buildup is let go too long the wiper will smear or scratch the deposits. The only way to perform an assured cleaning is by removal of the quartz sleeve.
Fail Safe Systems
Applications for public water systems often require compliance with federal and or state guidelines which require a fail-safe system, attached please find a copy of the federal guidelines. The major compnents to be incorporated in the system would include a narrowband UV meter with audible alarm, manual quartz sleeve wiper system, normally closed solenoid valve, and a flow control valve.
The equipment should be designed for ease of service. Minimal time should be required to remove the quartz sleeve and lamp. Maintenance usually is limited to an annual changeout of the lamp and cleaning of the quartz sleeve. Depending on the water quality and pretreatment equipment, the cleaning of the quartz sleeve may be more frequent. Upon installation and whenever the system is serviced all downstream piping must be santized prior to start-up.
There are many variables which can affect the choice of ultraviolet equipment and sizing criteria. Flow rate, water quality, space requirements, and water test results are some of the factors to be considered. The quality of models and component designs for the potable market may sometimes be confusing. The answer to the question you have is just a phone call away. We provide technical assitance for application sizing, equipment specifications, and free water analysis for ultraviolet transmission testing.