What is UV disinfection and how does it work?
Ultraviolet disinfection systems are mysterious to many people — how can “light” kill bacteria? But the truth is it can. Ultraviolet (UV) technology has been around for 50 years, and its effectiveness has been well documented both scientifically and commercially. It is nature’s own disinfection/purification method. With consumers becoming more concerned about chlorine and other chemical contamination of drinking water, more dealers are prescribing the ultraviolet solution suitable for both small flow residential applications as well as large flow commercial projects.
Ultraviolet is a means of killing or rendering harmless microorganisms in a dedicated environment. These microorganisms can range from bacteria and viruses to algae and protozoa. UV disinfections are used in air and water purification, sewage treatment protection of food and beverages, and many other disinfection and sterilization applications. A major advantage of UV treatment is that it is capable of disinfecting water faster than chlorine without cumbersome retention tanks and harmful chemicals. UV treatment systems are also extremely cost efficient!
What technology is used exactly?
Ultraviolet is one energy region of the electromagnetic spectrum, which lies between the x-ray region and the visible region. UV itself lies in the ranges of 200 nanometers (nm) to 390 nanometers (nm). Optimum UV germicidal action occurs at 260 nm.
Since natural germicidal UV from the sun is screened out by the earth’s atmosphere, we must look to alternative means of producing UV light. This is accomplished through the conversion of electrical energy in a low-pressure mercury vapor “hard glass” quartz lamp. Electrons flow through the ionized mercury vapor between the electrodes of the lamp, which then creates UV light.
As UV light penetrates through the cell wall and cytoplasmic membrane. It causes a molecular rearrangement of the microorganism’s DNA, which prevents it from reproducing. If the cell cannot reproduce, it is considered dead.
How is it designed and what is a sufficient dose?
The design of an ultraviolet sterilizer has an extremely important bearing on how the UV Dose is delivered. The dosage is the most critical function of UV disinfections. As individual UV lamps emit a set amount of ultraviolet energy, it is important that a system is sized correctly. Flow rates are the determining factor and must not be overstated.
Contact time, which is the time the water is within the sterilization chamber, is directly proportional to Dosage, which is the amount of energy per unit area (calculated by dividing the output in watts by the surface area of the lamp), and thus the overall effectiveness of microbial destruction in the system. This product of intensity and time is known as the Dose and is expressed in microwatt seconds per centimeter squared (uWsec/cm²).
What are the advantages of UV technology?
Following are the advantages of UV sterilization:
• Environmentally friendly, no dangerous chemicals to handle or store, no problem of overdosing.
• Low initial capital cost as well as reduced operating expenses when
• compared with similar technologies such as ozone, chlorine, etc.
• Immediate treatment process, no need for holding tanks, long retention times, etc.
• Extremely economical, hundreds of gallons may be treated for each penny of operating cost.
• No chemicals added to the water supply – no by-products (i.e. chlorine + organics = trihalomethanes).
• No change in taste, odor, pH or conductivity or the general chemistry of the water.
• Automatic operation without special attention or measurement, operator friendly.
• Simplicity and ease of maintenance, periodic cleaning (if applicable) and annual lamp replacement.
• No moving parts to wear out.
• No handling of toxic chemicals, no need for specialized storage requirements, no WHMIS requirements.
• Easy installation, only two water connections and a power connection.
What are the factors affecting UV disinfection?
Because UV does not leave any measurable residual in the water it is recommended that the UV sterilizer be installed as the final step of treatment and located as close as possible to the final distribution system. Once the quality of your water source has been determined, you will need to look at things that will inhibit the UV from functioning properly (e.g., iron manganese, TDS, turbidity, and suspended solids).
Iron and Manganese will cause staining on the quartz sleeve and prevent the UV energy from transmitting into the water at levels as low as 0.03 ppm of iron and 0.05 ppm of manganese. Proper pretreatment is required to eliminate this staining problem.
Total Dissolved Solids (TDS) should not exceed approximately 500 ppm. There are many factors that make up this equation such as the particular makeup of the dissolved solids and how fast they absorb the sleeve, again impeding the UV energy from penetrating the water.
Turbidity is the inability of light to travel through water. Turbidity makes water cloudy and aesthetically unpleasant. In the case of UV, levels over 1 NTU can shield microorganisms from the UV energy, making the process ineffective.
Suspended Solids need to be reduced to a maximum of 5 microns in size. Larger solids have the potential of harboring or encompassing the microorganisms and preventing the necessary UV exposure. Pre-filtration is a must on all UV applications to effectively destroy microorganisms to a 99.9% kill rate.
Additional Factors affecting UV is temperature. The optimal operating temperature of a UV lamp must be near 40°C (104°F). UV levels fluctuate with temperature levels. Typically a quartz sleeve is installed to buffer direct lamp-water contact thereby reducing any temperature fluctuations.
What is the common use of UV technology?
One of the most common uses of ultraviolet sterilization is the disinfection of domestic or rural water supplies due to contaminated wells and ground water aquifers. Coupled with appropriate pre-treatment equipment, UV provides an extremely simple, economical, efficient and user-friendly means of producing potable water.