What is UV Disinfection?
UV light, which continues to be a reliable means of disinfection, involves exposing contaminated water to radiation from UV light. The treatment works because UV light penetrates an organism’s cell walls and disrupts the cell’s genetic material and making reproduction impossible. A special lamp generates the radiation that creates UV light by striking an electric arc through low-pressure mercury vapor. This lamp emits a broad spectrum of radiation with intense peaks at UV wavelengths of 253.7 nanometers (nm) and a lesset peak at 184.9 nm. Research has shown that the optimum UV wavelength range to destroy bacteria is between 250 nm and 270 nm. At shorter wavelengths (e.g. 185 nm), UV light is powerful enough to produce ozone, hydroxyl, and other free radicals that destroy bacteria.
Advantages:
Generally, UV is simple to install and requires little supervision, maintenance, or space. Improved safety, minimum service time, low operation and maintenance costs, and the absence of a chemical smell or taste in finished water are primary factors for selecting UV technology rather than traditional disinfection technologies.
UV treatment breaks down or removes some organic contaminants. UV achieves 1-log reduction of Giardia lamblia at an intensity of 80-120 mWs/cm2, and 4-log reduction of viruses at an intensity of 90-140 mWs/cm2. Only recently has the scientific community begun to accept UV as a highly effective tool for Cryptosporidium control.
UV light disinfection does not form any significant disinfection byproducts, nor does it cause any significant increase in assimilable organic carbon (AOC).
Research has confirmed that UV effectiveness is relatively insensitive to temperature and pH differences. In addition, researchers found that UV application does not convert nitrates to nitrites, or bromide to bromines or bromates.
Recent pilot studies show that UV-treated drinking water inhibits bacterial growth and replication in the distribution system; however, conditions within distribution systems, such as leaks, still require additional residual disinfection (e.g., free chlorine).
The advantages of using UV, rather than chemical disinfection, include:
– Has no known toxic or significant nontoxic byproducts
– Has no danger of overdosing
– Removes some organic contaminants
– Has no volatile organic compound (VOC) emissions or toxic air emissions
– Has no onsite smell and no smell in the final water product
– Requires very little contact time (seconds versus minutes for chemical disinfection)
– Does not require storage of hazardous material
– Requires minimal space for equipment and contact chamber
– Improves the taste of water because of some organic contaminants and nuisance microorganisms are destroyed, Does not affect minerals in water
– His little or no impact on the environment except for disposing of used lamps or obsolete equipment
Limitations:
Microbial and chemical characteristics are two major water quality factors that affect the UV unit performance. Microbial characteristics of water include type, source, age, and density. Chemical water characteristics include nitrites, sulfites, iron, hardness, and aromatic organic levels.
UV radiation is not suitable for water with high levels of suspended solids, turbidity, color, or soluble organic matter. These materials can react with UV radiation, and reduce disinfection performance. Turbidity makes it difficult for radiation to penetrate water.
Disadvantages of UV disinfection include:
• No disinfection residual
• No technical database exists on how well UV systems perform for various water quality conditions
• No standardized mechanism measures, calibrates, or certifies how well equipment works before or after installation
Systems also should consider using different kinds of microbial testing. Laboratories typically test for total coliform to judge microbiological
activity in drinking water—but coliforms are sensitive to UV light. Because of this sensitivity, microbial tests for UV treated finished water
should include a Heterotrophic Plate Count (HPC) test. HPC microorganisms may provide a better disinfection assessment than the UV sensitive coliforms.
Process Description UV light effectively destroys bacteria and viruses.
However, how well the UV system works depends on the energy dose that the organism absorbs.
If the energy dose is not high enough, the organism’s genetic material may only be damaged rather than disrupted.
An effective dose is measured as a product of the lamp’s intensity (the rate at which photons are delivered to the target), including radiation concentration, proper wavelength, exposure time, water quality, flow rate, and the microorganism’s type, and source, as well as its distance from the source light.
Typical UV light components include:
• A stable high-voltage source of electricity because low-line voltage would result in a lower UV dose
• A chamber made of stainless steel or any other material that is opaque and will not corrode
• UV lamps that are properly secured inside quartz sleeves, easing installation replacement, and maintenance
• Quartz sleeves with sufficiently high transmission rates to deliver the UV energy produced by UV lamps
• Mechanical wipers to maintain optimum transmission between scheduled cleaning and maintenance work
• Sensors to monitor the UV intensity passing through the water. These sensors need to be connected to alarm systems to alert the operator in case of low UV intensity. The operator must have easy access to these sensors for necessary installation, replacement, calibration, and maintenance
• Safety control to shut off UV lamps in case of low-flow levels and elevated lamp temperature
• Arc and lamp-out monitors to alert the operator of system failure
• Electronic ballasts
UV units are currently used as stand-alone treatment systems or as part of a series of other drinking water treatment processes or multiple barrier system. A common treatment that uses UV light to remove and disinfect contaminants from groundwater sources involves a combined ozone or hydrogen-peroxide process along with UV application. So, it is common to find that manufacturers of UV equipment also manufacture ozone equipment.
Furthermore, the drinking water treatment industry provides UV equipment (mainly closed chamber units) for short-term uses. Rental units are used in cleanup and emergency situations, such as if groundwater is contaminated by spilled toxic organic compounds.
Monitoring and Operation Requirements :
Factors that mainly affect UV light system performance are:
UV lamp output, aging of UV lamp, and fouling of unit surfaces. To better control these factors, operators must ensure continuous dose measurement (i.e., accurate intensity and flow-rate measurement) and proper maintenance (cleaning as well as maintain lamp and sleeve replacement regimes).
A typical high Flow UV installation is shown below.
Applications of UV Disinfection:
– Effluent Treatment
– Swimming Pool water disinfection
– Domestic water disinfection
– Pharmaceutical applications
– Sugar Syrup treatment
– Ozone destruction
