Ozone & Ultraviolet Disinfection Systems: Keeping pure water pure...
Get out, and stay out!
Once the contaminants in a feedwater stream are purified to the desired level, the last thing that is needed is an invading hoarde of biological pests. After the water treatment technologies, like reverse osmosis (RO) have done their jobs, the water that is not immediately used is vulnerable to attack. Ozonation and Ultraviolet (UV) sterilization systems are designed to make sure purified water remains free from biological contaminants.
Ozonation Systems are often used to:
q Keep purified water free from biological contaminats
q To kill biiological contaminants that populate a non-purified water source
Ozonation is very effective for destroying cryptosporidium and also reduce the formation of trihalomethanes (THM's) which result from the interaction of chlorine and naturally-occurring organic material in feed water.
Ozone is produced by subjecting oxygen molecules to high voltages. Chemically, ozone is O3 versus the O2 we breathe. In the environment, ozone is created by the Sun's ultraviolet rays and also by lightning, which produces the particular "fresh scent" after a thunder storm. O3 posesses two stable oxygen atoms and one unstable oxygen atom, which makes it reactive with other substances. In the case of bacterium, ozone that comes in contact with the cell wall of the bacteria manifests an oxidative burst whic creates holes in the cell wall. In a matter of seconds, the bacteria can no longer maintain its shape and dies.
Ozone is produced and injected into a recirulating water stream which ensures that the water supply remains free from harmful levels of bacteria.
Advantages of Ozonation include:
q Strongest oxidizer and disinfectant commercially available
q Decomposes into harmless oxygen
q Ozone reacts with a large variety of organic compounds
q Generated and used at point of use
q Kills bacteria over 3,000 times faster than chlorine
Ultraviolet (UV) Sterilization is often used to:
q Maintain RO product water, or permeate, water in a Germ-Free state for subsequent downstream processes
q Insure that any microorganisms that find their way into product water are prevented from multiplying
Ultraviolet Light, or UV light is an energy band within the electromagnetic spectrum. Its wavelength is between that of X-rays and visible light. At the 254nm (254 millionths of a meter) wavelength, UV light reaches its near peak germicidal effectiveness against most microorganisms.
UV light destroys germs by causing a molecular change in their DNA that prevents germs from multiplying, and therefore, destroys the ability to spread disease. When germs cannot multiply, they are considered dead.
UV Systems are sized according to application requirements.
Advantages of UV lights include:
q Environmentally friendly, no dangerous chemicals to handle or store, no problems of overdosing.
q Low initial capital cost as well as reduced operating expenses when compared with similar technologies such as ozone, chlorine, etc.
q Immediate treatment process works, no need for holding tanks, long retention times, etc.
q Extremely economical, hundreds of gallons may be treated for each penny of operating cost.
q No chemicals added to the water supply - no by-products (i.e. chlorine + organics = trihalomethanes).
q No change in taste, odor, pH, or conductivity or the general chemistry of the water.
q Automatic operation without special attention or measurement, operator friendly.
q Simplicity and ease of maintenance, periodic cleaning (if applicable) and annual lamp replacement, no moving parts to wear out.
q No handling of toxic chemicals, no need for specialized storage requirements, no WHMIS requirements.
q Easy installation, only two water connections and a power connection.
q More effective against viruses than chlorine.
q Compatible with all other water processes (i.e. RO, filtration, ion exchange, etc.)
The UV system, mounted on the wall (above right) is used to maintain a bacteria-free water supply in this clean-room environment. Here, stored RO permeate is continuously recycled through the UV system to prevent biological growth.
Factors Affecting UV:
The effectiveness of a UV system in eliminating microbiological contamination is directly dependant on the physical qualities of the influent water supply.
Suspended Solids or Particulate Matter cause a shielding problem in which a microbe may pass through the sterilizer without actually having any direct UV penetration. This shielding can be reduced by the correct mechanical filtration of at least five microns in size.
Iron / Manganese will cause staining on the lamp or quartz sleeve at levels as low as 0.3 ppm of iron and 0.05 ppm of manganese. Proper pretreatment is required to eliminate this staining problem.
Calcium / Magnesium Hardness will allow scale formation on the lamp or quartz sleeve. This problem will be especially magnified during low flow (or no flow) times when the calcium and magnesium ions tie-up with carbonates and sulfates to form hard scale build-up inside the sterilizer chamber and be deposited on the lamp or sleeve.
Other Absorbing Compounds such as humic and fumic acids as well as tannins will reduce the amount of UV energy available to penetrate through the water to affect the genetic material, DNA of the molecule.
Additional Factors Affecting UV:
Temperature is a determining factor. The optimal operating temperature of the UV lamp must be near 40°C (104°F). UV levels will fluctuate with excessively high or low temperature levels. A quartz sleeve is typically employed to buffer direct lamp - water contact thereby reducing any temperature fluctuations. A typical method employed in a system without a quartz sleeve is to engineer the system to take into account these fluctuations and typically de-rate the regular flow rate by the corresponding amount.
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