Ultra Violet Disinfection

Ultraviolet Sterilization Technology

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 80 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.

What is UV and how does it work?

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 nanometres (nm) to 390 nanometres (nm). Optimum UV germicidal action occurs at 260 nm.

Since natural germicidal UV from the sun is screened out 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.

Design and Dosage

The design of an ultraviolet sterilizer has an extremely important bearing on how the UV Dose is delivered and 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 be 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 micro watt seconds per centimeter squared (uWsec/cm²).

Divide by 1000 to express the dose in mJ/cm², the preferred notation.

For maximum UV transmission a “hard glass” quartz sleeve is recommended for two main reasons. It isolates the lamp from the water to offer more uniform operating temperatures and allows for higher UV output into the water.

A variety of optional features may be added on to the UV sterilizers. They include UV monitoring devices that measure the actual UV output, solenoid shut-off devices that will stop the water flow in the event of a system failure, flow control devices to properly limit the water flow in the units, audible and visual alarms (both local and remote) to warn of lamp failures, high temperature sensors to monitor excessive temperatures in the reactor chamber or control panel, and hour meters to monitor the running time of the UV lamps.

Following are the advantages of UV sterilization:

• Environmentally friendly, no dangerous chemicals to handle or store, no problem of overdosing (it’s impossible).
• 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.
• Compatible with all other water processes (i.e., RO, filtration, ion exchange, etc.)

Factors Affecting UV

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).