Disinfection by Ultraviolet Light

Contents:

- Effect of Ultraviolet
- UV-C Production
- Ultraviolet Dose
- Dose/Destruction Relationship
- Applications
- Medium and High Pressure Arc Tubes
- Low Pressure Lamps
- Irradiation Chambers
- Single Arc Tube Configuration
- UV Intensity Monitoring
- System Sizing

For the past 100 years science has recognized the bactericide effects of the ultraviolet area of the electromagnetic spectrum.

The specific wavelengths responsible for this reaction are situated between 240 - 280 nanometers (referred to as nm) with a peak wavelength at 265 nm. They are known as UV-C.

Fig. 1 - UV-C in the spectrum of electromagnetic radiation.

Fig. 2 - spectral energy distribution curve for germicidal action and spectral power distribution for low and medium pressure UV lamps.

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EFFECT OF ULTRAVIOLET

When a micro-organism is exposed to UV-C, the nuclei of the cells are modified, due to photolytic processes. In result, cell division and, by extension, reproduction is prevented.

UV-C PRODUCTION

The Ultra Violet source is basically a fused silica quartz tube, typically l5mm to 25mm diameter ranging from 100mm-1200mm long. The inert gas with which the tube is filled, provides the primary discharge and the necessary action to excite and vaporize the miniscule deposits of mercury within.

The low pressure UV lamp is only capable of producing lines at 185nm and 254 nm. An increase in the current supplied would cause the UV lamp to rapidly heat up thus increasing the mercury pressure to produce the typical medium pressure spectral output shown in Diagram 2.

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ULTRAVIOLET DOSE

The UV dose is the product of UV intensity (expressed as energy per unit surface area) and residence time.

Therefore: DOSE = I x T

This is commonly expressed as 1mJ/cm2=1000 micro Watt second/cm2

The minimum dose expressed by Willand gives the user the guaranteed assurance of success. Average and cumulative doses offered by others depend on turbulent flow characteristics which can disappear when flow is variable.

Willand recommend the appropriate UV dose for each application taking into account water quality, arc tube ageing, industry specifications, as well as microbiological standards.

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DOSE / DESTRUCTION RELATIONSHIP

The relationship between the dose and the destruction achieved of a target micro-organism can be summarized as follows:

N/N_o = e^-KD

Where:
N = Initial Number of target organisms
No = Number of target organisms after treatment
K = Constant associated with target organisms
D = Dose

From the above relationship doubling of the dose applied will increase the destruction by a factor of 10. Therefore doubling the dose required for 90% destruction will produce 99% destruction of the target organism. tripling the dose will produce a 99.9% destruction of the target organism and so on.

Some 90% destruction values are shown in Fig. 3 and the relationship between UV dose and destruction are shown in Fig. 4

Fig. 3 - DOSE REQUIREMENTS - COMMON MICRO-ORGANISMS

Fig. 4 - E.coli (Waterborne indicator Pathogen) DOSE = 5.4 mJ/cm2

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APPLICATIONS

DISINFECTION

• LIQUIDS: water, syrups, emulsions, brines.
• SURFACES: packaging, conveyors, food, working surfaces.
• GASES/AIR: food preparation, clean rooms, air conditioning.

PHOTOCHEMICAL REACTIONS

• OXIDATION: TOC reduction, ozone destruction, chlorine removal.
• CATALYSIS: pesticide removal, effluent treatment, ground Recovery.
• DEODORISATION: sewage and industrial emissions.

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MEDIUM AND HIGH PRESSURE ARC TUBES

Power ratings are from 0.4kW to 7.0kW with a maximum treatment capacity of 600 m3/hour with a single lamp.

The high energy output is equally effective on both hot and cold fluids.

The broad spectrum output performs more efficiently than low pressure lamps on flows > 13 m3/hour. Conversion of power input to biocidal output is > 15%.

Useful arc tube life between 4000 - 8000 hours depending on operating conditions.

Full spectrum output 185 - 480 nm available for photochemical reactions.

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LOW PRESSURE LAMPS

Ideal for low flow situations with power ratings from 15w to 200w.

Single wavelength output at 254 nm.

Conversion to UV-C typically 30% - 35%.

120 - 200 watt lamps unaffected by water temperatures.

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IRRADIATION CHAMBERS

The disinfection process involves the exposure of fluids with microbiological contamination to a UV energy source which is mounted centrally in an irradiation chamber.

Lenntech have always believed that correct chamber design is a significant part of effective disinfection and to this end computer modeling is used to establish turbulent flow, which ensures good mixing and balanced exposure at high and low flows and residence time characteristics.

Lenntech UV Systems design equipments so that the dose is AT THE WALL, AT THE END OF LAMP LIFE. This protects the process from possible inadequate treatment which may occur through short circuiting when average and cumulative doses are is used.

High quality internal finishing avoids shadowing and other bacterial traps.

Chambers have integrally fabricated sample ports, drain and air vents as standard.

The inlet and outlet orientation, size and end termination are to customers specification to aid installation.

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SINGLE ARC TUBE CONFIGURATION

Single arc tube configuration greatly enhances performance. One high intensity lamp is capable of disinfecting up to 600m3/hour. UV intensity monitoring is positive, simple, effective and proven.

Multi-tube designs utilizing quantities of low pressure lamps housed in one chamber present both hydraulic and mechanical problems. Maintenance is time consuming and expensive. Baffles are required to introduce turbulence and through the shadowing effect of these it is possible for untreated water to pass through the chamber shielded from the monitor.

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UV INTENSITY MONITORING

Willand intensity monitors respond to UV-C. The monitor is the customer's safeguard the unit is operating at efficient output. When low UV-C output threshold is reached an alarm is initiated. Monitor output can be linked to BEM or PLC units to ensure optimum plant operation.

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SYSTEM SIZING

To ensure customers are supplied with the correct unit for the intended application we recommend they contact Lenntech UV to arrange an appointment with a trained Lenntech consultant.

Information necessary to correctly size a Lenntech unit includes, but is not limited to:

1. Peak flow (liters per second or gallons per minute). 2. Fluid sample for free transmission test.

3. Microbiological challenge.

4. Required standard after UV treatment.

All information supplied is in the strictest confidence.