DISINFECTION BY ULTRAVIOLET - GENERAL INFORMATION

What is UV light?

For the past 100 years science has recognised the bactericidal effects of the ultraviolet area of the electromagnetic spectrum. Ultraviolet light is the section of the electromagnetic spectrum with a wavelength shorter than that of visible light and is usually divided into UVA, UVB and UVC.

UVC and UVB induces changes in the nucleic acids of DNA (RNA in the case of viruses) altering the molecules to a more reactive state. DNA bases are prone to absorption of UV light at around 260nm and the major biocidal role of UV light is disruption of DNA.

The major effect of UV absorption is the dimerisation of pyrimidine. The dimer produces a kink in the DNA preventing replication and protein synthesis. If the damage to the DNA is sufficient, that is the UV dose is sufficiently high, the repair mechanisms will be overwhelmed and the organism cannot replicate.

The effectiveness of the UV wavelengths is dependent on the DNA absorption curve. This peaks at around 260 – 265nm which are the most efficient wavelengths for disinfection. However, as DNA absorbs at all wavelengths between 200nm and 315nm any light emitted over these wavelengths will have a level of germicidal effect.

Relationship between uv energy & population reduction

The disinfection effect is based on the quanta of UV energy applied. This is usually referred to as the dose and is calculated as .

Dose = Intensity x Time

The units of UV dose are: 1mJ cm-2 = 1000μWs cm-2

In addition the effect of the dose on the organism is dependent on the species itself. Simple bacteria (E.coli) require relatively small doses of energy and other organisms (Hepatitis A) can require higher levels. Some doses for the 90% destruction of common organisms are shown in Fig.1:

Fig 1: Dose Requirements for 90% or 1log Reduction of Common Micro-organisms

Raising or lowering the dose for a specific organism has a logarithmic effect such that doubling the dose for a 90% kill will give a 99% kill. The survival rate for a given organism is therefore a function of the initial numbers of organisms and the applied dose.

Generating uv light

UV light is conventionally generated in mercury vapour discharge lamps. There are different types of discharge lamps all with different characteristics, see Fig 2.
Low pressure lamps emit UV energy at 254,7nm and the medium pressure lamp produces a continuum of energy in the bactericidal region 200 to 315nm.

Due to the high level of UV output and its lack of sensitivity to water temperature, medium pressure lamps are usually utilised in all but the smaller flow applications. A large number of low pressure lamps, as would be required to treat flows above 50-100m3/hr, calls for excessive maintenance. Monitoring is also less precise for a plurality of lamps. The UV output of a low-pressure lamp is sensitive to the temperature of the lamp, which in turn is dictated by the temperature of the fluid being treated. For this reason care must be excercised when specifying a UV unit to treat water below 5°C or above 40°C.

The efficiency of conversion of electricity into UVC is higher with low pressure lamps, about x2 to x2,5 the efficiency of a medium pressure lamp. High output low pressure lamps using amalgams are expensive but have double the power output of conventional low pressure lamps. They are less sensitive to fluid temperatures and can be used on water of 60°C.

Fig 2: UV Lamp Types

The effectiveness of UV treatment is dependant on the transmission of UV light through the air or water to be treated.  Typical transmission rates of various substances are shown in Fig 3.

Fig 3: Typical Transmission Values for a 10mm Path Length

APPLICATION OF UV LIGHT

The disinfection process involves the correct exposure of the fluid to be disinfected to a UV source.  The source must be powerful enough to overcome the natural absorbance of the fluid. We believe that correct chamber design is a significant part of the sizing formula and our manufacturer has employed extensive computer modelling (CFD) to establish the flow pattern through treatment chambers.

The most conservative calculation of UV dose is measured at the chamber wall (i.e. furthest from the lamp), which protects the process from possible inadequate treatment which may occur through short circuiting.  Other methods of calculation are based on average intensities or cumulative  dose.

Multi-lamp designs utilising numbers of low pressure lamps housed in one chamber, can present both hydraulic and mechanical problems. Flow patterns are random and the chamber often requires a baffle system to attempt to improve chamber hydraulics.

UV INTENSITY MONITORING

The efficiency of the UV lamp is measured at the chamber wall, through a dry seal, by a UV monitor. This responds only to bactericidal UVC and can be calibrated to meet stringent QA and validation procedures of certain industries.  The monitor performance can be validated on-line using a portable UV calibration instrument.
                                               
Multi-lamp units have a single UV monitor studying a single lamp. Often this lamp is degrading differently from the others which gives a false overall reading.

SYSTEM SIZING

To ensure customers are supplied with the correct unit for the intended application, we recommend an applications engineer spends time with the client studying the intended use. The following information is required for a successful conclusion:

1. Peak flow  ( litres per second  or gallons per minute )
2. 50 ml fluid sample for UV suitability testing (where water is non potable)
3. Micro-biological challenge to be overcome
4. Required micro-biological standard required after UV treatment
5. Sketch of water distribution ( this is not absolutely necessary but will help our engineer decide the best place for the installation) and the proposed process control.

APPLICATIONS

Disinfection

LIQUIDS
- Water, Swimming Pools, Sucrose Syrups, Emulsions, Brines, Final Effluent.
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, Swimming Pools

OUR MANUFACTURER’S CREDENTIALS

With over two decades of experience in UV Technology, they are an innovator who has supplied many thousand units to the industrial market.  Their products and procedures are certified to the highest standards, and designed for performance and reliability.