Disinfection by UV Radiation
This is designed to be a short guide to Ultraviolet Irradiation (UV) used in the treatment of drinking water for those responsible for monitoring private water supplies and the owners and users of such
supplies. The main text is aimed primarily at technical practitioners involved in the management and regulation of water supplies and covers basic principles, design, operation and maintenance. It also
includes a checklist for maintenance and a Frequently Asked Questions section, which can stand alone and may be more appropriate for the non-technical user .
Ultraviolet irradiation (UV) uses ultraviolet light in order to deactivate a wide range of pathogens found in drinking water supplies. Unlike chlorine, UV can also have the advantage of being effective against protozoa, including Cryptosporidium. The process relies on UV light being able to pass through the whole column of water requiring treatment with sufficient energy to inactivate the
pathogens. Effectiveness is significantly impaired if water is coloured or contains particulate material. It is often the disinfectant treatment of choice for private water supplies, especially small
ones. UV treatment can be located centrally at the source of the supply or at individual properties immediately before the water is consumed, or both. Unlike chlorine, UV does not have any residual
disinfectant effect in the water.
UV light will also photo-oxidise some organic compounds, breaking them down into smaller molecules. This has some application in the treatment of compounds causing tastes and odours, pesticides and algal toxins. The shorter wavelengths of UV, around 185 nm, tend to be more effective in this application.
Strengths of UV
- Cheap and effective disinfection
- Chemical free
- Relatively simple to install, operate and maintain
- Inactivates Cryptosporidium
- Compact footprint
- Minimal concerns over by-products
Weaknesses of UV
- No lasting disinfectant residual
- Cannot operate without power
- Requires water to have low levels of colour and turbidity
- Is ineffective if the dose and contact time are not correct
- Hard to verify water has been adequately disinfected
Principles of UV Disinfection
UV radiation inactivates microorganisms by penetrating cell walls and disrupting vital cell functions. If sufficient energy reaches the cell it results in the death or impairment of that cell, and consequently the organism itself. The most effective wavelength of UV radiation for damaging cell DNA is 254nm, although in practice wavelengths between approximately 200 – 300 nm are generally
considered biocidal. As with chlorine disinfection, some organisms are more susceptible than others. Bacteria are more susceptible than larger protozoan parasites such as Cryptosporidium and
Giardia, although with a sufficient dose and contact time UV can be considered effective at deactivating these. There is less agreement on vulnerability of viruses, although UV undoubtedly has some effect. Sometimes UV is used in combination with other treatments such as ozone or hydrogen peroxide.
Generation of UV Radiation
Special lamps are used to generate UV radiation, and these are enclosed in a reaction chamber made of stainless steel or, less commonly, plastics. Low pressure mercury lamps, which generate 85% of their energy at a wavelength of 254 nm, are most commonly used; their wavelength is in the optimum germicidal range of 250 to 265 nm. These lamps are similar in design, construction and operation to fluorescent light tubes except that they are constructed of UV-transparent quartz instead of phosphor-coated glass. The optimum operating temperature of the lamp is around 40 Degrees Celsius so the lamp is normally separated from the water by a ‘sleeve’ to prevent cooling by the water. The intensity of UV radiation emitted decreases with lamp age; typical lamp life is about 10 to 12 months after which the output is about 70% of that of a new lamp, and lamp replacement is required.
The usual UV reactor configuration is a quartz-sleeved low pressure mercury lamp in direct contact with the water; water enters the unit and flows along the annular space between the quartz sleeve
and the wall of the chamber. Other configurations include lamps separated from the water, for example lamps surrounded by ‘bundles’ of PTFE tubes through which the water flows.
Several new treatment technologies have been developed for inactivation of Cryptosporidium. These include pulsed UV or white light systems and combined filtration-irradiation or adsorption irradiation techniques that increase exposure to UV, for example by trapping the micro-organisms on a filter then subjecting them to UV irradiation. Pulsed UV and pulsed white light devices that
generate high intensity, short duration, pulses of radiation are reported to give more effective inactivation of oocysts than conventional UV systems.
Application and Design of Systems
Design and Installation
Careful consideration needs to be given to the design a UV system, and this should be done in the context of the whole water supply system, from source to tap. The production of a draft Drinking
Water Safety Plan, or management plan, for the system may be helpful at this stage to highlight actual or potential risks and to try to minimise them through good design.
One of the key things to remember about UV is that there is no lasting residual disinfectant effect imparted to the water, unlike chlorine disinfection where a residual concentration remains.
Consequently, there remains the risk that UV disinfected water may become re-contaminated post-treatment before it is supplied to the user. Ingress of contaminants is a particular risk if the integrity
of any tanks and pipework is not satisfactory. Additionally, long retention times in storage tanks or long lengths of pipework in buildings can lead to biological activity and present a risk. This is
particularly the case where water is allowed to heat up, perhaps due to pipes running in close proximity to hot water pipes. One option is to install UV treatment at the point of use, as close to
the tap as possible. On a larger supply, consideration could be given to installing UV treatment at a central point on the whole supply to minimise microbiological growth in the system, with further
small UV units to ensure continued disinfection at the point of use.
Although relatively simple to operate, UV disinfection will only be effective if certain constraints are met. The ideal water for use with UV has minimal dissolved substances, is free from turbidity an
suspended solids and low in organic compounds and colour. Few raw water supplies, certainly in the North of the UK, meet these criteria without pre-treatment and such treatment should be considered a pre-requisite otherwise UV disinfection will not be consistently effective.
The exact pre-treatment required will depend on the quality of incoming water and the specific requirements of the UV system in use. Cartridge filters are the most common and, probably, the
simplest treatment solution for preparing water for UV disinfection. It is common for a sequence of filters, decreasing in pore size, to be used in series so that the first filter removes coarse material and
each successive filter removes increasingly fine particles. The use of too fine a filter can result in filters blocking or needing to be changed at an impractically high frequency. Filter pore sizes of 20
micron, 10 micron and 5 micron are common, but specialist advice should be sought. When choosing filters it should be remembered that incoming water can vary significantly, especially after heavy
Where organic and coloured compounds are a problem, ordinary cartridge filters are unlikely to help and filters containing activated carbon or another adsorbant substance may be required. Where
colour is particularly high, even these may struggle to be effective and more complex treatment may be needed, on specialist advice.
Selection of UV System
The choice of UV system will depend on the circumstances of each individual supply, and professional advice should be sought. Any product intended for use should have been proven to be
effective via an accredited validation method. This is often undertaken by the manufacturer.
Common validation schemes are:
Austrian : ÖNORM M 5873
Part one of this standard covers low pressure mercury lamp units. Part two covers medium pressure.
German: DVGW Standard W294
Products are tested at the DVGW’s testing facility in Germany and issued with a certificate if they meet the standard.
American: NSF. 2004. NSF/ANSI 55 Ultraviolet Water Treatment Systems
This standard has two classes: Class A covers equipment capable of providing full disinfection of micro-organisms including Cryptosporidium. This standard is most appropriate to most PWS
applications. The Class B standard is designed for supplementary disinfection, where the water has already been thoroughly treated.
Further information and a maintained list of accredited equipment may be found here: