By Charles Nicolson
The term ‘ultraviolet’ refers to radiations beyond violet radiation in the radiation wavelength spectrum, starting where the visible section ends, continuing to the beginning of X-rays at wavelengths of around 100 nm.
Beyond ultraviolet, as wavelengths continue to diminish, radiation becomes X-rays, then gamma rays and finally cosmic rays. Figure 1 shows how UV between 400 and 200 nm wavelengths is divided into three bands denoted UV-A, UV-B and UV-C. (Note – ‘nm’ denotes nanometre – one thousand-millionth of a metre).
The ability of ultraviolet radiation to control or kill micro and macro-organisms is contained mostly in the UV-C band (with some overlapping into UV-B) which from now on will be referred to simply as ‘UV’ or ‘ultraviolet’. UV is effective at sufficient dosing levels against all micro-organisms including bacteria, fungi, moulds, viruses, yeasts, and algae present in water, air and on surfaces.
All UV wavelengths between 200 nm and 320 nm are effective biocides and a specific wavelength of 265 nm has the greatest killing effect on the micro-organisms most commonly occurring in water circuits used in HVAC installations. In terms of frequencies rather than wavelengths, the frequency range of ultraviolet radiation goes from just below to just above 1017 Hertz.
UV works as a fast-acting biocide by penetrating directly into the cells of organisms and destroying their DNA which terminates the ability of organisms to propagate. However, the fact that UV has to penetrate right into the cells of organisms to accomplish this means that there must be minimum dosages, measured by combinations of UV strength and exposure time.
UV radiation is generated by lamps containing small amounts of mercury. Wavelengths and overall emission strengths depend on gas pressures inside the lamps which are either low (0.001 to 0.01 bar) or high (1.0 to 2.0 bar) with corresponding differences in electrical power demands.
Water circuits in HVAC installations are either ‘once through’ such as used in humidifiers and hot water supply sections, or ‘re-circulating’ as chilled water and condenser cooling circuits. UV dosing is sized on water flow rates and therefore may have to be installed as side-stream configurations in re-circulating water circuits.
Earlier, it was stated that UV is capable of killing all micro-organisms and for air conditioning considerations this includes legionella bacteria. Also, corrosive anaerobic bacteria such as Sulphate-reducing bacteria (SRB) is controlled indirectly by the UV exterminating all the other aerobic bacteria which use up any dissolved oxygen in circulating water thereby creating favourable conditions for anaerobic bacterial growth.
The intensity of UV is the amount of energy per unit area which needs to be present at the required maximum penetration distance through the water flow and is normally measured in mW/cm2. Assuming that the intensity is sufficient, the next consideration is whether the micro-organisms in the water flow are subjected to this UV intensity for sufficient time to receive lethal doses of the UV.
Doses are measured as: intensity x exposure time – so the units used are mJ/cm2. Intensity reductions as the UV travels through water flow depend on the quality of the water according to:
Discolouration of water is normally measured in Hazen units where less than 10 Hazen units is clear, and 100 Hazen units is a distinctive brown/yellow colour.
- Iron (and sometimes manganese): Rapid corrosion in a water circuit can be produced by sufficient iron in corrosion products to reduce UV transmission.
- Organic matter
In HVAC water circuits, organic matter is usually the water treatment chemical package used. Therefore, with UV doing micro-organism control no organic biocides need to be added to the water.
- Suspended solids
Suspended solids affect UV transmission by blocking, scattering and absorption. Therefore, water flowing through UV units may have to be filtered to reduce solids concentration to below 20 mg/litre.
Turbidity is due to suspension of finely divided matter which is not removed by normal filtration and is seen as lack of clarity or brilliance. Turbidity should not be confused with colour since water may have a dark colour but still be clear and not turbid.
These are the five main causes of reduction of UV transmissibility in water. The actual transmissibility of water is measured by a UV intensity meter as a percentage of the UV intensity emitted by a standard 254 nm UV lamp after passing through a 10 mm quartz cuvette holding a sample of the water as shown in Figure 2.
Pathogenic bacteria are generally susceptible to relatively low UV dosing levels. For example, the lethal dosage for one of the most commonly occurring organisms, e-coli, is 19 mJ/cm2. Other familiar pathogens such as the bacteria which causes cholera, vibrio cholera, and the bacteria pneumophilia, the cause of Legionnaires’ disease, require similarly small lethal UV doses of 21 mJ/cm2 and 15 mJ/cm2 respectively.
The types of bacteria and other micro-organisms which can coagulate and form biofilms, however, can only be killed by far more powerful UV emitters which are available but usually more costly both to install and operate than established biocide dosing programs.
UV units have regained their former popularity for sanitising water but only if the water is sufficiently clean and clear as described above. In new and upgraded HVAC installations UV units are being increasingly seen in both cold and hot potable quality water flows and systems as well as make-up to adiabatic and heated water humidifiers.
Closed cooling and heating water circuits are still treated with initial doses of bio-dispersants and subsequently re-dosed at six- or 12- month periods which is more economical than installing UV units.