1.7: Disinfection with ultraviolet light
Germicidal ultraviolet (UV) light is generated by low and medium pressure mercury vapour lamps. UV irradiation disrupts the chemical bond of many organic molecules and damages nucleic acid and hence can be a potent disinfectant. The UV light effective for inactivating microorganisms is in the UV-B and UV-C ranges of the spectrum (200–300 nm), with maximum effectiveness around 265 nm.
The mechanism of disinfection by UV light differs considerably from the mechanisms of chemical disinfectants such as chlorine and ozone. Chemical disinfectants inactivate microorganisms by destroying or damaging cellular structures, interfering with metabolism, and hindering biosynthesis and growth (Snowball and Hornsey 1988). UV light inactivates microorganisms by damaging their nucleic acid, thereby preventing them from replicating and disrupting their ability to infect hosts.
UV irradiation has a minimal effect on the chemical composition or taste of water. Unlike chemical disinfectants, high dosage or over-dosing with UV light presents no danger, and is sometimes considered as a safety factor.
Application
UV light disinfection is a treatment option that can contribute to the effective implementation of a multi-barrier approach that reduces microbial risk in drinking water supplies. UV light disinfection can be used as the primary disinfectant for the inactivation of chlorine resistant pathogens (e.g. Cryptosporidium and Giardia), thereby reducing disinfection by-product formation.
However, UV light disinfection typically should not completely replace the use of chemical disinfection. This is because there are a number of other aspects to consider in managing the microbial risk of drinking water supplies, such as:
maintaining a disinfection residual within the distribution system;
management of taste and odour compounds;
controlling cyanobacteria;
deactivation of viruses that are not easily treated by UV light alone; and
ensuring there is an adequate multi-barrier approach for the entire drinking water system.
Practical considerations
The equipment required for UV irradiation is fairly reliable, the technology required is relatively simple and controls for the process are being developed.
There are a number of factors that should be considered in relation to UV light, such as:
reliability of power supply, in particular the start-up and restart times should be factored into operational and response plans;
water quality aspects, such as algae, high colour and turbidity, hardness and organic matter, as they can reduce the amount of UV irradiation reaching microorganisms and necessitate higher doses of applied irradiation for effective disinfection (which can be managed if the percentage UV transmittance is known);
a site-specific mercury spill response plan should be established to minimise mercury release in the rare event of a lamp breakage;
units require regular cleaning and maintenance to remain effective; and
once the appropriate UV dose is determined and matched to the flow rate, exceeding the validated flow rate could result in the application of an insufficient UV dose, as a result of short-circuiting.
Performance validation
Table IS1.7.1 presents published dosage rates for UV light that have been demonstrated as achieving a four log reduction in the target microorganism. These values are supplied for illustrative purposes only and is consistent with Table 5.6. For UV dosage rates that achieve a greater log reduction, the cited references should be consulted. Further information can be obtained from a review of existing data on the effectiveness of UV light against a range of specific pathogens undertaken by WaterVal (2017). The UV dosage rate that is applied at a particular water treatment plant should be based on the microbial risk assessment for that particular water supply system.
Table IS1.7.1 Published dosage rates to achieve 99.99% (4 log) inactivation of various microorganisms by UV irradiation
Protozoa
22
Viruses
186
E.coli
22
Based on WaterVal (2017)
The important conclusion to draw from Table IS1.7.1 is that, UV light will effectively inactivate protozoa and bacteria, but is less effective against viruses.
Wherever possible, a validated UV light system should be used, and preferably those systems that have been validated in accordance with the requirements of the USEPA Ultraviolet disinfection guidance manual for the final long term 2 enhanced surface water treatment rule (UVDGM) (2006). Other validation processes for UV light systems also exist (DVGW, 2006a, 2006b, 2006c; ONORM, 2001, 2003; and NWRI, 2012).
Water quality considerations
The performance of UV disinfection is not affected at turbidity levels of up to 1 NTU, and UV light may remain effective at higher turbidities greater than 1 NTU, as long as the transmittance of UV light through the water is not compromised. However, the lower the turbidity of the water the more effective the performance of UV light will be. This reinforces the importance of percentage UV transmittance as a measure of the effectiveness of the applied UV dose to inactivate targeted pathogens.
UV irradiation is not pH dependent and the temperature effect between the ranges of 5 to 35°C is minimal (USEPA 2006). The presence of algae in the water being treated may reduce the UV transmittance and interfere with the UV disinfection process and should be considered in the design phase if the supply is prone to algal blooms.
Highly coloured water is not suitable for UV disinfection as the dissolved organic matter which gives the water its colour strongly absorbs the UV light, greatly reducing the effectiveness of the UV disinfection process. This again highlights the importance of measuring UV transmittance.
Persistence
At the proper dosage, UV light requires only a short contact time, but has the disadvantage that it leaves no residual disinfectant, which would provide an additional barrier within the distribution system.
By-products
Few data are available on the by-products of UV disinfection. At the UV doses typical for drinking water supplies (less than 200 ), there is no evidence of the formation of by-products (DBP) or exacerbation of DBP if post UV disinfection occurs (USEPA 2006).
UV light has been reported to convert nitrate to nitrite (Sharpless and Linden, 2001). Given the typical values of Australian waters, the nitrate to nitrite conversion is unlikely to result in the exceedance of health guidelines for drinking water.
Operational considerations
Given that the UV process will be a critical control point (CCP), other important issues that will need to be considered to ensure the effectiveness of the process are:
establishing target criteria (section 3.4.2) and critical limits for the UV irradiation process, including UV transmittance and intensity;
preparing and implementing operational procedures (section 3.4.1) and operational monitoring (section 3.4.2) for the process;
preparing corrective action procedures (section 3.4.3) in the event that there are excursions in the operational parameters; and
undertaking employee training (section 3.7.2) to ensure that the UV irradiation process operates to the established target criteria and critical limits.
It is recommended that validation is undertaken for each system to ensure appropriate treatment is in place for the water quality and level of risk.
Operational monitoring
As there is no disinfection residual to measure following UV light treatment, other operational aspects of UV light systems should be monitored to ensure that the treatment system is operating as expected. Examples of monitoring parameters are listed below, recognising that each system will need to develop its own operational monitoring specifications reflecting its unique circumstances.
UV dose
Flow rate
UV transmittance
Lamp outage
Lamp age
UV intensity
Another issue that needs to be considered with respect to UV light is that the performance of the UV lamps deteriorates over time, so that lamps should be changed at the frequency recommended by the manufacturer. Typically the loss of UV light output is around 25% over 12,000 hours operation.
Furthermore, biofilm, which can accumulate on the sleeve surrounding the lamp, should be regularly removed from the sleeve. Many UV light systems now have automated cleaning systems.
References
Austrian Standards Institute (ÖNORM) (2001). ÖNORM M 5873-1: Plants for the disinfection of water using ultraviolet radiation: Requirements and testing. Part 1. Low pressure mercury lamp plants. Österreichisches Normungsinstitut, A-1021 Vienna, Austria.
Austrian Standards Institute (ÖNORM) (2003). ÖNORM M 5873-2: Plants for the disinfection of water using ultraviolet radiation: Requirements and testing. Part 2. Medium pressure mercury lamp plants. Österreichisches Normungsinstitut, A-1021 Vienna, Austria.
German Association for Gas and Water (DVGW) (2006a). UV Devices for Disinfection in the Water Supply Part 1: Requirements Related to Composition, Function and Operation, in Technical Rule, Code of Practice W294-1. Deutsche Vereinigung des Gas- und Wasserfaches, Bonn, Germany.
German Association for Gas and Water (DVGW) (2006b). UV Devices for Disinfection in the Water Supply Part 2: Testing of Composition, Function and Disinfection Efficiency in Technical Rule, Code of Practice W294-2 Deutsche Vereinigung des Gas- und Wasserfaches, Bonn, Germany.
German Association for Gas and Water (DVGW) (2006c). UV Devices for Disinfection in the Water Supply; Part 3: Measuring Windows and Sensors for the Radiometric Monitoring of UV Disinfection Devices: Requirements, Testing and Calibration in Technical Rule, Code of Practice W 294-3 Deutsche Vereinigung des Gas- und Wasserfaches, Bonn, Germany.
National Water Research Institute (NWRI) (2012). Ultraviolet Disinfection Guidelines for Drinking Water and Water Reuse, Third Edition, California, United States of America.
Sharpless CM and Linden KG. (2001). UV photolysis of nitrate: Effects of natural organic matter and dissolved inorganic carbon, and implications for UV water disinfection. Environmental Science and Technology 35:2949–2955.
United States Environmental Protection Agency (US EPA) (2006). Ultraviolet disinfection guidance manual for the final long term 2 enhanced surface water treatment rule. US EPA Office of Water: Washington D.C.
WaterVal (2017). WaterVal Ultraviolet disinfection: Guidance document. Australian Watersecure Innovations LTD 2017.
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