In its earliest days, UV water treatment was something of a black art, where the predicted performance was based on some fairly large assumptions. Today, as the industry has matured, that has changed. Regulatory authorities have adopted methods for analysing the performance of a particular UV system to answer the critical question – how effectively does it prevent dangerous pathogens from affecting humans?
How tough is your organism?
When measuring the effectiveness of a UV system, we must first understand how a measured amount of UV light affects a particular organism. The output of a UV lamp is normally measured as an intensity value, the power over a particular area. However, it is the UV dose (the intensity multiplied by the exposure time) that governs the biological effect on the organism, which is the critical parameter in quantifying the effectiveness of the system.
The organism in water is exposed to a beam of UV light of known intensity for a specific time in a laboratory experiment. The sample is analysed to show how many of the organisms have been inactivated (prevented from reproducing), and compared to a sample that hasn’t been exposed to the UV light beam. The term used is log inactivation: a 1-log inactivation means that 90% of the organisms are rendered unable to reproduce and grow colonies in a culture. 2-log equals 99%, 3-log is 99.9%, etc. By performing measurements over a range of exposure times, you can construct a graph for your organism which shows the level of inactivation for a measured UV dose – known as a dose-response curve.
Constructing a dose-response curve for an organism
This work, carried out over many years in the mid-20th century, showed that many harmful (pathogenic) organisms, such as Giardia and Cryptosporidium, are susceptible to inactivation under UV light. Which is all very well, but how can the results of this lab experiment be related to a UV treatment chamber?
Common water-borne pathogens Cryptosporidium (left) and Giardia (right) as seen under an electron microscope
Better safe than sorry
UV chambers come in a variety of shapes and sizes, but they all have a continuous flow of liquid past the UV lamps. Originally, the average UV dose delivered by the system was calculated by multiplying the UV intensity of the lamps by the residence time of the water in the chamber, which implicitly assumed a smooth, even flow pattern inside the chamber. This was then related to the dose-response curve for each organism to give the expected performance.
In an ideal world, this might have worked – but the world is not ideal. Flow patterns through chambers are not uniform. Lamp outputs reduce as the lamp ages. Substances in the water absorb the UV light and prevent it penetrating as far as might be expected (see our article on UV transmittance for more details). So a more accurate measure of the effect on the water is needed. But how can this be done without introducing a dangerous organism into a water supply?
One way is to use a safe organism. A number of non-pathogenic microorganisms have been introduced which can be safely added to a water supply and act as “test subjects” for the UV system. Popular examples of these so-called “challenge microorganisms” include spores of the Bacillus subtilis bacterium and the bacteriophage MS2 virus. These have well-characterised UV dose-response curves and are easily analysed to measure the log inactivation achieved by the UV system. Once this is known, the dose-response curve can be applied ‘in reverse’ to give the effective dose that is actually being delivered by the system (known as the Reduction Equivalent Dose, or RED).
Calculating the effective dose delivered by UV systems using a challenge microorganism
All this work is carried out at industry-approved test centres, to ensure that the challenge microorganism is correctly administered and mixed into the test stream, and to allow the flow rate and transmittance to be precisely adjusted. This also permits adequate treatment of the water before sending to the municipal sewerage system.
Knowing the RED allows us to predict the log inactivation that the system can achieve (under a given set of operating conditions) for any pathogen using its own dose sensitivity. REDs give a scientific measure of the system’s performance that is now recognised by authorities in many countries.
Proving fitness for purpose
Just as existing conventional UV systems are being ever more closely scrutinised for evidence of their effectiveness, so new products brought to market will also need to prove their worth by meeting or exceeding the same standards. It will be critical for Eco-UV to be tested thoroughly if it is to be accepted by the industry and its customers, and that is exactly what Eco-UV partner DVGW, the German gas and water association, will be doing at its Water Technology Centre, TZW. The new lamp technology will be integrated into a series of pilot systems which will be subjected to rigorous testing over a period of many months using the kind of analysis described above. The results will be of fundamental importance to the success of the project.
As the applications of UV treatment have expanded to include more demanding processes in the food, beverage and pharmaceutical industries as well as the treatment of municipal drinking water supplies, improved testing and validation of the performance of UV systems has helped increase confidence in the technology, and allows companies to prove that it really can do what it says on the tin – keeping water supplies safer for all of us.
Ultraviolet Disinfection Guidance Manual for the Final Long Term 2 Enhanced Surface Water Treatment Rule, United States Environmental Protection Agency Report EPA 815-R-06-007, November 2006.