How is a UV system selected
There are three key parameters to consider when selecting a UV disinfection system:
- Water quality
- Water flow rate
- Pathogen(s) to be inactivated
To understand the principles of UV system selection fully, each of these parameters need be examined in more detail.
The nature and quality of the water to be disinfected is critical, not only in selecting an appropriate UV system, but also in deciding if UV disinfection is suitable or even possible for the application. Of all the water quality parameters, UV transmittance (UVT) is the most important. UVT determines how well UV-C light penetrates the water, and therefore how much the pathogens in the water are exposed to sufficient UV-C light to be inactivated.
UVT is determined by taking a water sample in a quartz cuvette and passing UV light at 254nm through the sample. The percentage of UV light that penetrates this sample is referred to as the sample’s ‘UVT’. Most typically, the cuvette used has a path length of 10mm, in which case the UVT reading is referred to as being the ‘T10’ value. The UVT of the sample is critical for a very simple reason: although other parameters such as biological oxygen demand (BOD), chemical oxygen demand (COD), turbidity and total dissolved solids (TSS) all influence the extent to which UV-C light penetrates the water, and although correlation to UVT is difficult if not impossible, they are all effectively accounted for by the single UVT reading.
Of all other water quality parameters apart from UVT, TSS and total dissolved solids (TDS)/salinity are also important. TSS is particularly important because anything in excess of 20mg/l TSS can result in a phenomenon known as ‘shielding’, whereby the pathogens are shielded from the UV-C light by particles suspended in the water. TDS/salinity is important because, at very high levels, attention must be paid to the construction materials of the UV system to avoid corrosion.
Another key factor in determining how effective UV-C light will be in deactivating a given pathogen is the length of exposure time the pathogen has to the UV-C light of a given UV intensity. The longer the exposure time, the more UV-C radiation will penetrate the pathogen’s cells and the more effective the inactivation process will be.
The slower the flow rate of the water through a UV system, the longer the UV exposure time and vice versa, and so the maximum and minimum flow rate of the water should be considered. This is why many UV systems now have the ability to adjust the power output of the lamps in relation to changes in water flow rate. By doing so, energy may be conserved when water flow rates are lower than peak flows.
When determining maximum and minimum flow rates, it is important to establish the instantaneous flow rates, as this determines the instantaneous minimum and maximum UV exposure times. Daily and hourly flow rates are usually misleading in this respect, as they can mask important ‘peaks and troughs’ in the instantaneous flow rate, resulting in erratic calculations of the true UV exposure time during these peaks and troughs.
Pathogens to be inactivated
Different pathogens have differing resistance to UV; some are more susceptible than others and so require different amounts of UV-C exposure for inactivation. In order to correctly size and select a UV system, it must be established which pathogen(s) are to be inactivated.
But what does inactivation truly mean? Does it mean that every single pathogen that ever passes through the UV system will be inactivated? In reality, this is impossible. Indeed, this is impossible regardless of what disinfection method is used, whether it is UV, chlorine or anything else. What is possible is that the pathogen is reduced by a predictable amount. This predictable amount is referred to as a ‘log’ reduction (as in logarithmic reduction). A ‘one log’ (most commonly referred to as 1 log) reduction will see the pathogen of interest reduced by 90% from the influent level. A 2 log reduction will see a 99% reduction, 3 log by 99.9%, and so on. Scientists have calculated the amount of UV exposure required to inactivate a whole range of different pathogens by various log reductions. Examples appear in figure 2.
Figure 2 – Excerpt from the USEPA UV Disinfection Guidance Manual showing log reductions applicable to common water-borne pathogens, page 1-7.
The amount of UV-C delivered to inactivate a pathogen has been referred to as ‘UV exposure’. In fact the correct term for this exposure is ‘UV dose’ or ‘UV fluence’. The relationship between UV fluence and log reduction, as illustrated in Figure 2, is described as a pathogen’s ‘Dose Response Curve’. As UV dose is the most common term for UV exposure, this is what will be used from here onwards.
It is important to note from Figure 2 that the UV dose required to inactivate a given pathogen to a given log reduction is rarely linear. A common mistake is to take the UV dose required to achieve a 1 log inactivation and simply multiply it in order to calculate a higher log reduction. Although one very common pathogen, E. coli, has a dose response curve that is almost linear, most are not.