While UVC is a proven and potent anti-microbial technology, effective water disinfection depends on delivering the proper UV dose (a product of UV intensity and time) within a system. Understanding the ultraviolet transmittance (UVT) of the water source is an important factor to ensure that the delivered UV dose is sufficient to inactivate pathogenic microorganisms.
UVT is a measure of water quality and indicates the percentage of UV light at a specific wavelength that is able to pass through water and be used for microbial inactivation. The higher the UVT, the better the water quality (i.e. less UVC light is being scattered or absorbed before reaching the microbe). A UVT of 100 percent would indicate that, for a given wavelength, all of the light is able to pass through water with no losses from contaminants like dissolved organics or particulate matter.
Traditional UV water disinfection systems relied on low-pressure mercury lamps which emit exclusively at 253.7 nm. However, as the benefits of UVC LEDs lead to increasing application in point-of-use drinking water, purifier OEMs can benefit from the ability to select stronger germicidal wavelengths and correspondingly should consider the impact UVT may have on UVC dose requirements and system performance.
Figure 1 (above) shows some typical UVT levels on common source water. Immediately noticeable is the large impact that pre-treatment has on UVT. Filtered tap water, bottled water and de-ionized water all show high transmissibility, >95 percent for all wavelengths above 260 nm.
UVT is a concern…but only if you’re not using pre-treatment before UV
In contrast, the UVT of regular unfiltered tap water in this example shows a range from just under 60 percent at 250 nm to 80 percent at 285 nm. This absorption of the UV light is due to residual chlorine and normal salts (hard water). In these cases, as with the mercury lamp system, it will be necessary to compensate the delivered UV.
You can calculate compensation by using the following formula:
[Dose to achieve reduction in target microbe] x 100 percent / UV Transmission percent
Example: To achieve a 6 log reduction (LRV) of E. coli requires a dose of 15 mJ/cm2 using an LED with a peak wavelength of 265 nm. If a system uses direct tap water which has a UVT of 75 percent at 265 nm, then the required compensation dose calculation would be:
(15 mJ/cm2) x 100 percent / 75 percent = 20 mJ/cm2
In systems where source water is filtered to address non-microbial contaminants (silt, heavy metals, petroleum products) before UV disinfection, the water is likely to have a high UVT and require little dosage adjustment. Purifier OEMs should always consider the performance of any pre-filtration and whether water quality could diminish over the full lifetime of the filter.
The impact of pathogen’s absorption spectra will always be high
In terms of UVC wavelength, disinfection performance is not equal across the UV range. It can vary quite widely based on target microbe and small differences in peak wavelength. In the case above, tap water showed better UVT at longer wavelengths (280 nm) than shorter (250 nm). However, determining dosage at different wavelengths and under different UVT conditions always requires considering the spectral sensitivity of the target microbe. In Figure 1 (above), E. coli absorption spectrum by wavelength is superimposed on the UVT plots. This illustrates how, based on the spectral sensitivity of E. coli, light delivered by a 280 nm LED is only 60 percent as effective at disinfection as light delivered by a 265 nm LED.
Table 1 (below) shows the impact of wavelength on UVT and spectral sensitivity on dose when targeting a 6 LRV for E. coli using standard tap water without filtration.
|Log Reduction||Reduction Factor||Percent Reduced|
The table shows that an LED emitting at 265 nm with UVT of 75 percent requires a dose of 20 mJ/cm2 while an LED emitting at 280 nm with UVT of 80 percent would require a dose of 31.3 mJ/cm2. That is, while slightly more UV light may be transmitted at 280 nm than at 265 nm the germicidal performance advantage of 265 nm over 280 nm makes this a far more effective disinfection wavelength.
As UVC LEDs gain traction in point-of-use drinking water disinfection, purifier OEMs can now take advantage of more effective germicidal wavelengths - particularly those less impacted by the UVT. LEDs allow you to more closely select a wavelength in the application to minimize this impact and maximize the germicidal performance.