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mong the various disinfection technologies, ultraviolet (UV) disinfection has emerged as a highly popular alternative to chlorination. The application of UV disinfection, through the traditional low-pressure (LP) UV systems and the newly developed medium-pressure (MP) UV systems, for drinking water has become |
increasingly popular in recent years due to its excellent disinfection performance over a wide range of microorganisms without the production of harmful disinfection by-products (DBPs) (Figure 1).
Unlike chemical disinfectants, UV disinfection employs a unique mechanism of DNA damage to prevent the pathogens from reproducing and thereby achieves reduction in pathogen concentrations. In particular, UV disinfection inactivates microorganisms through the formation of cyclobutane pyrimidine dimers in their DNA. However, these cyclobutane pyrimidine dimers may be removed via photoreactivation, which allows inactivated microorganisms to recontaminate the water. Studies have found that photoreactivation of Escherichia coli following MP UV disinfection is lower than that following LP UV disinfection. A hypothesized explanation for this is damage to photolyase, because it contains a tryptophan-rich flavin adenine dinucleotide (FAD) cofactor that has a peak absorbance at 280 nm, a wavelength emitted by MP, but not LP UV lamps. Nevertheless, there is currently no evidence to validate the hypothesis, and research on this effect has been limited. This study therefore aims to investigate the effects of LP
and MP UV radiation on photolyase activity and to propose the mechanism for photoreactivation suppression by MP UV radiation reported previously.
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Figure 1: Pilot scale UV disinfection systems. (a) LP UV disinfection reactor (b) P UV disinfection reactor (inside the chamber). |
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Figure 2: SDS-PAGE image showing presence of photolyase in various steps of purification. Lane 1: Marker; Lane 2: Cell lysate; Lane 3: Crude extract; Lane 4: (NH4)2SO4 precipitate; Lane 5: After chromatography; Lane 6: After dialysis into storage buffer; Lane 7: Marker. The dotted section indicates the position of photolyase. |
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Photolyase was extracted and purified from E. coli containing plasmid with the phr gene (Figure 2). The calculated rates of dimer repair by LP and MP UV-irradiated photolyase based on absorbance changes in the substrate are summarized in Figure 3. For LP UV-irradiated photolyase, the dimer repair rate was unaffected up to a dose of 10 mJ cm−2 (rates varied 3% between 0.467 and 0.480 M dimer M−1 photolyase min−1) and then decreased significantly such that the dimer repair rate at 40 mJcm−2 was only 72% of that of unirradiated photolyase. This observation suggests that a minimum radiation threshold is required before the dimer repair ability of photolyase is affected; this is analogous to the multihit inactivation model. In contrast, the dimer repair rates of MP UV-irradiated photolyase decreased by 20% at a UV dose as low as 2 mJ cm−2 and remained relatively constant above
10 mJ cm−2. This phenomenon may be due to the high absorption of photolyase at wavelengths such as 280 nm and 384 nm that are produced by MP UV lamps, which could have caused photolyase damage either structurally (irreversible) or via oxidation (reversible), thereby affecting its dimer repair ability. The results provide evidence in favor of the hypothesis that photolyase damage results
in reduced photoreactivation rate when both LP and MP UV radiation are applied and also suggest mechanisms by which the damage occurs in microorganisms.
The information obtained through the research work on UV disinfection provides an in-depth understanding of repair suppression which is of great importance in the water industry.
Figure 3: Repair rates of photolase exposed to various doses of LP and MR UV radiation. |
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Dr Hu Jiangyong obtained the PhD from Tsinghua
University in 1996. She worked in Tsinghua
University as a lecturer before she joined the
Department of Civil Engineering, NUS as a
NSTB postdoc in 1996. From 1998 onwards, she
worked in the same Department as an Assistant
Professor till June 2004. She is currently working
in the Division of Environmental Science and
Engineering as an Associate Professor. Her
research focuses on water reclamation and water
quality.
Email: esehujy@nus.edu.sg |
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