Comparison of the disinfection effects of vacuum-UV (VUV) and UV light on Bacillus subtilis spores in aqueous suspensions at 172, 222 and 254 nm.

The efficacy of UV and vacuum-UV (VUV) disinfection of Bacillus subtilis spores in aqueous suspensions at wavelengths of 172, 222 and 254 nm was evaluated. A Xe(2)* excilamp, a KrCl* excilamp and a low-pressure mercury lamp were used as almost monochromatic light sources at these three wavelengths. The first-order inactivation rate constants at 172, 222 and 254 nm were 0.0023, 0.122 and 0.069 cm(2) mJ(-1), respectively. Therefore, a 2 log reduction of B. subtilis spores was reached with fluences (UV doses) of 870, 21.6 and 40.4 mJ cm(-2) at these individual wavelengths. Consequently, for the inactivation of B. subtilis spores, VUV exposure at 172 nm is much less efficient than exposure at the other two wavelengths, while exposure at 222 nm is more efficient than that at 254 nm, which is probably because triplet energy transfer from DPA to thymine bases at 222 nm is higher than that at 254 nm. This research indicated quantitatively that VUV light is not practicable for microorganism disinfection in water and wastewater treatment. However, in comparison with other advanced oxidation processes (e.g. UV/TiO(2), UV/H(2)O(2) or O(3)/H(2)O(2)) the VUV-initiated photolysis of water is likely more efficient in generating hydroxyl radicals and more effective for the inactivation of microorganisms.