Biodosimetry, that is the application of microorganisms for the measurement of radiation, is already in wide use in the field of UV disinfection. The measurement of the mean fluence in flow-through systems with a biodosimeter (microorganisms with calibrated UV sensitivity) results in the Reduction Equivalent Fluence (REF). In the case of quasi monochromatic radiation (mercury low pressure lamp, 253.7 nm), the flow pattern of the water through the inhomogeneous radiation field, together with some other parameters, strongly influences the REF but the spectral sensitivity of the biodosimeter plays no role. If microorganisms with shouldered survival curves are used as biodosimeters two parameters (k and d) are necessary to describe their survival curves. In general, both parameters are wavelength dependent and the functions k(λ) and d(λ) must be known if medium pressure mercury lamps are used, which emit polychromatic UV radiation. The knowledge of k(λ) also is necessary for selecting an appropriate UV sensor which controls the function of the UV lamps during the operation of the disinfection plant. In literature many different spectral sensitivity curves were published but they all differ somehow. The functions k(λ) and d(λ) were measured for spores of Bacillus subtilis ATCC 6633 in using quasi-monochromatic UV radiation and the results were tested in using polychromatic UV radiation.

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