High irradiance, medium pressure UV disinfection systems are becoming widely accepted as an alternative to conventional low pressure UV technology, especially for high flow rate applications where lower equipment and maintenance costs outweigh the higher electrical cost. However, the
effect that the characteristic broad spectral output has on the germicidal effectiveness of the system has not been well understood. This paper presents germicidal dose response results from laboratory studies with selected wavelength bands produced by filtering all other wavelengths from
a broadband medium pressure lamp output. Tests were conducted using a standard collimated beam apparatus. Both radiometry and ferrioxalate actinometry were employed to measure the actual UV doses. E. Coli was used as the test organism because of its well-known absorption characteristics
and reliable analytical methodology. Previous work by Gates (1930) compared UV disinfection of S. Aureaus and B. Coli at various wavelengths using a quartz monochromator. He observed that a wavelength of 265 nm was optimal with 254 nm being somewhat better than 280 nm and
230 nm. He also found that the response at 230 nm was similar to that at 280 nm. By estimating the relative efficiencies based on the absorption spectrum of E. Coli and the relative number of photons at each wavelength the ratio of 230:254:280 (normalized to 254 nm) would be 0.53:1.0:0.74.
This study confirmed these ratios. This paper details the experimental procedure and results obtained from the laboratory wavelength study comparing the relative efficiency of the 230 nm, 256 nm and 280 nm wavelengths for E. Coli treatment. Theoretical calculations of relative efficiencies
are also presented. By representing applied energy in terms of watts rather than photons per second, considering the entire absorption curve, and correcting for the absorbance of a typical secondary effluent, there is a shift to higher wavelength light being more germicidally effective in
secondary effluent treatment than light emitted at the 254 nm wavelength, where low pressure lamps emit.
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