AWWA WQTC58922

A 1 um diameter fluorescent microsphere was selected and characterized for use as a photobleachable probe of ultraviolet (UV) fluence in a disinfection reactor. Loss of fluorescence (photobleaching) was calibrated as a function of fluence, and this relationship was found not to be significantly affected by water quality parameters, if oxidants such as hypochlorite were excluded. Photobleaching was also determined to be caused by the UV portion of the medium pressure lamp spectrum, especially wavelengths below 300 nm. Microspheres were injected into the inlet of a pilot scale UV reactor and recovered in outlet samples for subsequent analysis by flow cytometry. Data from flow cytometry, together with the calibration data, were used to construct a histogram of UV fluence for the reactor, showing behavior consistent with previous predictions in other reactors. Improvement of the sensitivity of the technique is required to permit direct measurement of the minimum UV fluence. Includes 14 references, figures.