ULTRAVIOLET DISINFECTION OF SECONDARY WASTE-WATER EFFLUENTS - PREDICTION OF PERFORMANCE AND DESIGN

An empirical ultraviolet (UV) disinfection model was developed to pred ict coliform inactivation in unfiltered secondary wastewater effluent. The model was developed in the tailing region of the UV dose-response curve for log survival values less than -3. Input parameters to the m odel include the suspended solids concentration, the unfiltered UV tra nsmittance measured at a wavelength of 253.7 nm, the influent coliform concentration before exposure to UV light, and the applied UV dose. I n the model, UV dose is calculated as the average UV intensity within the reactor (using the point source summation method) multiplied by th e exposure time (based on the assumption of approximate plug flow cond itions). Because the model is empirical, it should be calibrated to es tablish the statistical significance of each input parameter before us e in a particular situation and to implicitly account for other factor s influencing disinfection performance. The model was calibrated with data collected from a small-scale UV reactor operated at two northern California wastewater treatment plants. The values of suspended solids concentration, unfiltered UV transmittance, and UV dose were found to be statistically significant with the water quality data set consider ed. The correlation coefficient (R(2)) of the calibrated model was 0.7 9. The calibrated model was then used to illustrate a design approach that integrates model uncertainty, wastewater variability, and variabl e permit requirements. In this approach, the number of UV lamps, modul es, banks, and channels can be optimized to provide cost-effective des igns. Several design examples are used to illustrate both disinfection and headless considerations in the recommended design approach.