Ultraviolet disinfection of secondary wastewater effluents: Prediction of performance and design

An empirical ultraviolet (UV) disinfection model was developed to predict 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 model include the suspended solids concentration, the unfiltered UV transmittance measured at a wavelength of 253.7 nm, the influent coliform concentration before exposure to UV light, and the applied UV dose. In the model, UV dose is calculated as the average UV intensity within the reactor (using the point source summation method) multiplied by the exposure time (based on the assumption of approximate plug flow conditions). Because the model is empirical, it should be calibrated to establish the statistical significance of each input parameter before use in a particular situation and to implicitly account for other factors 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 considered. The correlation coefficient (R(2)) of the calibrated model was 0.79. The calibrated model was then used to illustrate a design approach that integrates model uncertainty, wastewater variability, and variable permit requirements. In this approach, the number of UV lamps, modules, banks, and channels can be optimized to provide cost-effective designs. Several design examples are used to illustrate both disinfection and headless considerations in the recommended design approach.