Synergistic effect of ultrasonic pre-treatment combined with UV irradiation for secondary effluent disinfection

Abstract

The ultraviolet (UV) disinfection efficiency is often affected by suspended solids (SS). Given their high concentration or large particle size, SS can scatter UV light and provide shielding for bacteria. Thus, ultrasound is often employed as a pre-treatment process to improve UV disinfection. This work investigated the synergistic effect of ultrasound combined with UV for secondary effluent disinfection. Bench-scale experiments were conducted in using samples obtained from secondary sedimentation tanks. These tanks belonged to three wastewater treatment plants in Beijing that use different kinds of biological treatment methods. Several parameters may contribute to the changes in the efficiency of ultrasound and UV disinfection. Thus, the frequency and energy density of ultrasound, as well as the SS, were investigated. Results demonstrated that samples which have relatively higher SS concentrations or higher percentages of larger particles have less disinfection efficiency using UV disinfection alone. However, the presence of ultrasound could improve the disinfection efficiency because it has synergistic effect. Changes in the particle size distribution and SS concentration notably affected the efficiency of UV disinfection. The efficiency of Escherichia coli elimination can be decreased by 1.2log units as the SS concentration increases from 16.9mg/l to 25.4mg/l at a UV energy density of 40mJ/cm2. UV disinfection alone reduced the E. coli population by 3.4log units. However, the synergistic disinfection of ultrasound and UV could reach 5.4log units during the reduction of E. coli at a 40kHz frequency and an energy density of 2.64kJ/l. The additional synergistic effect is 1.1log units.