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 1.2 log units as the SS concentration increases from 16.9 mg/l to 25.4 mg/l at a UV energy density of 40 mJ/cm(2). UV disinfection alone reduced the E. coli population by 3.4 log units. However, the synergistic disinfection of ultrasound and UV could reach 5.4 log units during the reduction of E. coli at a 40 kHz frequency and an energy density of 2.64 kJ/l. The additional synergistic effect is 1.1 log units.
The inhibitory effect of ozone and hydrogen peroxide (HP) on urea hydrolysis in stored urine was investigated and compared. Ozone showed less effect on urea hydrolysis due to the complicated composition of urine (including a large amount of urease-producing bacteria) and bacteria regeneration. Ozone concentration and total heterotrophic bacteria analysis demonstrated that residual ozone concentration decreased by 43% within 15 hr from 13.50 to 7.72 mg/L in the one-time ozonation urine test, and finally completely decomposed within 4 days. In addition, bacteria regenerated quickly after ozone completely decomposed. However, HP showed a significant effect on inhibiting urea hydrolysis not only in stored urine but also in fecal-contaminated urine. The suitable doses of applied HP to inhibit urea hydrolysis in stored urine, concentrations of 0.5 and 1.0 g feces per liter of fecal-contaminated urine, were 0.03, 0.16 and 0.23 mol/L, respectively. The urea concentrations after 2 months stored were 7,145, 7,109 and 7,234 mg/L, respectively.
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