Ultrasonic/O 3 combined process was employed to pretreat heterocyclic pesticide wastewater for increasing biodegradability and reducing biological toxicity. Influences of ultrasonic frequency, ultrasonic power, probe diameter, initial pH and O 3 dosage on the COD removal were studied. The results showed that the ultrasonic/O 3 process significantly improved the biodegradability and reduced the biological toxicity of the wastewater. The ratio of BOD 5 /COD was increased from 0.03 to 0.55 and the EC 50 increased from 11% to 52% under ultrasonic/O 3 treatment. Low ultrasonic frequency brought better COD removal. Initial pH was found to have a high influence on the COD removal and alkaline conditions were more favorable. The influences of ultrasonic power and probe diameter were small. With an increase in O 3 dosage, COD removal was effectively improved. The optimal operational parameters for the combined process on COD removal were ultrasonic frequency 20 kHz, initial pH 9.00, ultrasonic power 300 W and dosage of O 3 454.8 mg/(L·min), under which the efficiency of COD removal reached 67.2%.
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