Study On Pesticide Wastewater Treatment Using O₃/UV Process

Effluents from the production of sulfonylurea herbicides are characterized by high concentrations in organic matter, low biodegradability and high toxic effects on microorganisms. The traditional biological process has many problems in degrading this kind of wastewater. Hence, a chemical pretreatment is required before the herbicide wastewater discharged into biological treatment process. O3/UV is a promising way of pretreatment.The oxidation of sulfonylurea herbicide wastewater by means of O3 was first discussed. The influences of different experimental parameters including initial orgainc concentration, pH and dosage of ozone were observed and the mechanism of O3 was studied. The optimum conditions were found to be as follows:pH 13.59, used ozone dosage 81.35mg/min and contact time 120min. Under these optimum conditions, a COD reduction of 58.26% could be reached and an increase of EC50, as a measure of acute toxicity, from 11%(untreated wastewater) to 48% was observed while biodegradability of wastewater, measured as BOD5/COD, increased from 0.03 (untreated wastewater) to 0.42. TBA (tertiary butanol) was used as a radical scavenger to investigate the contributions of direct and indirect oxidation for treatment of pesticide wastewater by O3. It was found that the removal efficiency of COD decreased rapidly with the increase of the concentration of tert-butanol, which demonstrated the existence of-OH in this system. Initial pH of solution had an obvious effect on the ratio of indirect oxidation of O3. When pH was increased from 2.06 to 13.59, the ratio of indirect oxidation was enhanced from 4.47% to 33.12%.O3/UV process was then used to treat sulfonylurea herbicide wastewater.The optimum conditions were found to be as follows:pH 13.59, used ozone dosage 65.08mg/min and contact time 80min. Under these optimum conditions, a COD reduction of 63.47% could be reached and an increase of EC50 from 11% (untreated wastewater) to 55% was observed while BOD5/COD increased from 0.03 to nearly 0.56. The BOD5/COD and EC50 of the wastewater treated by O3/UV process was improved by 0.18 and 7%, respectively when the ozone dosage decreased 16.27mg/min and the reaction time was shorten by 40min compared to the treatment by O3. The removal efficiency of COD decreased rapidly with the increase of the concentration of tert-butanol, which indicated the existence of-OH in O3/UV system. The influence of initial pH of solution on the ratios of indirect oxidation in O3/UV system was small and the proportion of the indirect oxidation induced by radicals in O3/UV maintained at 40%-50%. Indirect oxidation played a more important role in O3/UV process.