Study Of Pesticide Wastewater Treatment By H/A/O Bio-contact Oxidation Process

The treatment of pesticide production wastewater has always been the focus of social attention because of its special characters such as complicated composition, high concentration of organic compounds, low biodegradability and strong bio-toxicity and so on.Wastewater produced by a pesticide plant in Hebei, mainly including two kinds of pesticide wastewater containing thiamethoxam and pretilachlor contained a large number of heterocyclic and benzodiazepines organic substances. The wastewater was of high suspended solid and nitrogen concentration, and a large number of benzodiazepines/heterocyclic materials were difficult to biodegrade, so direct biochemical treatment was not feasible. Now that, a combined process of coagulation, O3oxidation, H/A/O bio-contact oxidation and sand/activated carbon filtration was employed to treat the pesticide wastewater in order to make sure that the effluent COD was less than80mg/L, pH was between6to8, NH4+-N concentration was less than5mg/L and TN concentration was less than15mg/L, which met the integrated wastewater discharge standard (GB8978-1996) level one emission standards. The entire research and engineering commissioning were divided into four stages.1. The pretreatment stage. The pretreatment capacity of coagulation was examined by changing flocculant dosing quantity, wastewater pH and coagulation mixing time to examine respectively. The biodegradability improving capacity was also examined by changing the O3concerntation and the wastewater pH.2. The hydrolytic acidification (H) stage. The effect of COD volume load to hydrolytic acidification was studied both in the lab-scale experiment and engineering commissioning.3. A/O bio-contact oxidation stage. The effect of the reflux ratio to the A/O bio-contact oxidation was examined by adusting the reflux ratio regulation in the wastewater treatment.4. The operation of the combined process. The treatment efficiencies of the combined process were examined by the COD, NH4+-N and TN removal ratios of the final effluent.The optimum reaction conditions of coagulation determined by single factor experiment were as follows:influent pH was11, PFS dosing quantity was250mg/L, PAM dosing quantity was200mg/L and mixing time was20min. The effluent COD and NH4+-N of coagulation was4512and100mg/L, respectively, and the COD removal ratio was60.8%. The optimum reaction conditions of O3oxidation were as follows:wastewater pH was10, concentration of O3was20mg/L and the reaction time was90min. The COD effluent was under1031mg/L and the COD removal was more than30.2%under such conditions. The biodegradability of pesticide wastewater was improved effectively after O3oxidation pretreatment. The BOD5/COD ratio of wastewater increased from0.03to0.34, which laid a foundation for subsequent biochemical treatment.Maintain COD volume load at1.5kg COD/(m3-d), set HRT12h and keep temperature between20-25℃as the optimum operation parameters of hydrolysis acidification pool. The hydrolysis acidification effect was not good at the beginning of the commissioning because the bio-filming didn’t finish. COD removal capacity increased, COD removal ratio was between48%-51%and the effluent COD decreased from1000-1200mg/L to450-550mg/L when the fill finished bio-filming.The optimum operation parameters of A/O bio-contact oxidation process were as follows:keep HRT of A/O pool4h and12h, respectively, maintain the DO of A pool lower than0.5mg/L while O pool near2mg/L and keep sludge concentration at3670mg/L. Add15kg/d glucose as carbon source to A pool and20kg sodium hydroxide to D pool as alkalinity. COD, NH4+-N and TN removal ratio achieved high levels when the mixture reflux ratio R was set200%.The biodegradability of pesticide wastewater improved a lot by adopting the combined process containing coagulation, O3oxidation, H/A/O bio-contact oxidation and sand/carbon filtration. The COD, NH4+-N and TN of final effluent was63mg/L,0.5mg/L and8mg/L, and their removal ratio was99.5%,99.7%and95.1%, respectively. The effluent quality could meet the integrated wastewater discharge standard (GB8978-1996) level one emission standards after the treatment of the combined process.