Construction Of Electro Catalytic -Biological Oxidation Combined Process And Treatment Of Herbicides Wastewater

The pesticide wastewater can not be treated in a conventional biological way due to the characteristics (high chemical oxygen demand, unbiodegradability and toxicity) of the wastewater. Electro-catalytic oxidation which is one method of Advanced Oxidation Processes was confirmed to treat the bio-refractory toxic organic pollutants effectively. Three-dimensional electrode reactor which solved problem of high consumption and low efficiency in the electro-catalytic oxidation process particularly becomes a research hotspot in the field of electrochemistry in recent years. In this paper, the electro-coagulation-electrocatalytic oxidization- biologicalcontact oxidation combined process system was founded with the core technology of electro-catalytic oxidation, and the herbicides wastewater treatment research was conducted.The feasibility study of electro-catalytic oxidation degradation of lab-synthetic pretilachlor, clomazone and 2,4-D wastewater was carried out with Sb doped Ti/ SnO₂ electrode as anode in this paper. The herbicide removal, TOC removal and energy consumption were investigated in order to determine the reaction conditions of the electro-catalytic. Degradation mechanism of pretilachlor was analyzed with the help of HPLC and GC-MS. The biodegradability and toxicity of processed wastewater were investigated by means of microbial respiration curve and algal growth inhibition test. The results showed that electro-catalytic oxidation technology can effectively remove clomazone, pretilachlor, 2,4-D and some intermediate in wastewater. Under the conditions of current density of 20 mA/cm2, Na2SO4 dosage of 0.1 mol/L,initial pretilachlor concentration of 50 mg/L, clomazone concentration of 100 mg/L, 2,4-D concentration of 100 mg/L, treatment time of 60 min, the removal of pretilachlor, clomazone and 2,4-D were 98.8%,73% and 98.7,and the TOC removal were 43.1%, 34.3% and 52.3%; and the energy consumption were 12.4 kWh/m3,12.48 kWh/m3 and 14.88 kWh/m3,respectively. The wastewater was bio-refractory and inhibiting the growth of algae. After electrolysis 120min, all herbicides (pretilachlor, clomazone, 2,4-D) were removed, TOC removal increased to 70.2%, 57.9% and 75.4%, respectively. And the biodegradability of wastewater was improved significantly, algal growth inhibition was significantly reduced. Accordingly it is necessary to establish the new three-dimensional electrode system in order to achieve high efficiency low consumption treatment herbicide wastewater.Quartz ceramic ring particle electrodes loaded with Sb doped SnO₂ were prepared by dip coating method. The micrograph, crystal phase structure of the particle electrodes were characterized by means of scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction techniques.The three-dimensional electrode system was established with Sb doped Ti/SnO₂ electrode as anode and stainless steel as cathode. The COD removal and energy consumption were investigated in order to compare with the efficiency of electrochemical degradation of herbicide wastewater between the three-dimensional electrode and the two-dimensional electrode. The electro-catalytic oxidation degradation mechanism of three-dimensional electrode was investigated be the means of anodic polarization curve, cyclic voltammetry and fluorescence spectroscopy. The results showed that the amount of hydroxyl radicals generated in three-dimensional electrode was much more than in the two-dimensional electrode. Therefore, the COD removal of herbicide wastewater was higher than the two-dimensional electrode. But the energy consumption was less than the two-dimensional electrode. After electrolysis 120 min, the difference of the COD removal and energy consumption between the two systems was 28.7% and 20.2 kWh/ kgCOD, respectively. Determination the potential distribution was measured in the three-dimensional electrode reactor. The results showed that the potential distribution was uniform in the reactor. Fluent software to simulate the flow field distribution was used in the reactor. And the results show that the flow field was uniform in the reactor. Thus a new type of bipolar three-dimensional fixed-bed reactor was designed.The electrocoagulation-electrocatalytic oxidization-biological contact oxidation combined process was constructed and the process parameters were optimized. The results showed that effluent COD and turbidity were 1700 mg/L and 25 NTU, under the conditions of initial COD concentration of 3000 mg/L, current density of 15 mA/cm2 and upstream speed of 4 L/h after electrocoagulation 30 min, meanwhile the energy consumption was 0.78 kWh/m3.The three-dimensional electrode reactor was used to treat the herbicide wastewater in the next step. When the cell voltage was 30 V, the amount of aeration was 50 mL/min, the volume of particle electrodes was 2/3 of the packed bed,the hydraulic retention time was 120 min, the effluent COD was 400 mg/L and the biodegradability of the wastewater was significantly improved and the ratio of B/C was larger than 0.38, meanwhile the energy consumption was 6.9 kWh/m3.Through startup, domestication, operation of bio-contact oxidation reactor, the effluent COD﹤150 mg/L in the biological oxidation process of wastewater biological treatment. On this basis, the establishment of continuous operation of the electrocoagulation- electro-catalytic oxidation - biological contact oxidation process for actual herbicide wastewater. The results showed that the COD removal rate was 95%, wastewater discharge standards to achieve. All of this will provide the basic research data for the practical treatment of the herbicide wastewater.