Study On Electrochemical Treatment Of Salinity Organic Wastewater With Low Energy Consumption

The present study aims at the treatment of industrial saline organic wastewater.The process of organic degradation in the wastewater with low and high conductivity was investigated by directly anodic oxidation and indirectly anodic oxidation by active chlorine and electro-Fenton.Furthermore,the microfluidic reactor and reverse electrodialysis reactor were introduced and the reaction mechanism and mineralization efficiency of these two reactors in treating low and high salinity organic wastewater were explored,respectively.The technicality and economy were also analyzed,aiming to providing theoretical and technical basis for the development of low energy consumption electrochemical organic wastewater treatment technology.In conventional electrochemical reactor,hydroxyl radicals produced by BDD（Boron-doped diamond electrode）anode can completely mineralize phenol in low conductivity wastewater which lead to higher energy consumption due to higher cell voltage in this process.After 6 hours of electrolysis with 16.4mA current intensity,the TOC removal rate achieved 83%with an energy consumption of 57kWh?m³.The indirect oxidation based on chlorine evolution is the main mechanism of phenol degradation in high conductivity using DSA（Ti/RuO₂）as electrode material.However,the refractory intermediates produced in the reaction coupled with partially removal of the TOC resulted in lower current efficiency.After 6 hours of electrolysis with 16.4mA current intensity,the TOC removal rate was 44%while the current efficiency was 21%.In cathodic electro-Fenton process of 6 hours with current intensity of 16.4mA,the accumulative dissolved H₂O₂limited the degradation efficiency of phenol which caused lower TOC removal rate（41%）with a current efficiency of 20%.To optimize the conversion efficiency of H₂O₂ in undivided reactor with carbon felt cathode,the effect of many parameters linked to the anodic process were here evaluated.It was demonstrated that the performances of the process strongly depend on the ratio between the cathode and the anode surface area,the nature of the anode,the mixing ratio and the current density.Higher concentrations of H₂O₂ were obtained under higher mixing rate and surface area ratio between the cathode and anode at-0.9 vs.SCE working potential.The increasing H₂O₂generated in cathode,and the decomposition of H₂O₂ on the anode surface was decreased.This new research model provides an effective way of improving current efficiency and reducing the cost caused by the modification of complex electrolytic equipment and electrode materials.The accumulated concentration of H₂O₂ reaches to 7.3 mM under the area of cathode and anode（Ti/IrO₂-Ta₂O₅）is8cm² and 2cm²,working potential-0.9V vs.SCE and mixing rate 600 rpm.When the surface area ratio of cathode and anode increased from 1 to 4,the accumulation of H₂O₂ increased by 3.4 times.The real wastewater coming from the separation of oil and water phases,characterized by low conductivity was performed by anodic oxidation using boron doped diamond（BDD）.For the electrolysis performed in the conventional cell without supporting electrolyte,a d70%abatement of the TOC was coupled with a high cost of treatment closed to 302.4￥/m³.The addition of sodium sulphate,as supporting electrolyte,obviously reduced the cell potentials and consequently the energetic consumptions and the operating costs.In the presence of 0.05 M Na₂SO₄at 60 mA current intensity,the cost dropped drastically to 27￥/m³ with an abatement of TOC of 69%.However,the addition of SO₄^2-brought high concentration of S₂O₈^2-under high currents,which significantly lowered the concentration of hydroxyl radicals closing to the electrode surface,thus reduced the strength of the direct oxidation by hydroxyl radicals.In the case of the micro cell with a micro-scale inter-electrode distance（50μm）,a significant abatement of the TOC was achieved in short period comparing to the macro cell with low cell potential.Therefor reduced the operating cost to about 15.7￥/m³ with 80%TOC abatement with no addition of supporting electrolyte.Different concentration of wastewater containing NaCl in the reverse electrodialysis stacking reactor were used to produce electric energy,which based on the use of many pairs of anion and cation exchange membranes situated between DSA-Cl₂（Ti/RuO₂）anode and carbon felt cathode.The wastewater with high concentration of NaCl were treated by chlorine indirectly anodic oxidation.The effect of the number of cell pairs of membranes and the NaCl concentrations on the wastewater treatment were investigated.When the salinity gradients simulated the real petroleum wastewater with 143 g L^-1 and 78.77 mg L^-11 NaCl in concentrated and diluted solutions,under 60 cell pairs of membranes and 3.7?external resistance,a continuously formic acid removal of 70%was obtained in concentrated solution in sequence reaction.Here we provided an efficient and stable technology to remove organics form high salinity wastewater while the energy consumption was controlled well without external electric field.