Study On The Performance Of Coagulation-advanced Oxidation Process For Treating NF/RO Concentrate Of Biologically Treated Landfill Leachate

After biological treatment,the effluent quality of landfill leachate usually can’t meet the discharge standard requirements,and it needs to be further treated by nanofiltration（NF）and reverse osmosis（RO）technologies.As a result,landfill leachate NF concentrate（LLNC）and RO concentrate（LLRC）containing high content of inorganic ions and dissolved organic matter（DOM）are produced which account for 20%-30%of the leachate.In the past,LLNC and LLRC were usually treated by recirculation method.However,it resulted in the accumulation of salt ions in the leachate and affect the following biological treatment efficiency of the leachate.Therefore,to find a suitable treatment and disposal technology for the concentrate of biologically treated leachate is the key problem needs to be solved urgently.Based on this demand,the first step of this study was to make clear the characteristics of LLNC and LLRC to provide basic information for the selection of treatment methods.Then,LLNC and LLRC were treated by coagulation method,and the organic matter removal efficiency by the traditional Fe Cl₃ and Al Cl₃ coagulants was evaluated.After that,adsorption treatment was used to further remove the organic matter from the coagulation effluent,and the DOM removal mechanism during coagulation and adsorption process was discussed.In order to improve the DOM removal efficiency during LLNC and LLRC coagulation process,a new coagulant ZrCl₄was selected to replace Fe Cl₃ and Al Cl₃,and then was applied for LLNC and LLRC treatment,and its performance was evaluated accordingly.Finally,after ZrCl₄ coagulation,the effluent was further treated by UV/SPC advanced oxidation system to remove the residual DOM and scaling ions at the same time.The main results of this study are as follows:（1）The characteristics of LLNC and LLRC was systematically analyzed,and it can be defined by the high content of salt ions and refractory organics,and also the deep color in the concentrate.The organics in LLNC and LLRC were mainly small molecular organics and hydrophobic organics.The proportion of small molecular organics in LLRC organics was significantly higher than that in LLNC.（2）The DOM in LLNC and LLRC can be efficiently removed by coagulation treatment,and the coagulation removal performance of iron salt was obviously better than that of aluminum salt,and Fe Cl₃ was selected as the optimal coagulant.During coagulation treatment of LLNC and LLRC with Fe Cl₃,the optimal initial p H was 6.0,and the optimal coagulant dosage was 3.0 m M and 4.0 m M,respectively.Under this condition,the TOC removal efficiency reached 59.62%and 60.92%,respectively.The coagulant aid showed little effect on the coagulation removal of DOM from LLNC and LLRC.The removed DOM was mainly macromolecular hydrophobic organic substances in LLNC and LLRC during coagulation treatment,and the DOM remained in the coagulation effluent was mainly small molecular organic matter.These remained small molecular organic matter in coagulation effluent can be further removed by adsorption with coal-based powder activated carbon,and the adsorption removal effect was better under acidic conditions.Under the optimal dosage of powdered activated carbon of 2.0 g/L and 0.5 g/L in LLNC and LLRC coagulation effluent,the TOC removal rate reached 88.02%and 84.06%,respectively.The DOM removed by adsorption was mainly hydrophobic organic compounds,especially small molecular organic compounds.Therefore,the organics in LLNC and LLRC can be efficiently removed by the coagulation-adsorption combined process,and it showed complementary advantages of coagulation and adsorption for the removal of organics with different molecular weight.（3）The new coagulant ZrCl₄ was adopted to treat LLNC and LLRC,and the optimal conditions of ZrCl₄ coagulation treatment was determined,that is,the dosage of ZrCl₄ was 5.0m M,the initial p H was 6.0,no coagulation aid was added.Under this condition,the removal rate of TOC in LLNC and LLRC reached 73.30%and 74.16%,respectively,and it was significantly better than that of traditional Fe Cl₃ and Al Cl₃ coagulants.The ZrCl₄ coagulation process mainly removed the hydrophobic organics in LLNC and LLRC.Compared with the traditional Al Cl₃ and Fe Cl₃ coagulants,the superiority of ZrCl₄ coagulant was due to not only the higher removal rate of macromolecular and medium molecular organic matter but also the relatively higher removal efficiency for small molecular organics in the concentrate.This can be attributed to the higher Zeta potential and stronger electrical neutralization ability of ZrCl₄flocs,the larger size of flocs formed by ZrCl₄ and the stronger organic matter adsorption capacity,and at the same time,the formed ZrCl₄ hydrolysates with more positive charge and stronger electric neutralization ability under the optimal coagulation conditions.Therefore,ZrCl₄ showed the best coagulation removal effect on DOM.In addition,the residual metal content in the ZrCl₄ coagulation system of LLNC was much lower than that of Fe Cl₃ and Al Cl₃,which was only 2.11%of the added Zr.After regeneration and being recycled for three times,the TOC removal efficiency only decreased by 2.73%in the coagulation process with the recovered Zr coagulant from ZrCl₄ coagulation sludge,and which confirmed the feasibility of the regeneration and recycling of the zirconium coagulated sludge.（4）UV/SPC system can be used for advanced treatment of coagulation effluent of LLNC and LLRC to further remove the contained DOM.Under the optimal condition that the dosage of SPC was 20.0 m M,the removal rate of TOC in LLNC and LLRC coagulation effluents reached to 65.18%and 28.13%,respectively.In the UV/SPC system,the initial p H and SO₄^2-content had no obvious effect on the removal of organic matter,while Cl^-concentration showed a certain inhibitory effect.Moreover,affected by the higher Cl^-content,under the same oxidation conditions of UV/SPC system,the removal efficiency of DOM in LLNC coagulation effluent was significantly higher than that of the LLRC coagulation effluent.After being oxidized by UV/SPC system,the refractory organic compounds（humic acid and fulvic acid）in LLNC and LLRC were completely degraded and removed,and part of the residual hydrophobic small molecular organic compounds in the coagulation effluent were oxidized,decomposed and mineralized,while the other part of them was decomposed into hydrophilic organic matter with smaller molecule.Therefore,all the DOM in the final effluent were small molecule organic matter,and the content of hydrophilic DOM was significantly improved.In the UV/SPC advanced oxidation system,the detected active radicals were·OH,CO₃˙^-and O₂˙^-,and·OH played the most important role in the oxidation process.In addition,scaling ions in the leachate can be removed by UV/SPC system.When the dosage of SPC was 20.0 m M,the removal rate of Ca²⁺in LLNC and LLRC reached 90.17%and 86.9%,respectively,and the removal rate of Mg²⁺in LLNC and LLRC reached 96.7%and 97.7%,respectively.The scaling problem that may exist in the subsequent desalination process of the leachate concentrate coagulation effluent could be solved.