| Membrane separation is typically used for advanced treatment of leachate in municipal solid waste（MSW）incineration plants.The membrane concentrated leachate produced by this process contains high concentrations of pollutants and must be properly treated to prevent environmental problems.The commonly used treatment methods for concentrated leachate,such as spraying it into the incinerator or recirculating it through the stored MSW,will generate secondary pollutants that damage the treatment system or significantly reduce its efficiency.Advanced oxidation processes（AOPs）are alternative treatment methods that can completely remove many pollutants.However,when using only AOPs to treat concentrated leachate,the effect can be poor,and the treatment cost is relatively high due to the highly concentrated pollutants and complex quality of the leachate.Therefore,treatment that combined coagulation and electrochemical enhancement was examined in this research.The efficiencies of removing pollutants such as chemical oxygen demand（COD）,biochemical oxygen demand（BOD）,ammonia nitrogen（NH3-N）,total nitrogen（TN）,color number（CN）,turbidity（TUB）and solution conductivity（SC）were used to determine the optimal treatment conditions.A variety of analytical methods,such as spectral analysis,gas chromatography-mass spectrometry and flow field-flow fractionation analysis,were used to demonstrate the feasibility of the combined treatment process.In addition,the transformation characteristics and removal mechanisms of pollutants（especially organic compounds）were investigated.The research results not only provide a theoretical basis and process guidance for the effective treatment of membrane concentrated leachate,but also provide reference and have practical value for the effective treatment of other highly concentrated organic wastewaters with high salinity and poor biochemical properties.The main achievements of this research were the following.（1）The characteristics of membrane concentrated leachate from six MSW incineration plants were studied.The COD concentrations were extremely high(2000–30000 mg L-1),and due to the high content of refractory organic matter,tthe biodegradability was very low（BOD/COD in the range 0.035 to 0.056）.Many metallic elements were detected,such as K,Na,Ca,Mg,Fe,Zn,Cr and Co.The concentrations of Cl-and SO42-were also high,(Cl-concentration as high as 140,000 mg L-1).Three principal components（PC1,PC2 and PC3）were identified that reflected the characteristics of the membrane concentrated leachate.Among them,PC1 and PC2 were significantly correlated to the characteristics of dissolved organic matter（DOM）in the leachate.（2）The optimum condition and main technical parameters for treatment of reverse osmosis membrane concentrated leachate（RO CL）,nanofiltration membrane concentrated leachate（NF CL）and tight ultrafiltration concentrated leachate（TU CL）by the combined coagulation and UV-enhanced electrochemical technique（“coagulation+UV-TSE”）were determined through single-factor experiments.UV radiation was applied to enhance the treatment effect during the UV-TSE process.First,coagulation was applied as the pretreatment process.Poly-silicate aluminum ferric salt（PSAF）and polyacrylamide（PAM）were used as the optimal coagulant and coagulant aid,the dose ratios between PASF and PAM for RO CL,NF CL and TU CL respectively were 833:1,500:1 and 625:1.Subsequently,the coagulated effluent was treated by the UV-TSE process.The UV-TSE process involved two stages,the first of which was electrochemical oxidation（EO）.Results showed that during the EO process,Ti/Ru-Ir should be used as the anode and graphite as the cathode.Furthermore,when the initial p H of the coagulated RO CL,NF CL and TU CL samples was 7,7 and 9,respectively,the optimal current densities were 60m A cm-2,80 m A cm-2 and 80 m A cm-2,respectively,and the corresponding electrode gaps were15 mm,15 mm and 10 mm.The EO stage of the electrochemical reaction was carried out for180 min.In the second stage of the process,it was determined that Al should be used as the anode and graphite as the cathode.Furthermore,when the p H of coagulated RO CL,NF CL and TU CL samples was 7,the current density should be 40 m A cm-2,60 m A cm-2 and 60 m A cm-2,respectively,the electrode gap should be 10 mm（for all three leachates）,and the electrolysis time should be 60 min,180 min and 180 min,respectively.（3）The characteristics and efficiencies of removing pollutants from RO CL,NF CL,and TU CL samples were studied.More than 70%of COD and light absorbing substances at 254nm wavelength(UV254),more than 90%of CN and more than 80%TUB were removed during the coagulation process.However,the removal of TN was relatively poor,and the NH3-N concentration increased（by as much as 300%）.Following UV-TSE treatment of the coagulated leachates,more than 97%of COD and TOC,more than 99%of UV254,CN and TUB and more than 93%of TN were removed.Furthermore,the concentrations of NH3-N and Cl-were less than 5 mg L-1 and 400 mg L-1,respectively,while the p H of effluent was 6–7.Notably,the p H and concentrations of NH3-N,CN,Cl-and TUB satisfied commonly used environmental standards in China,such as GB8978-1996,GB16889-2008 and GB18918-2002.（4）The transformation characteristics and removal mechanism of organic substances were analyzed.The coagulation process mainly relied on adsorption bridging and adsorption neutralization to remove organics,but the distribution of DOM molecular weights was not significantly changed.Organics were removed by direct oxidation and/or indirect oxidation during the UV-TSE process.Some macromolecular organics were completely oxidized or decomposed into relatively small molecular weight organics,and the average molecular weight of DOM was significantly reduced.UV radiation stimulated the electrochemical system to generate free radicals（i.e.,·Cl,·OH,·Cl O and·Cl2-）that had high oxidation potential and easily reacted with refractory pollutants（i.e.,humus）,thus improving the overall pollutant removal efficiency.（5）The transformation mechanism of Cl during the UV-TSE process was preliminarily explored through monitoring the change of concentrations of Cl-,free chlorine and total chlorine during treatment.Chlorides were transformed to free chlorine and total chlorine by the electric current,and these products subsequently reacted with pollutants.UV radiation prompted chemical reactions between substances such as Cl-,HCl O,and Cl O-to form free radicals.Some free chlorine was consumed to generate free radicals with higher oxidation potential that more easily reacted with pollutants.In addition,UV radiation improved the utilization rate of the oxidized species in the system,thereby improving the pollutant removal efficiency.Therefore,the concentrations of free chlorine and Cl-during UV-TSE treatment were lower than those during treatment by TSE alone.After treating the membrane concentrated leachate by the“coagulation+UV-TSE”process,the Cl-concentration was reduced from 7000–25000 mg L-1 to 100–400 mg L-1,which satisfied the discharge standard for Cl-concentration as stated in Discharge Standards for Water Pollutants in Sichuan Province（DB51/190-93）. |
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