Study On The Treatment Process Of Landfill Leachate Reverse Osmosis Concentrate By Bipolar Membrane Electrodialysis

Sanitary landfill is one of the main methods of domestic waste disposal in China,and landfill leachate is a reaction product of this process.The coupled process of "biological treatment-nanofiltration-reverse osmosis" is widely used to treat landfill leachate because of its good effluent quality.However,the landfill leachate reverse osmosis concentrate（LLRC）,which is produced after the treatment of leachate by reverse osmosis,is more difficult to be treated than leachate because of containing a large amount of inorganic salts,refractory organics,and toxic and hazardous substances.Bipolar membrane electrodialysis（BMED）is highly efficient in separating inorganic salts and obtaining inorganic acids and bases and has certain advantages in treating highsalinity organic wastewater.In this paper,BMED was used to treat LLRC for the separation and recovery of inorganic salts,and the effect of voltage,initial acid-base concentration,and membrane surface linear velocity was investigated.It was found that at optimum operating parameters（18 V applied voltage,0 mol/L acid-base initial concentration,14.52 cm/s membrane surface linear velocity）,the acid-base concentration,desalination rate and current efficiency were higher and energy consumption was lower.This process recovered 0.41 mol/L of acid-base production from the LLRC.The recovered acid contained 88 wt% HCl and 12 wt% HNO₃.Meanwhile, the recovered bases were 52 wt% Na OH and 45 wt% KOH.A protonation approach was taken to control the electromigration of negatively charged organics.In addition,the arsenic and heavy metal levels were below the relevant standards,which ensured the safety of the recovered acids and bases.Subsequently,the economics of the BMED process for LLRC treatment was analyzed,and a net profit was $27.630 for the treatment of 1 m³ LLRC by BMED after excluding the fixed costs,which showed remarkable economic benefits.To control the migration of organics into the acid-base chamber,the existence states and transmembrane transport mechanisms of organics during the BMED process were investigated.First,a total organic carbon analyzer,fluorescence spectrophotometer,ultraviolet spectrophotometer（UV-Vis）,Fourier infrared spectrometer（FT-IR）,and gas chromatography-mass spectrometry（GC-MS）were used to qualitatively and quantitatively analyze the status of organics transport across membranes.The results showed that negatively charged organics and a small fraction of electrically neutral organics in LLRC were transported across the membrane to the acid chamber by electric field forces and free diffusion,respectively.These organics exhibit a complex structure as well as strong toxic and harmful properties,reducing the safety and value of the acid.The content of organics in the base chamber is low and they were structurally simple and electrically neutral substances（hydrocarbons,protein-like substances,etc）.Additionally,the effect of organics on the BMED membrane modules was investigated using field emission swept surface electron microscopy（SEM）with energy spectrometry（EDS）and Fourier infrared spectroscopy（FT-IR）.It was found that the surfaces of the ion exchange membranes were slightly contaminated by organics and inorganics.The cation exchange membrane（CEM）was contaminated worse than the anion exchange membrane（AEM）because of the co-contamination of calcium and magnesium with protein-like substances.The organics in the LLRC were controlled to be electrically neutral by pH adjustment,and then the type and concentration of organics in the acid and base chambers were similar.The study elucidated the existence states and transmembrane transport mechanisms of organics in the LLRC during BMED operation.To the zero standards,the Fenton-Cl system was adopted to degrade refractory organics in the desalted LLRC.First,the parameters of Fenton oxidation were optimized and the optimum parameters were selected by the amount of residual TOC and TOC removal rate.It was found that suitable pH（pH=3）,hydrogen peroxide（1000 mg/L）,ferrous catalyst（250 mg/L）,oxidation time（60 min）,and chloride concentration（1000 mg/L）could promote the production of hydroxyl radicals or other free radicals.Under the optimum conditions,TOC and COD in the effluent were 25 and 34.61 mg/L after the oxidization,respectively,which met the pollution control standards for domestic landfills and the reuse of urban recycling water-Water quality standard for industrial uses in China.The organic species and fractions before and after oxidation were analyzed by fluorescence spectrophotometer and GC-MS.It was found that the organic species were converted from large molecules of fulvic acid,humic acid,and protein-like substances to small molecules of tryptophanlike substances before and after oxidation.GC-MS analysis showed that the organic fractions（15）were significantly lower than those before oxidation（28）,and were mostly unbranched n-alkanes with simple structures and minimal toxicity.This result demonstrates the good water quality and safety of the effluent after the oxidization and proves that Fenton-Cl is an effective method for the treatment of refractory organic wastewater.In this paper,the migration of organics across the membrane was controlled by protonation.It is demonstrated that the treatment of LLRC by BMED has the advantages of high desalination efficiency,good treatment effect,product safety,and significant economy.The desalted LLRC was treated by Fenton-Cl oxidation,and the effluent has good water quality and low hazardous substances.The study provides a new method to achieve resource recovery and zero discharge of LLRC.