Removal of Complicated Pollutants from Coking Wastewater by Membrane Distillation: Mechanism and Process Enhancement

Author:Ren Jing

Supervisor:cheng fang qin lou hui ru


Degree Year:2019





Coking wastewater contains multiple organic compounds such as phenols,polycyclic aromatic hydrocarbons and heterocyclics,as well as high concentrations of inorganic salts.It possesses significant biological toxicity,dark color and irritating odor,which is one of the touchy industrial wastewaters to be treated worldwide.Membrane distillation(MD)is a novel and efficient membrane separation process,which is thermally driven by transmembrane vapor pressure difference.It has advantages in the desalination and purification of wastewater,because of its high rejection rate,light membrane fouling and strong adaptability to wastewater with complex contaminants.Furthermore,with the developments of MD integrated technologies and new membrane materials,the application of MD has been continuously expanded in recent years.For instance,it has been successfully applied to the advanced treatment of various industrial wastewaters including textile wastewater,dyeing wastewater,pharmaceutical wastewater,and oily wastewater.In addition,the use of waste heat further reduces the operating costs of MD.However,there is still a lack of systematic research on the treatment performance and removal mechanisms of MD in treating coking wastewater.Therefore,this paper studied the diffusion behaviors and removal mechanisms of various complex pollutants in coking wastewater during MD process.Biodegradation and coagulation were integrated with MD to improve its performance in treating coking wastewater.In addition,a novel hydrophilic/ hydrophobic composite membrane was constructed to enhance the anti-fouling and anti-wetting properties of MD process,in order to meet the reuse purpose of coking wastewater.The main contents and conclusions of this thesis are as follows:Firstly,the feasibility of raw coking wastewater treatment by MD was investigated and the effect of coking wastewater temperature on the MD performance was studied.The migration and diffusion of pollutants during MD process were analyzed by excitation-emission matrix(EEM)fluorescence spectroscopy.The interaction between the contaminants and membrane surface was evaluated using a scanning electron microscope-energy dispersive X-ray spectroscopy(SEM-EDS).It has been found that MD could effectively retained humic acid-and fulvic acid-like organic matter in coking wastewater.The removal rate of TOC in wastewater reached more than 85% by MD,and the decolorization effect was remarkable.However,some small molecular weight pollutants such as phenols and aromatic proteins could penetrate into the distillates and became the main factor affecting the distillate quality.In addition,the feed temperature of coking wastewater also played a significant role in the pollutant’s rejection and diffusion during MD process.When the feed temperature was 30-40 °C,the adsorption of pollutants on the membrane surface was light and the inorganic pollutants were predominant,but the permeate flux was relatively low.Permeate flux at 50 °C was obviously improved,and the pollutant removal rate was above 90% to guarantee a good permeate quality.When the feed temperature was increased to 60 °C,the diffusion of phenols was intensified,and the distillate was deteriorated.For a comprehensive comparison,it was preferred that 50 ° C could be a suitable feed temperature of MD process.Secondly,a biodegradation-MD integrated system was applied to strengthen the coking wastewater treatment,since the MD performance was found to be mainly affected by the small molecular weight and volatile organics,which are generally biodegradable.The coking wastewater was sequentially subjected to anaerobic and aerobic biodegradation,and the effluents from anaerobic and aerobic tanks were used as the feed solutions of MD,respectively.High-throughput sequencing of Illumina Mi Seq was used to analyze the relationship between microbiological community distribution and characteristic pollutants removal,in order to further explore the effects of different biochemical processes on the MD performance.Results showed that biodegradation could effectively reduce the concentration of pollutants in wastewater,which might be beneficial to relieve membrane fouling and increase the permeate flux.It should be noted that different biological processes had distinct enhancement mechanisms for MD performance.In the anaerobic process,Thauera,Hyphomicrobium and Comamonas dominated the microbiological community,leading to a better removal effect of polycyclic aromatic hydrocarbons(PAHs).However,the relative abundance of phenolic compounds in the anaerobic effluent increased,which might contribute to the enhanced diffusion of volatile organics and wetting phenomenon in the MD process.However,the permeate quality of the anaerobic-aerobic-MD system was significantly improved,and membrane wetting was drastically mitigated.This was mainly attributed to the effective removal of volatile pollutants including phenols and ammonia from the wastewater by the aerobic bacteria such as Thiobacillus and Pseudomonas.Thirdly,coagulations with polyaluminum chloride(PAC)and polyaluminum chloride/polyacrylamide(PAC/PAM)were used as control strategies of membrane fouling and wetting during a prolonged MD process of 72 h.This because our previous studies have found that a small amount of organic and inorganic salts still could be detected in the distillates even with the assistance of bio-degradation.Thus,EEM spectroscopy and gas chromatography-mass spectrometry(GC-MS)were used to further explore the correlation between organic diffusion and membrane wetting.The study found that phenols and small molecular aromatic proteins in coking wastewater preferentially penetrated into the distillate,while humic acid-and fulvic acid-like pollutants diffused relatively slowly.Interestingly,membrane wetting occurred coincidently with the penetration of phenolic and heterocyclic organics.The wetting rate was obviously correlated with the feed composition and membrane surface properties.PAC pre-coagulation was beneficial to increase initial membrane flux and shorten the wetting time.PAC/PAM pre-coagulation not only removed contaminants which could accelerate membrane wetting,but also retarded membrane wetting by the complexation with organics.The deposition of these complexes on the membrane surface introduced a gel-like hydrophilic layer on the hydrophobic substrate,which established a hydrophilic-hydrophobic composite membrane structure with superior wetting resistance.These novel insights would be beneficial to wetting control in MD process for coking wastewater treatment.Finally,a novel hydrophilic-hydrophobic composite membrane was fabricated by coating graphene oxide(GO)on the polytetrafluoroethylene(PTFE)substrate,since this structure was found to be beneficial to improve the membrane resistance towards fouling and wetting based on our previous study.GO nanoparticles with two-dimensional sheet structure were prepared by Hummers method,and the interlamellar spacing of GO was 0.762 nm calculated by XRD characterization.Polyvinylidene fluoride(PVDF)was used as a binder to uniformly coat GO on the surface of the hydrophobic membrane.The morphology,structure and properties of the composite membrane were analyzed through atomic force microscope(AFM),SEM-EDS,contact angle and Fourier transform infrared spectroscopy(FTIR).It was found that GO surface modification introduced multiple oxygen-containing groups,including hydroxyl,carboxyl and epoxy,on the surface of the composite membrane,rendering the surface layer of the composite membrane to be more hydrophilic.Therefore,the composite membrane surface exhibited the characteristics of in-air hydrophilic and underwater super-oleophobic,which effectively prevented the adsorption and diffusion of hydrophobic organics from coking wastewater to the membrane surface.In addition,due to the selective sorption and nanocapillary effect of GO,the mass transfer of water vapor through the micro-porous membrane was enhanced,while the rejection of pollutants was also improved.Moreover,the good thermal conductivity of GO was beneficial to reduce the temperature polarization and to relieve the flux decay.As a consequence,GO/PTFE composite membrane applied in MD is a competitive technology for coking wastewater treatment with high flux and less fouling propensity.