Hydrophobic Modificaton Of Hollow Fiber Membrane And Its Application To Concentrate Landfill Leachate In Membrane Distillation

Landfill leachate is a kind of wastewater containing high chemical oxygen demand（COD）,ammonia nitrogen and heavy metal ions.Dual membrane method（membrane bioreactor+nanofiltration/reverse osmosis）is the mainstream treatment process nowadays,but it produces highly polluted concentrates,and nanofiltration and reverse osmosis concentrates account for about 20-40%of the treated volume.Finding a treatment process with good treatment effect and high water recovery rate is extremely important for landfill leachate reduction.Membrane distillation（MD）,as an emerging membrane technology,can achieve deep concentration due to its advantages of operating at low temperature and atmospheric pressure,high salt cut-off rate,no evaporator corrosion problem,and the use of renewable energy,and most importantly,the driving force of membrane distillation is the vapor pressure difference,which is less affected by the salinity of the solution.Therefore it can be used as an alternative process to recover water from high saline wastewater.However,due to the complex water quality of leachate,membrane contamination and membrane wetting are easily caused.From the perspective of preventing membrane contamination and membrane wetting,this thesis explores effective pretreatment measures,prepares contamination-resistant hydrophobic membranes and uses them for the concentration of actual waste leachate.Firstly,single-factor experiments were used to determine the optimal coagulant as well as the optimal coagulation conditions.Chemical oxygen demand（COD）,ammonia nitrogen（NH₃-N）,and absorbance(UV₂₅₄)were used to evaluate the removal effect of typical pollutants.Three coagulants,polymeric aluminum chloride（PAC）,polymeric ferric sulfate（PFS）and titanium tetrachloride（Ti Cl₄）,were compared,among which PFS showed the best treatment effect.After that,the optimum coagulation conditions were optimized:PFS dosage 8 g/L,polyacrylamide（PAM）dosage 0.1 g/L,initial p H 6,fast stirring 300 rpm/min for 2 min,slow stirring 80 rpm/min for 10 min,and coagulation at room temperature,where COD,ammonia nitrogen and UV254 were removed by 45%,21%and 83%.The coagulated waste leachate was then heated,and the best removal of COD,NH₃-N and UV₂₅₄ was achieved at a heating time of 3 h,with31.4%,45.2%and 16.2%removal,respectively.Next,polyvinylidene fluoride（PVDF）hollow fiber membrane is used as the base membrane,and the emulsion is fixed on the surface of PVDF membrane by impregnation-curing method to improve the hydrophobicity of PVDF hollow fiber membrane.The effects of coating time on the morphology and properties of the composite membranes were investigated,and the contamination resistance and wetting properties of the composite membranes were studied.When the coating time was 20min,the water contact angle and LEPw value of the composite membrane reached 115°and 0.75 MPa,respectively.In the direct contact membrane distillation（DCMD）concentration experiment,the composite membrane treated with 3.5 wt.%Na Cl,50mg/L HA and 14.7 m M Ca SO₄ aqueous solution,respectively,the membrane flux was stable and the condensate conductivity was kept within 10μS/cm,showing excellent anti-fouling and anti-scaling performance.Finally,in the study of deep concentration of waste leachate by membrane distillation,it was found that pretreatment can effectively remove humus from waste leachate and can effectively alleviate membrane contamination and membrane wetting.The permeate side conductivity of both the original and composite membranes was above 1000μS/cm before concentration pretreatment,and the permeate side conductivity of both the original and composite membranes was less than 25μS/cm after pretreatment.The decay rate of flux of the modified composite membranes was less than that of the original membrane,which showed good anti-pollution ability.In the scale-up test,the degree of membrane flux decay was basically the same as that of the panel.Due to the long operation test,the membrane surface showed different degrees of contamination,which led to a decrease in the initial flux in each cycle,but the COD removal rate in the permeate side condensate was still above 92%and the conductivity was kept below 100μS/cm,maintaining excellent treatment capacity.Chemical cleaning can partially restore the membrane flux with some effect.