Structure And Filtration Characteristics Of Membrane Fouling Layer For Wastewater Treatment

Membrane bioreactor(MBR)is a highly effective wastewater treatment technology,but its wide application is restricted by membrane fouling.Membrane fouling is mainly caused by a fouling layer formed on the membrane surface.Membrane fouling layer has complex pore structure and interior fluid flow during filtration process.In this work,submerged membrane fil tration experiments were conducted to investigate the filtration characteristics of fouling layer.Then three dimensional structure of the fouling layer was reconstructed and the computational fluid dynamics(CFD)method was used to analyse the flow behavior with different pore structures.Pretreatment methods were also studied to control membrane fouling.First of all,a submerged membrane filtration system was set up to conduct filtration experiments.Active sludge was divided into two parts: suspended solids and supernatant and three filtration curves were obtained.The whole filtration process was divided into two stages according to Hermia blocking law.Filtration processes were analysed under different areation intensities.Membrane fouling layers were obtained after the filtration experiments.Scanning electron microscope(SEM)and confocal laser scanning microscopy(CLSM)were used to observe microstructure of these layers.Slice images of the fouling layer were acquired after fluorescent staining and the three dimensional structure of the fouling layer was reconstructed.Structure parameters were obtained and the CFD method was used to calculate permeability of the fouling layer.The permeability value from CFD results agreed well with that from seepage experiments,showing the validity of the image processing and the CFD calculation.Specific area,volume ratio,and aggregation state of four kinds of pollutants were analysed based on the image processing results.Sodium alginate and yeast power were used as model pollutants for filtration experiments.The composition change of fouling layers were analysed.Flow through the fouling layers were calculated with different pore structures.Large pores contributed most of effective permeability of the fouling layer.Coexistence of the two pollutants caused a decline of large pores and permeability loss of the fouling layer.Calcium chloride was used as coagulant to control membrane fouling.From the results of filtration experiments and CLSM images,it was concluded that coagulants increased the flow flux in a certain range of doses.Add potassium permanganate with calcium chloride could enhance the coagulation process.Besides,sodium hypochlorite was used as oxidant to control membrane fouling.It was found that addition of sodium hypochlorite two times had better results than one time.