Cake Properties In Microfiltration Of Activated Sludge Suspension

Membrane bioreactor(MBR)has been widely used in wastewater treatment and reuse due to its advantages such as good effluent quality,simple operation and maintenance,and small space requirement.However,the appearance of membrane fouling limits its wide application.Sludge cake formation on the membrane surface is generally considered as the dominant reason for membrane fouling in MBR.Therefore,in order to alleviate membrane fouling in MBR,it is significant to acquire extensive knowledge about cake properties.In this study,microfiltration experiments were performed with activated sludge suspension as the feed solution and 0.1?m PAN as the filter medium to analyze cake properties from both time and location,and the influence of operating conditions on cake properties.Corresponding results were obtained as follows:(1)A method was proposed to determine characteristic parameters in Tiller's power-law expressions,which made it available to determine local cake properties in microfiltration using conventional cake filtration theory.(2)An equation to determine cake thickness was developed.Results determined by this equation showed that the variation trend of cake thickess with time could be divided into two stages.In the first stage,the growth rate of cake thickness decreased fast with time;while in the second stage,it rarely changed with time.(3)An equation to determine relative cake resistance(Rx/ Rc)was developed.Results determined by this equation showed that when the relative thickness(x/L)< 0.2,Rx/ Rc increased fast with x/L;it rarely changed with x/L when x/L > 0.2.(4)Cake properties such as porosity,specific cake resistance,permeability,solidosity,pore liquid pressure and solid compressive pressure varied with time and location.Specially,as the distance from the membrane surface increased,cake porosity,cake permeability and pore liquid pressure increased while specific cake resistance,cake solidosity and solid compressive pressure decreased.As time went by,cake porosity,cake permeability and pore liquid pressure decreased while specific cake resistance,cake solidosity and solid compressive pressure increased.(5)The cake structure was asymmetric and consisted of a skin layer and a loose layer.The loose layer was above the skin layer which was close to the membrane surface.More than 70% of cake resistance concentrated upon in the skin layer,while its thickness was only 20% of cake thickness.Cake properties such as cake porosity,specific cake resistance,pore liquid pressure and solid compressive pressure varied sharply with location within the skin layer;however,it was reverse for the loose layer.