Abstract
In the submerged- type membrane separation equipments, membrane bioreactors or membrane separation drinking water production plants, air scrabbling in many cases plays a major role in the membrane surface cleaning. This study is aimed at elucidating mechanism how rising air bubbles induce shear stress. Shear stress acting on a vertically set flat sheet is directly measured changing airflow rate and bulk water viscosity. Effect of air flowrate and bubble diameter on shear stress was investigated. Time-series data of shear stress variation working on flat-sheat membrane surface caused by bubble flow showed intense fluctuation, which might be caused by the behavior of rising bobbles. Calculating time-averaged value plus 3 times of standard deviation for each time-series data as "maximum" effective shear stress to detach substances off membrane, it was shown that higher viscosity liquid can give higher effective shear stress, which tend to show plateau if air flow flux is more than 1.3cm3/cm2/s.
