Effect of Velocity Gradient and Floc Characteristics on Membrane Fouling

Abstract

Effects of coagulation-flocculation pretreatment and settling on the flux performance of ultrafiltration membranes were studied and results were interpreted in terms of structural characteristics of flocs and membrane cakes formed by the flocs. To find out the fractal dimension of membrane cake experimentally, a new non-invasive method was proposed and tested by applying to experimental data of membrane filtration. In first phase (section A) of this study, two types of clay suspensions (medium and low turbidity) were used within a pH range of 5.5-7.5. Clay flocs were produced under low and high conditions with an alum dose range of 3-6.5mg/L. In second phase (section B), humic acid flocs were produced under three conditions (50,400, 17,820 and 1,890) within a pH range of 4.8 to 5.5 at an alum dose of 70 mg/L. Different coagulation-flocculation conditions produced flocs of different microstructures and ultimately membrane cakes of different permeabilities. Larger flocs with higher fractal dimension , were produced by high and conditions of medium and low produced flocs of relatively low . Experiments where the settling was provided to suspended clay and humic acid flocs, prior to ultrafiltration, led to the production of very dense membrane cakes with less porosity and high specific cake resistance (αm). In experiments where the settling was not provided to suspended clay and humic acid flocs prior to ultrafiltration, the membrane cakes were highly porous having lower specific cake resistance (αm). As a result, high -S experiments translated into highest specific cake resistance (αm) and medium -N experiments translated into lowest specific cake resistance (αm). Fractal dimension of the flocs produced using humic acid solution were well correlated with fractal dimension of membrane cakes of constituent flocs whereas, fractal dimensions of clay flocs were poorly correlated to fractal dimensions of membrane cakes