Detailed characterisation of fouling deposition and removal on a hollow fibre membrane by direct observation technique

Direct observation (DO) technique allowed the visualisation of fouling deposition and removal in real time and in a non-invasive manner. The DO setup was relatively simple and featured a crossflow hollow fibre membrane module, a microscope and a video camera. Transmembrane pressure (TMP) and fouling height were observed to characterise in detail the mechanisms of fouling deposition. At the end of the bentonite filtration at crossflow velocity (CFV) of 4 mm/s, the fouling thickness was measured as 85 μm. Specific cake resistance was calculated by taking into account the cake height measurements with this technique. The DO technique also allowed unique insights into the mechanisms related to the fouling removal. The effect of bentonite concentrations were assessed to provide credibility of the technique.

This setup provided in-situ observation tangentially to the surface of the hollow fibre membrane, therefore the cake height during filtration and cleaning was clearly quantified. The hydraulic membrane performances were correlated with the deposit thickness and allowed the calculation of the specific cake resistance during membrane filtration. Using the value of fouling thickness from the DO experiment, allowed more accurate result compared to the fouling thickness estimation from TMP–flux correlation. It was observed that the movements of particles near the membrane surface during filtration generally followed this process: (1) particles movements in the bulk flow, (2) descending motion, (3) moving in-contact and (4) stopping on the membrane surface. Exciting phenomena occurred during fouling removal, the backwashing force and crossflow shear caused cake expansion followed by fluidisation of the fouling layer. This DO technique has significant potentials to be explored for the MBR study, such as filtration of activated sludge and effect of bubbling. As with all optical microscopy, solution turbidity is a limiting factor in its application. Acknowledgments The authors gratefully thank the Australian Research Council for the financial support of this study.