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Abstract
To date, membrane fouling characterisation has been most commonly assessed via hydraulic performances and rarely by visualisation techniques. However, fouling visualisation can provide more detailed characterisation on the morphological structure of the material deposition on the membrane surface. The experimental approach using direct observation (DO) technique allows the visualisation of fouling deposition and removal from hollow fiber membrane in real time and in a non-invasive manner for model and real mixtures of particulates and macromolecules.
Since the specific cake resistance of fouling layer can estimated from the
transmembrane pressure (TMP) and fouling thickness values measured directly, the number of assumptions is reduced significantly and appeared to be consistent with a number of other studies. The observations during membrane cleaning showed that the fouling layer was removed via cake expansion and gradual erosion mechanisms depending on cross-flow velocity.
The fouling mechanisms during crossflow filtration of bentonite-biopolymers mixtures were observed as three-stage-fouling: hindered transport diffusion, pore plugging and rapid cake deposition. The presence of alginate on bentonite fouling caused the increase of hindrance bentonite movement, cake density and more severe pore blocking phenomenon. The particles and biopolymers deposited homogeneously along the membrane surface. During the characterisation of the fouling removal mechanisms, it was demonstrated that the fouling layer of mixtures containing BSA has higher adhesive and cohesive characteristics than the alginate based mixture.
The DO technique was also used to characterise the effect of aeration on the fouling layer. By analysing two captured image sequences with MatPIV image analysis, more detailed particle velocity movement within membrane vicinity could be also characterised. The heterogeneous localised particle velocity was observed during crossflow and crossflow plus aeration filtration modes due to distribution of fluid velocity within the channel. The localised particle velocity at a given area during crossflow sparging was higher than the result obtained during CF without aeration due to flow wake caused by bubble flow and lateral membrane movement. These two phenomena, which were observed during crossflow plus air sparging, promoted the fouling thickness distribution along the fiber membrane.
Video clips of selected experiments are provided.