Initial stages of bacterial fouling during dead-end microfiltration.
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Constant pressure experiments were performed using track-etched polycarbonate membranes and rod-shaped bacteria (viz., Brevundimonas diminuta and Serratia marcescens) to study flux decline and backwashing during the early stages of microfiltration. The intermediate blocking law originally derived for spherical particles was modified to account for the approximate cylindrical shape of the selected bacteria. A deposition factor was introduced to empirically account for the morphology of bacterial deposits. The initial stages of flux decline prior to the secretion of new extracellular polymeric substances (EPS) was quantitatively described by the intermediate blocking law before transitioning to cake filtration at later times. Scanning electron microscopy (SEM) provided additional visual evidence that bacteria simultaneously deposited directly on the membrane and on each other during early stages of filtration as assumed bythe intermediate blocking law. Empirical deposition factors decreased with initial permeate flux indicating its effect on bacteria deposition patterns, which was also confirmed by SEM. Bacteria were easily removed following short filtration times before significant secretion of new EPS by simply rinsing with ultrapure water, thereby completely restoring the clean membrane permeability. In contrast, this rinsing procedure did not completely recover the membrane permeability following longer durations when significant amounts of new EPS proteins and polysaccharides were secreted. Consequently, backwashing effectiveness during water and wastewater microfiltration will be high prior to EPS production whereas flux recovery may not be possible solely by hydrodynamic means once EPS are secreted.
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