FOULING OF A HYDROPHOBIC MICROFILTRATION MEMBRANE BY ALGAE AND ALGAL ORGANIC MATTER: MECHANISMS AND PREVENTION
Spinette, Rodrigue F.
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Microfiltration is a membrane filtration process increasingly used in potable water treatment because it effectively removes turbidity and particles which include pathogens. However, the permeability of microfiltration membranes usually decreases during filtration as a result of the accumulation of aquatic material on or within the membrane, a phenomenon referred to as fouling. While the fouling of microfiltration membranes by surface waters has largely been attributed to the presence of natural organic matter (NOM) in these waters, the fouling caused specifically by algae and their associated algal organic matter (AOM) has not been rigorously investigated. The fouling of a hydrophobic microfiltration membrane system typical of those used in potable water treatment was therefore tested with a series of model organic compounds, selected to represent AOM, and three genera of algae grown in the laboratory, Scenedesmus, Asterionella, and Microcystis. Finally, the use of coagulation/flocculation pre-treatment using alum to reduce fouling by algae and AOM was examined. Model compounds including proteins and polysaccharides were capable of significant and irreversible fouling of the hydrophobic microfiltration membrane. Two fouling ii i mechanisms by these compounds were identified: (1) limited pore constriction caused by the adsorption of small organic molecules onto the membrane pore walls and (2) pore obstruction caused by large organic aggregates that form under specific solution conditions. Fouling of the hydrophobic membrane by the different algae varied significantly. Algal cells accumulated on the membrane surface during filtration and caused no fouling or fouling which was mostly reversible. By contrast, AOM was capable of significant and mostly irreversible fouling. Because the AOM produced by each alga varied significantly in concentration and size, their fouling varied significantly as well. Similarities in fouling mechanisms were observed between AOM and the model compounds. Coagulation/flocculation pre-treatment using alum may be an effective method to reduce the fouling caused by several types of organic matter, including AOM. Reductions in fouling are achieved when the alum dose exceeds the minimum effective alum dose (MEAD) established by jar test for removal of the organic matter by settling.