FOULING OF A HYDROPHOBIC MICROFILTRATION MEMBRANE BY ALGAE AND ALGAL ORGANIC MATTER: MECHANISMS AND PREVENTION
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Date
2008-02-06T21:25:25Z
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Johns Hopkins University
Abstract
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
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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.