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Study of membrane fouling in a membrane enhanced biological phosphorus removal process Geng, Zuohong

Abstract

Membrane fouling is an inherent problem that has been associated with membrane processes since the emergence of membrane technologies. In the present research, membrane fouling in a membrane enhanced biological phosphorus removal (MEBPR) process was, for the first time, investigated systematically with respect to membrane fouling mechanisms, roles of sludge constituents in membrane filtration, characteristics of membrane foulants, and the relation of sludge properties to fouling. It was revealed that membrane fouling in the pilot scale MEBPR process was hydraulically irreversible and was mainly due to the adsorption of dissolved organic matter in activated sludge mixed liquor, particularly extracellular polymeric substances (EPS) such as carbohydrates and humic or humic-like substances. Large sludge floes were found to likely exert dual effects on membrane filtration. At low flux, they tended to induce hydraulic resistance via sludge deposition. At high flux, large sludge floes seemed to be able to mitigate fouling by sterically hindering the transport of soluble and colloidal sludge components to membrane surfaces. Unlike in other wastewater treatment membrane bioreactors, biofouling in the form of microbial growth was not significant in the studied MEBPR process due to the vigorous aeration and frequent backflushing applied. Inorganic fouling (i.e. precipitation of struvite) was also not observed. The accumulation of foulants (i.e. carbohydrates and humic or humic-like substances) on membrane surfaces resulted in an increase of surface hydrophobicity, which in turn might have accelerated the fouling process. The physical and biochemical properties of sludge, including floe size distribution, zeta potential, relative hydrophobicity, bound and unbound (soluble) EPS content, were also examined in the present research. Compared to a conventional sludge, the higher content of soluble EPS in activated sludge mixed liquor and the smaller floe size distribution were very likely responsible for the higher fouling propensity of the MEBPR sludge. Thus, the content of soluble EPS in mixed liquor was suggested to be a key property to evaluate the fouling potential of activated sludge mixed liquor. Contrary to the literature, the content of EPS bound in the sludge matrix was found not to be a direct influencing factor to membrane fouling.

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