The objective of this study was to test the hypothesis that the operating conditions of bioreactors influence the structure of the microbial community leading to differences in irreversible membrane biofouling. For a period of 265 days, four lab-scale sequencing batch bioreactors treating
a synthetic pulp and paper mill wastewater were operated to mimic: (a) plug flow hydraulic loading mode followed by gravity sedimentation to achieve solids separation; (b) continuous stirred flow hydraulic loading mode followed by sedimentation; (c) plug flow mode followed by membrane filtration
to achieve solids separation; and (d) continuous stirred flow mode followed by membrane filtration. Mixed liquor was removed, and membrane biofouling was examined in a membrane flow cell with microfiltration and ultrafiltration membranes operated at tangential surface velocities of 0.1 m/s
and 3.5 m/s. Amplified Ribosomal Dexoyribonucleic acid Restriction Analysis (ARDRA) and 16S rDNA sequence analyses were used to identify predominant bacterial populations within the mixed liquor and on the surface of biofouled microfiltration membranes operated at tangential surface velocities
of 3.5 m/s. The structure of the bacterial community in the four bioreactors highly varied during the course of the experiment. The bioreactors employing membrane filtration to achieve solids separation demonstrated a greater degree of irreversible biofouling as compared to the systems
operated with gravity sedimentation, suggesting that the lack of selective pressure for sedimentation in integrated membrane bioreactor systems may produce an increased tendency for biofouling. The predominant bacterial populations identified in the mixed liquor were significantly different
as compared to the predominant bacterial populations identified on the surface of biofouled microfiltration membranes suggesting that the presence of certain bacterial populations may produce an increased tendency for biofouling. The results of this study support our hypothesis suggesting
that future designs for membrane bioreactor systems should consider the relationships among microbial ecology and irreversible biofouling with regard to bioreactor operating conditions.
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