The succession of glycogen accumulating organisms (GAOs) has been observed in an acetate-fed, anaerobic-aerobic sequencing membrane bioreactor (MBR) operated for 260 days without enhanced biological phosphorus removal (EBPR) activity. Semi-quantitative fluorescence in situ hybridization results showed that a gammaproteobacterial lineage GB frequently observed in EBPR processes was initially the numerically dominant species (50–66% of total cells) of the GAO in the MBR from day 1 to day 38. During this period, succession of two different subgroups of group GB was also observed. On day 85 onward, a population shift from GB group to ‘Defluvicoccus’-related tetrad-forming organisms (TFO) occurred. This microbial succession was suspected to be related to the applied operating conditions (long hydraulic retention time and long solid residence time) which favored the proliferation of ‘Defluvicoccus’-related TFO rather than the GB group. Application of terminal restriction fragment length polymorphism on selected samples further revealed that the microbial diversity of the seeding sludge as determined by the number of terminal restriction fragments was higher than that of sludge samples taken after day 85.

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