Role of filamentous microorganisms in activated sludge foaming: relationship of mycolata levels to foaming initiation and stability

The relationship between the levels of mycolic acid-containing actinomycetes (mycolata), Gordonia spp. and Gordonia amarae, and foam initiation and stability was characterized using: (1) batch tests involving addition of G. amarae cells to activated sludge, (2) analysis of a full-scale activated sludge plant that experienced seasonal foaming, and (3) a study of lab-scale activated sludge reactors augmented with G. amarae. Using batch tests, threshold Gordonia levels for foam formation and foam stability were determined to be approximately 2×108 μm ml−1 and 1×109 μm ml−1, respectively. In the full-scale plant, the levels of Gordonia spp. and G. amarae increased during the course of foaming, and the foam formation threshold of 2×108 μm ml−1 corresponded to the onset of foaming. This value was also verified in lab-scale reactor washout experiments, where decreasing mycolata levels were observed during the course of foam dissipation. The foam stability threshold of 1×109 μm ml−1 was verified in lab-scale reactor studies. The increase in the levels of Gordonia spp. and G. amarae in the full-scale plant corresponded to an increase in temperature, suggesting that the growth of Gordonia spp. was favored during warmer periods.