An examination of the mechanisms for stable foam formation in activated sludge systems

Screening pure cultures of 65 mycolic acid producing bacteria (Mycolata) isolated mainly from activated sludge with a laboratory based foaming test revealed that not all foamed under the conditions used. However, for most, the data were generally consistent with the flotation theory as an explanation for foaming. Thus a stable foam required three components, air bubbles, surfactants and hydrophobic cells. With non-hydrophobic cells, an unstable foam was generated, and in the absence of surfactants, cells formed a greasy surface scum. Addition of surfactant converted a scumming population into one forming a stable foam. The ability to generate a foam depended on a threshold cell number, which varied between individual isolates and reduced markedly in the presence of surfactant. Consequently, the concept of a universal threshold applicable to all foaming Mycolata is not supported by these data. The role of surfactants in foaming is poorly understood, but evidence is presented for the first time that surfactin synthesised by Bacillus subtilis may be important.

Research highlights
► This study presents data from 65 foaming Mycolata that the flotation theory can be applied to most of these to explain their role in stable activated sludge foam formation.
► The data demonstrate a clear distinction between the properties of foaming and scumming bacteria, and proposes that these terms should not be used synonymously.
► The data also demonstrates that when surfactants are absent but hydrophobic Mycolata are present, a scum is produced. On the other hand with no hydrophobic particles but a surfactant present, an unstable foam is generated.
► The concept of a universal threshold for foaming applicable to all plants is not supported by these data, where the numbers of cells required depends on the bacterial strain and the presence of surfactant.
► The study shows for the first time that Bacillus subtilis, shown here to be commonly culturable from foams may be an important participant, by its production of the powerful surfactant surfactin.