Impacts of structural characteristics on activated sludge floc stability

Activated sludge samples from seven full scale plants were investigated in order to determine the relationship between floc structure and floc stability. Floc stability was determined by shear sensitivity and floc strength. Floc structure was considered in terms of two size scales, the micro and macro-structure. The micro-structure refers to the organisation of the floc components, such as the individual microorganisms. The macro-structure refers to the overall floc. The floc macro-structure was characterized by filament index, sludge volume index (SVI), size, and fractal dimension. It had a significant impact on floc stability. Large and open flocs with low fractal dimensions, containing large numbers of filaments were more shear sensitive and had lower floc strength compared to small and dense flocs. Fluorescent in-situ hybridisation (FISH) analysis indicated that the organization of the bacterial cells might also have an affect on the floc stability