Distribution of extracellular polysaccharides and flotation of anaerobic sludge

Extracellular polysaccharides (EPS) were quantified in dense granules and loose flocs by chemical analysis of the uronic acid content. Their distribution within the aggregates was determined by microscopic staining. Granules contained a higher amount of EPS (1-1.6 mg g volatile suspended solids, VSS) than flocs (0.3 mg g VSS). In granules approximately 50% of the total amount of EPS was present in a 40-mu m-thick zone on the surface. The remainder was dispersed in the rest of the aggregate. In flocs the highest concentration was present in the centre and the EPS layer on the surface was not found. Tests showed that flocculent sludge was very sensitive to flotation, while the studied granules did not float. The lower susceptibility to flotation of granules as compared to flocs was attributed to the presence of the hydrophilic EPS coating that prevents attachment of gas bubbles.