Settling Properties of Activated Sludge Related to Floc Structure

Abstract

When characterizing sludge settling by means of sludge volume index and stirred sludge volume index it has been found that the former is normally higher, but that the difference varies greatly for different sludges. It was found to depend on difference in shape of the flocs. Spherical flocs give small differences and elongated flocs give large ones. Although both indices were positively correlated to the total length of filamentous organisms the differences were not. A general floc model is proposed on the basis of these findings together with colloid chemical considerations and measurements of floc strength, filtration resistance and amount of extracellular polymers. According to this model young flocs are of the elongated type, while flocs from a stable system are round. Sludge bulking is a consequence of stiff noncompressible flocs which may be due to high amounts of dissolved polymers, but most likely to filamentous organisms fixed in a network by binding polymers within the flocs. The amount of filaments inside the flocs is probably proportional to the amount outside which for that reason is correlated to the settling indices.