BIOFLOCCULATION IN ACTIVATED-SLUDGE - AN ANALYTIC APPROACH

A study on the physico-chemical structure of activated sludge flocs was carried out to get a better insight in its relationship with sludge settleability. For this purpose, 16 sludge samples from different origins were analyzed in order to provide information with regard to their settleability, biomass and exocellular composition, surface characteristics and internal hydrophobicity. The presence of filamentous microorganisms was observed in all samples but was not always associated with poor settleability, supporting to some extent the idea of their role as a backbone in the flocs. Relationships between the measured variables were studied through their linear correlations. A high amount of exocellular polymers (ECP) was associated with poor settling conditions. The DNA fraction and the C/N ratio of the ECP, had also a negative influence on the adsorption of a cationic molecule in the sludge samples. Finally, sludge settleability was described with a mathematical model which shows the opposition between ECP and the internal hydrophobicity of the flocs. From the model, the positive role of hydrophobic interactions should provide a new approach in the understanding of flocculation mechanisms in activated sludge.