FLOCKBILDNING I AKTIVSLAM – VIKTIGA FAKTORER OCH FRÅGOR / Flocculation in activated sludge – important factors and questions
Activated sludge is a complex ecosystem consisting of bacteria, protozoa, yeast and virus. Because bacteria are such a large and important component of activated sludge floes, bacterial adhesion is a crucial mechanism of the flocculation. We have studied how electrostatical interactions influence floe stability and sedimentation in water with different ionic composition. Changes in floe strength could be coupled to variations in ionic strength in the water. We also studied how cell surface hydrophobicity influence bacterial adhesion to sludge floes by using bacteria with well defined surface structures and bacteria isolated from activated sludge. The adhesion of bacteria was shown to be positively correlated to the hydrophobicity. The differences in adhesion between the bacteria could not be coupled to a specific combination of surface structures, the adhesion was a result of the net hydrophobicity. Hydrophilic bacteria does not adhere to floes, they may escape the sedimentation and is transported out of the plant with the effluent water.