Electrostatic interactions between bacterial surfaces, extracellular polymers (ECP) and polyvalent metal ions are important in activated sludge flocculation. An indirect study of these mechanisms was done by adding different concentrations of EDTA to activated sludge samples from 6 Swedish wastewater treatment plants. The effects on sludge properties were studied with sedimentation and filtration tests as well as analysis of released extracellular polymers.
EDTA had a significant effect on sedimentation velocity in all investigated sludges. This shows that charged polymers are important for the properties of the floc surfaces and in building up the sludge macroflocs. The effect on filtration resistance where the bulk properties of the primary flocs are more important varied considerably for the different sludges. Thus, both electrostatic and other interactions are involved to a varying extent in building up the primary flocs in the sludges investigated. Variations in sedimentation velocity, residual turbidity, filtration resistance and release of ECP with variations in EDTA concentrations could be explained by effects of polyvalent metal ions on ECP binding and conformation.
On the fluidization/sedimentation velocity of a homogeneous suspension in a low-inertia fluid
