Conformation of adsorbed macromolecules and solid-liquid separation in coagulation processes

The efficiency of solid-liquid separation in coagulation-sedimentation processes depends greatly upon the performance of the coagulant with respect to the production of suitable floc properties. High-molecular weight polymers that function by interparticle bridging can serve as very effective coagulant-aids, however, the dynamic processes of polymer adsorption and macromolecular rearrangement may have a decided effect upon the determination of floc characteristics. Ideally, adsorption would occur rapidly with the macromolecule in an extended, rod-like conformation; this configuration increases the effective collision radius and results in rapid growth. However, effective separation by sedimentation is best achieved by dense, compact flocs characterized by reduced interparticle distances. In addition, the resulting sludge must ultimately be dewatered; therefore, the ease with which water flows around and through the settled flocs is of considerable importance. These requirements call for floc properties that are not completely compatible with one another. Consequently, efficient coagulation processes must compromise and employ operational conditions that produce flocs suitable for the subsequent solid-liquid separations.