MECHANISMS OF FLOC FORMATION IN SLUDGE CONDITIONING WITH POLYMERS

In flocculation of dilute suspensions a period of slow mixing is neces sary to increase the probability of particle collisions that are neces sary for flocculation to occur. When flocculating sludges, the collisi on frequency is so high that flocs are formed immediately after rapid mixing. However, controlled mixing of the flocculated sludge for some time period does improve dewaterability. The mechanisms that are invol ved have been studied in detail and the subject of this paper. The res ults show that after rapid mixing and initial flee formation there mig ht still be polymer in solution, depending on polymer dose. Prolonged mixing results in breakage of large aggregates and in particle/floc su rfaces that are not yet covered with polymers. Subsequently the excess polymer adsorbs onto these surfaces. From changes in sludge theology one can see that within a period of about one half to one minute the b ulk properties change dramatically. After the initial flee formation t he suspension is in a state characterized by a three-dimensional netwo rk structure with strongly interacting, voluminous flocs. After the mi xing period there are more discrete and less interacting flocs. Especi ally the properties of the filter cake from pressure filtration are th en more favorable.