Mechanisms of floc formation in sludge conditioning with polymers

Abstract

In flocculation of dilute suspensions a period of slow mixing is necessary to increase the probability of particle collisions that are necessary for flocculation to occur. When flocculating sludges, the collision 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 involved have been studied in detail and the subject of this paper. The results show that after rapid mixing and initial floc formation there might still be polymer in solution, depending on polymer dose. Prolonged mixing results in breakage of large aggregates and in particle/floc surfaces that are not yet covered with polymers. Subsequently the excess polymer adsorbs onto these surfaces. From changes in sludge rheology one can see that within a period of about one half to one minute the bulk properties change dramatically. After the initial floc formation the suspension is in a state characterized by a three-dimensional network structure with strongly interacting, voluminous flocs. After the mixing period there are more discrete and less interacting flocs. Especially the properties of the filter cake from pressure filtration are then more favorable.

Settling behaviour of polymer flocculated water-treatment sludge II: effects of floc structure and floc packing

“…2 and Table 1). This result agrees with that reported by Langer et al [12] and Wen et al [13] and differs from the data reported by Wu et al [14] and Chu and Lee [15] who demonstrated a continuous increase of floc size with polymer dosage. The discrepancy may be attributed to the different polymer features, such as charge type and molecular weight, used by different investigators as suggested by Lee and Liu [16].…”

Dual polyelectrolytes incorporatingMoringa oleiferain the dewatering of sewage sludge

“…The van der Waals force could also be important in the adsorption reaction owing to the high molecular weight of polyelectrolyte and less negative value of ZP (−7.1 mV) on sludge surfaces. The adsorbed polyelectrolyte formed loops, trains, and tails on sludge surfaces [22,23]. Due to hindrance and electrostatic repulsion, it was difficult for the polyelectrolyte to adsorb on the loops, tails, or the surface of the particles at higher concentration.…”

Polyelectrolyte Flocculation of Waste Activated Sludge in Decanter Centrifuge Applications: Lab Evaluation by a Centrifugal Compaction Test

“…According to Christensen and coworkers (1993) the unreliability of the CST may be ascribed to the increased viscosity of the liquid phase when polymer overdosing is applied. At these high dosages, breakage of flocs by stirring leads to further consumption of polymer in solution (Langer et al, 1994), reducing its viscosity and yielding a lower CST value at longer shearing times.…”