Introduction

Flocculation processes with following solid/liquid separation are the basis of numerous industrial processes. Significant sectors, such as the paper industry, textile finishing, the detergent industry and the ecologically important sector of water treatment and protection, owe their technological progress in large measure to the use of organic flocculants. These are polymeric, water-soluble materials of different charge density having molecular weights up to 1 • 107 g/mol [1]. The most important product group for the waste water sector is that of ca-tionically and anionically modified polyacrylamides.

Flocculation processes involve a complex interaction of individual, elementary processes (Fig. 1) determined by aspects of colloid and surface chemistry:

— production of a homogeneous mixture of the polymeric flocculation aid and the substrate to be flocculated;

— adsorption of the polymer onto the particle surface which thus becomes destabilized after a particular degree of polymer coating;

— rearrangement of the adsorbed polymer on the particle surface to achieve an equilibrium conformation;

— collision of destabilized particles (before or after rearrangement) which initiates aggregation or flocculation;

— breakup of floes by shear stress.

The nature of the polymers determines their mode of action, which is explained by structural differences in

Adsorption and Flocculation Kinetics

Adsorption and Flocculation Kinetics

Bridging Flocculation

bridging model patch charge model

Fig. 1 Interaction of adsorption and flocculation kinetics bridging model patch charge model

Fig. 1 Interaction of adsorption and flocculation kinetics the adsorbed layers [3—5]. The flocculation mechanism of bridging is enhanced by high molecular weights and a low affinity between the flocculation aid and the surface. Destabilization and flocculation is achieved when a voluminous far-reaching polymer conformation is present and the surface is about half-coated. Polyelectrolytes with a high charge density tend to give a flat surface adsorption; the resulting mosaic structure at a particular degree of polymer coating is therefore characterized by regions having positive and negative charges (patch charge model). A correlation between flocculation effect and surface charge can only be derived for the charge-mosaic mechanism.

Differentiation of the two flocculation mechanisms is often possible by observing the shear stability. Polymer bridges are relatively shear-stable, the destruction of floes by increased shear stress generally being irreversible. Floes formed by the charge-mosaic mechanism, on the other hand, are substantially more compact and shear-labile; however, destroyed floes return reversibly to the original state after the stress is stopped [6, 7].

For those flocculation processes which are initiated by polymers, the depletion flocculation mechanism is also being discussed in addition to the mechanisms based on polymer adsorption. In the depletion mechanism, the osmotic pressure causes the displacement of polymer molecules from between neighboring particles as soon as these are closer than a particular minimum distance [8—10]. Theoretically, this mechanism is quite feasible, although it has as yet not been experimentally verified, in particular for real systems.

Besides the poly electrolyte structure, the dosing and mixing conditions play a decisive role in the flocculation result [2, 16]. Both ecological and economic factors are important in driving the targeted development of floc-culants, their specific selection and their optimal dosing. This requires effective measurement methods which allow assessment of the prevailing flocculation state under defined conditions which are close to those in practice. This paper describes a fiber-optic flocculation sensor (FOFS) for measuring the flocculation state in flowing systems, which has proved its value both for basic studies in the laboratory and as the measuring component of control systems for actual dosing equipment [6, 11—13, 16]. The focus of this contribution is the use of the FOFS for assessing the effectiveness of polymeric flocculants in the treatment of industrial and municipal waste water.

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