Reversible process flocculation

Conventional methods for treating wastewater containing dyes, aromatic compounds, or heavy metals are coagulation, flocculation, reverse osmosis, nanofiltration and pervaporation (Paul and Ohlrogge, 1998), and activated carbon adsorption, the latter of which is combined with membrane processes like nanofiltration (Eilers and Melin, 1999) or ultrafiltra-tion (Lenggenhager and Lyndon, 1997). 

To illustrate the effect of the reverse process on the rate of flocculation, we solved numerically the set of Equations 5.319, 5.331, and 5.332. To simplify the problem, we used the following assumptions (1) the von Smoluchowski assumption that all rate constants of the straight process are equal to Up (2) aggregates containing more than M particles cannot decay (3) all rate constants... 

Equation 5.320. We see that in an initial time interval all cnrves in Figure 5.55 touch the von Smoluchowski distribution (corresponding to h = 0), bnt after this period we observe a redaction in the rate of flocculation which is larger for the curves with larger values of b (larger rate constants of the reverse process). These S-shaped curves are typical for the case of reversible coagulation, which is also confirmed by the experiment. 

Aggregation The process of forming a group of droplets, bubbles, or particles that are held together in some way. Sometimes referred to interchangeably as coagulation or flocculation The reverse process is termed deflocculation or peptization. 

It was also found with non-ionic surfactants that flocculation of the system occurred either at or very close to the cloud point (the lower consolute boundary) of the surfactant. Moreover, this was found to be a reversible process in that on cooling below the cloud point the particles redispersed provided that the temperature was not taken too far above the cloud point (c. 5-10°C) [109]. 

If the depth of the primary minimum (that on the left from the maximum in Fig. 6a) is not so great, i.e., the attractive force which keeps the drops together is weaker, then the floes formed are labile and can disassemble into smaller aggregates. This is the case of reversible flocculation (3). For example, a floe composed of i+j drops can be split into two floes containing i and j drops. We denote the rate eonstant of this reverse process by (see Fig. 20a). In the present case bofli the straight process of flocculation (Fig. 19) and the reverse process (Fig. 20a) take simultaneously plaee. The kinetics of aggregation in this more general and eomplex case is described by the Smoluchowski set of equations, Eq. (96), where one is to substitute ... 

Between the two clearly distinguishable states of an emulsion lies flocculation, which refers to the mutual attachment of individual emulsion drops to form floes or loose assemblies of particles in which the identity of each is maintained (Fig. 11.2c), a condition that clearly differentiates it from the action of coalescence. Flocculation can be, in many cases, a reversible process, overcome by the input of much less energy than was required in the original emulsification process. 

From these two successive experiments it can be concluded that (i) flocculation can be induced by macromolecules (ii) this flocculation is a reversible process. The reversibility is due to the low energy of interaction between both components. This last point was obvious from the adsorption isotherm (Figure 2), where significant amounts of free polymer remain in equilibrium with unsaturated surfaces. 

Electrostatic or charge stabilization has the benefits of stabilizing or flocculating a system by simply altering the concentration of ions in the system. This is a reversible process, and is potentially inexpensive. 

Hi denotes the number of single particles per unit volume rij is the number of aggregates of k particles (k=2,3,...) per unit volume ay (i, j = 1, 2, 3,. ..) are rate constants of flocculation (coagulation see Figure 4.65) qi denotes the flux of aggregates of size k which are products of other processes, different from the flocculation itself (say, the reverse process of aggregate disassembly or the droplet coalescence in emulsions see Equations 4.346 and 4.350)... 

This is governed by the same forces as those described for other dispersion systems such as emulsions. There are differences, however, as coalescence obviously cannot occur in suspensions and the adsorption of polymers and surfactants also occurs in a different fashion. Flocculation, unlike coalescence, can be a reversible process and partial or controlled flocculation is attempted in formulation. 

The interaction of hydrocarbon and fluorocarbon surfactants on the surface of dispersed particles has been studied through a flocculation and redispersion process [65-67]. Dispersions of positively charged particles can be flocculated with an anionic surfactant. An excess of the anionic surfactant forms a bilayer on the particle surface and causes redispersion of the flocculated sol. This flocculation reversal was used to study the interaction between mixed surfactants on a solid surface. A dispersion of iron(ITI) oxide hydrate particles was flocculated with an anionic hydrocarbon or fluorocarbon surfactant at pH 3.5, where the sols had a positive zeta potential. Subsequently, a second fluorocarbon or hydrocarbon surfactant was added to the flocculated sol. The extent of redispersion depended on the interaction between the two surfactants on the solid particle surface. 

MethylceUulose with a methyl DS less than about 0.6 is alkali-soluble. Erom about 1.6 to 2.4, it is water-soluble (most commercial grades) above 2.4, it is soluble in a wide variety of organic solvents. MethylceUulose solutions in water start to gel at 55° C, independent of molecular weight. The gelation is a function of the DS, rate of heating, and type and amounts of additives such as salts. As the temperature increases, the viscosity initially decreases (typical behavior). When the gelling temperature is reached, the viscosity sharply rises until the flocculation temperature is reached. Above this temperature, the viscosity coUapses. This process is reversible with temperature (75). 

In this treatment process, unit operations such as chemical coagulation, flocculation, and sedimentation followed by filtration, activated carbon, ion exchange, and reverse osmosis are employed to remove significant amounts of nitrogen, phosphorus, heavy metals, organic matters, bacteria, and viruses present in wastewater.2 It is always the last process step in the wastewater treatment plant that finally renders the treated wastewater reusable and disposable into the environment without any adverse effect (Figure 22.1). 

With regard to reversible flocculation kinetics, the problem is even more challenging- Detailed models for the deflocculation process as well as the flocculation process are required computer simulation is probably going to be the only way forward here ... 

Treatment of dye wastewater involves physical, physico-chemical, chemical, and biological methods. Physical processes are dilution, filtration, and gamma radiation. Physico-chemical includes adsorption, coagulation, flocculation, precipitation, reverse osmosis, ion exchange, etc. 

Another full-scale process combines catalytic oxidation including biodegradation, adsorption, precipitation/flocculation, and reverse osmosis [120]. 

---