FILTRATION OF INORGANIC AGGREGATES

In this chapter, rejection and fractionation results have been presented for a fundamental study of the effect of inorganic aggregate structure on flux, the filtration of surface water like sy stems, and finally pretreatment of UF with coagulation. 

In summary, although the MF of coUoids is generally well understood, the literature is somewhat limited in the areas of filtration of colloids much smaller than the membrane pore size, and in systems where aggregation occurs. Systems are, in this regard, often poorly characterised, especially in the presence of humic substances. As shown in Chapter 2 organics stabilise inorganic colloids at sizes much smaller than pores, and their behaviour in MF or surface waters is largely unknown. 

In general, flocculants are used in solid-liquid separation processes such as thickening and filtration. Inorganic salts are also used sometimes to aggregate fine particles. Flocculation technique has been developed further for special applications of selective flocculation, selective dispersion and agglomeration flotation. 

Addition of soluble inorganic salts can also induce the precipitation from aqueous solutions of crystalline or amorphous crystalline surfactant precipitates. Consider then, for example, the increase in the Krafft temperature of SDS caused by the addition of a common ion, Na" [32-34]. This follows from simple mass action because the degree of micelle dissociation < 1 (i.e., the number of bound Na+ counterions is less than the micelle aggregation number) [35]. Peck [25] has shown, in measurements at 20°C, that the foamability of SDS declines markedly in the presence of added salt at concentrations > 0.3 M. The Krafft temperature of SDS under these conditions is >25°C [34], which means that crystalline SDS particles should be present as indicated by the onset of turbidity. Filtration to remove the turbidity partially restores the foamability [25] to a significant extent, which implies that the crystalline SDS particles exhibit some antifoam behavior. A combination of slow transport of surfactant to air-water surfaces and antifoam action by the crystalline surfactant would account for the almost total loss of foamability in the case of 0.01 M SDS in the presence of >0.3 M NaCl solution [25]. Antifoam action by crystalline particles... 

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