Stable Colloids in the Absence of Organics

After the investigation of filtration behaviour of well defined colloidal systems and organics, colloids are now examined in solutions which are closer to surface water conditions. In a preliminary section the effect of colloid size on flux will be examined. This includes the packing of stable primary colloids. A further section examined the filtration of aggregates in the absence of organics. The aggregates are now formed under conditions closer to surface waters, at a pH 7-8 and in the presence of calcium, b inally, the colloidal systems OPS and SPO as described in Chapter 4 are considered. 

For the 10 kDa membrane no flux decline was observed for either of the colloids at pH 3 or pH 12. Rejection is complete for both membranes as the colloids are much larger than the membrane pores. 

As shown in Chapter 4 the charge of the colloids at pH 3 (positive) and pH 12 (negative) is similar. Therefore an electroosmotic effect cannot explain this difference in filtration behaviour at pH 3 and 12. Diffusivity increases as colloid size decreases. This is of importance for the smaller colloids (40nm, 75nm) which are not influenced by lift or shear effects. This increased diffusivitj results in a thinner deposit layer due to a increased backtransport as described in Chapter 3. In the absence of charge effects the colloids are expected to pack denser (smaller voids) if the primaiy colloid size is smaller. 

According to Fane (1999) this void can be estimated to be of the order of one-sixth of the colloid size (see values in Table 6.9). 

This effect was not observed, although voids and membrane pore sizes are comparable for the 100 kDa membrane (9.1 nm). Particle charge, however, increased with decreasing particle size. 

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Results for the blocking law analysis are shown in the following sections for stable colloids in the absence of organics, and for both SPO and OPS cases. 

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