Colloidal fouling

Aromatic polyamide (aramid) membranes are a copolymer of 1-3 diaminobenzene with 1-3 and 1-4 benzenedicarboxylic acid chlorides. They are usually made into fine hollow fibers, 93 [Lm outer diameter by 43 [Lm inner diameter. Some flat sheet is made for spirals. These membranes are widely used for seawater desalination and to some extent for other process applications. The hollow fibers are capable of veiy high-pressure operation and have considerably greater hydrolytic resistance than does CA. Their packing density in hoUow-fiber form makes them veiy susceptible to colloidal fouling (a permeator 8 inches in diameter contains 3 M fibers), and they have essentially no resistance to chlorine. 

The buildup of biofilm on the membrane surface means an additional resistance to solvent flow as well as the possibility of enhancement of CP level by the biofilm, which is similar to the case of colloidal fouling [32,36], In general, the diffusivity is linked to the tortuosity factor of the biofilm [37]. Hence, it is likely that the backdiffusion of solutes in the biofilm on RO is hindered. The enhanced CP is important for two reasons. Firstly, the elevated concentration of solutes at the membrane wall means an increase in the osmotic pressure (CEOP) and hence a loss in the effective TMP. Secondly, the nutrient level is also enhanced and this will further accelerate the growth of the biofilm [32,36]. So, biofouling in RO becomes an interplay between C P and biofilm development. 

P. Bacchin, P. Aimar and V. Sanchez, Model for colloidal fouling of membranes, AIChEJ 41 (1995) 368-376. 

A number of factors can lead to high pressure drop, including membrane scaling, colloidal fouling, and microbial fouling. These three factors all involve deposition of material onto the surface of the membrane as well as onto components of the membrane module, such as the feed channel spacer. This causes a disruption in the flow pattern through the membrane module, which, in turn, leads to frictional pressure losses or an increase in pressure drop. 

Zhu, X. and M. Elimelech (1997). Colloidal fouling of reverse osmosis membranes Measurements and fouling mechanisms. Environ. Science Technol. 31, 12, 3654-3662. 

Bacchin P., Aimar P., and Sanches V., Model of colloidal fouling of membranes. AIChE Journal 41(2) 1995 368-376. 

Pulp and paper industry streams are mostly colloidal, i.e., they contain macromolecules and small particles in the range of 1 nm to 1 xm. Salt and pH affect colloidal stability, rate of precipitation, and colloidal fouling. The higher the surface charge... 

Vrijenhoek EM, Hong S, and Elimelech M. Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration membranes. J. Membr. Sci. 2001 188 155-128. 

Jbnsson AS and Jbnsson B. Colloidal fouling during ultrafiltration. Sep. Sci. Techn. 1996 31(9) 2611-2620. 

Colloidal fouling is caused by accumulation of particles and macromolecules on, in, and near a membrane. Materials accumulated at the membrane surface create an additional layer of resistance to permeation. Early work on colloidal fouling of RO membrane used to treat secondary effluents indicated that particles smaller than 5 im contribute more substantially to fouling than larger particles (56). It was postulated that as particle size increases, it is subjected to higher velocity and shear force at the membrane surface. Therefore, larger particles tend to be swept away in bulk flow rather than deposit on the... 

Colloids are defined as particulates in the size range of 1 nm to 1 pm (Potts et al. (1981)). Below that range, particulates are dissolved solids. While this size range includes organic matter, this section is mainly devoted to inorganic colloids. Fouling depends on colloid size and membrane pore size. A number of models exist for colloidal fouling and these were summarised by Bowen and Jenner (Bowen and Jenner (1995)). Here, major mechanisms and forces on colloids will be summarised. 

NF and RO membranes do not have pores larger than 1 nm and it is often the case that the double layer of charged particles is likely to be larger than this. Therefore, if colloids contribute to NF and RO fouling, then this is likely to occur via the formation of a dense gel layer on the membrane, which involves molecules which occupy the interparticle space. Colloidal fouling of NF and RO membranes is not as well understood as in MF and UF, where colloids and particulates are major foulants. 

Separation of charge and size effects for organics and ion separation and the effect of speciation Effect of pretreatment on membrane processes for rejection enhancemenr and fouling control. Colloidal fouling experiments with model colloidal systems and known aggregation behaviour... 

In the previous chapters, the effects of colloidal fouling of MF and UF membranes was thoroughly studied. While the effect of these relatively large colloids on NF were not expected to be significant, a brief series of experiments were performed to check this. 

Fouling also affects rejection. In MF and UF pores gradually fill up, while in NF a deposit of a higher charge than the membrane may form and enhance rejection. Colloid fouling can also increase the thickness of the unstirred boundary layer and increase concentration polarisation, which adversely affects rejection. 

Cohen R.D., Probstein R.F. (1986), colloidal fouling of reverse osmosis membranes. Journal of Colloid and Interface Science, 114, 1, 194-207,... 

Elimelech M., Zhu X., Childress A.E., Hong S. (1997), Role of membrane surface morphology in colloidal fouling of cellulose acetate and composite aromatic polyamide reverse osmosis membranes. 

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