Dense fouling layers

FIGURE 20.4 Scanning electron micrographs (SEM) micrographs of the cross section of a cellulose acetate membrane of 0.45 pm pore size after being used for beer CMF experiments. A dense fouling layer is observed on the membrane surface. (From Moraru, C.I., Optimization and membrane processes with applications in the food industry Beer microfiltration. PhD thesis. University Dunarea de Jos Galati, Romania, 1999.)... 

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. 

Riedl et al. [58] employed an atomic force microscopy (AFM) technique to measure membrane surface roughness and scanning electron microscopy (SEM) to assess the fouling layer. The smoothness of the membrane surface can influence the morphology of the fouling layer. It was shown that smooth membranes produced a dense surface fouling layer. 

In the woik of Olde Riekerink et al. (2002), one type of UF (cellulose triacetate, CTA) membrane and one type of RO (cellulose acetate blend, CAB) membrane were selected for CO2 plasma modification. CTA ultrafilters are well known for their low fouling properties and CAB membranes are widely used for desalination. CO2 plasma treatment resulted in the gradual etching of the membrane s dense top layer. 

Gehrke et al. [ 131 ] recently used a dip-coating process to deposit photocatalytic TiO nanoparticles (P25, Evonik) on a metallic filter material (micro-sieve). The fouling repellent and photocatalytic nanocoatings degraded the water impurities close to the micro-sieve surface before a dense cake layer was formed. This kind of surface activation is, however, restricted to chemically robust materials, excluding polymeric membranes that would be degraded by the induced oxidation process [125]. 

IS measurements were performed to determine the membrane variations associated with (i) Dense and porous layers of a commercial RO membrane (ii) Different PEG concentrations in the top dense layer of a polyamide/polysulfone experimental membrane (iii) Hydrophobic character of one layer in a composite or multilayer structure (iv) Membrane matrix material modification and (v) Protein (BSA) fouling of a porous commercial membrane. The results obtained with other characterization techniques, such as morphological, chemical, and adsorption analyses, have validated the information obtained from the IS results. 

Precipitation fouling may be defined as the phenomenon of a solid layer deposition on a heet transfer surface, primarily as a result of the presence of dissolved inorganic salts in the flowing solution which exhibit supersaturation under the process conditions. Deposits formed under various conditions have different mechanical characteristics. The term "scaling" is generally used to describe a dense crystalline deposit, well bonded to the metal surface. It is often associated with the crystallization of salts of inverse solubilities under heat transfer conditions. When the deposited layer is porous and loosely adherent, it is described by terms such as "soft scale," "powdery deposit," or "sludge."... 

The dense phase membrane always manifests a stronger stability, giving the reactor a constant behavior in time. This was explained in terms of carbon deposition (Gallucci et al., 2008) in fact, it was seen that the fouling phenomena are slower with this type of layer. The experimental evidence is shown in Table 4.22. 

Ultrasonic treatments also belong to the acoustic methods. Already, for more than ten years ultrasound has been used for the cleaning of membranes. In most cases the transducers have been situated outside the membrane, but the membrane has sometimes also been used as the transducer. Of crucial importance are the frequency of the waves and the way the waves have been applied (see Chapter 11). The method has mostly been used to prevent the build-up of a concentration polarization layer. Polarization of dense particles has been easier to prevent than fouling or polarization of small particles [17, 18]. 

---