Fouling layer

When we talk about this subject, the term foulant or foulant layer comes to the forefront. Foulant, or fouling layer, are general terms for deposits on or in the membrane that adversely affect filtration. The term "fouling" is often used indiscriminately in reference to any phenomenon that results in reduced product rates. "Fouling" in this casual sense can involve several distinct phenomena. These phenomena can be desirable or undesirable, reversible or irreversible. Different technical terms apply to each of these possibilities. 

Jackson, J.M., and Landolt, D., "About the Mechanism of Formation of Iron Hydroxide Fouling Layers on Reverse Osmosis Membranes," Desalination 12, 361-378 (1973). 

Several cleaning methods are used to remove the densified gel layer of retained material from the membrane surface. The easiest is to circulate an appropriate cleaning solution through the membrane modules for 1 or 2 h. The most common ultrafiltration fouling layers—organic polymer colloids and gelatinous... 

H. Strathmann, Criteria for Fouling Layer Disengagement During Filtration of Feed Containing a Wide Range of Solutes, J. Membr. Sci. 73, 181 (1992). 

Pitfalls of the different water treatment processes are the formation of extensive amounts of sludge, which has to be deposited off, as is the case with flocculation, the formation of fouling layers during membrane filtration, or DBP formation after disinfection of NOM-containing waters. 

The main problem in membrane usage for water purification is the fouling layer that adheres to the membrane. The source of the fouling layer is the different species existing in the feed water and their increased concentration next to the membrane wall. When water permeates through the membrane, all rejected species accumulate next to the membrane wall, their concentration increases in comparison to the bulk concentration, and the motion away from the membrane is controlled by diffusion to the bulk of flow against the flux of the water flowing to the membrane. 

Figure 3.4 Fouling on membrane surface creates an additional barrier to permeate transport that requires additional pressure to force permeate through the fouling layer.

Parametric studies of the effects of TMP, temperature and crossflow velocity on the permeate flux and protein retention rate have been conducted using 0.8 pm alumina membranes at a pH of 4.4. The maximum steady state flux is observed at a TMP of 3 bars. As expected, a higher crossflow velocity increases the steady state permeate flux, as illustrated in Figure 6.3 under the condition of 50 C, TMP of 5 bars and pH of 4.40 [Attia et al., 1991b]. The protein retention rate also improves with the inciease in the crossflow velocity. The permeate flux reaches 175 L/hr-m, accompanied by a protein retention rate of 97.5% when the crossflow velocity is at 3.8 m/s. This improvement in the flux corresponds to a reduction in the thickness of the external fouling layer. 

Measuring Fouling Layer Morphology and Cell Adhesion Kinetics.331... 

Schafer et al. [35] studied the role of concentration polarization and solution chemistry on the morphology of the humic acid fouling layer. Irreversible fouling occurred with all membranes at high calcium concentrations. Interestingly, it was found that the hydrophobic fraction of the humic acids was deposited preferentially on the membrane surface. This result is similar to the work of Ridgway et al. [31], who showed that the hydrophobic interaction between a bacterial cell surface and a membrane surface plays a key role in biofilm formation. The formation of two layers, one on top of the other, was also observed by Khatib et al. [36]. The formation of a Fe-Si gel layer directly on the membrane surface was mainly responsible for the fouhng. 

Recent research efforts brought about new and exciting developments in membrane technology, some with direct implications for the membrane filtration of beer. For example, Stopka et al. [21] reported flux enhancement in the microfiltration of a beer yeast suspension when using a ceramic membrane with a helically stamped surface. A relatively simple modification of the ceramic membrane surface resulted in modified hydrodynamic conditions and disturbance of the fouling layer. As compared with a regular, smooth ceramic membrane of the same nominal pore size, the stamped membrane leads to higher flux and lower power consumption per unit volume of permeate at the same velocity of the feed. 

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.)... 

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