Gel formation

Although pretreatment processes such as coagulation/flocculation and low-pressure membrane filtrations such as ultrafiltration (UF) and microfiltration (MF) have been used to remove larger particles and colloids, sometimes finer suspended particles and small colloidal matter still plague RO application [7,8]. Cake layer formation in RO process is influenced by various hydrodynamic and physiochemical parameters such as transmembrane pressure (TMP), cross-flow velocity (CFV), particle size, pH, and ionic strength [9]. According to that study, a significant permeate flux decline was observed as TMP increased and CFV decreased, which was attributed to the higher accumulative mass of particles on the membrane surface. The rate of flux decline decreased significantly with an increase in the ionic strength as well as particle size, while the flux decline rate was unaffected with the change in pH of the solution. The effect of hydrodynamic parameters on membrane performance deteriorated with an increase in the size of the solute particles. Various other studies on RO membrane filtration also determined that large particles do not contribute significantly to the membrane fouling and that fouling is usually controlled by small colloid particles [10,11]. Studies also showed that RO fouling was significantly caused by secondary water effluents than large suspended particles over size of 5.0 mm, leading to the conclusion that particles smaller that 0.45 mm, including true colloids and dissolved solids, contribute the most to the RO fouling [10].

Development of a colloidal cake layer on the membrane surface also significantly reduces the rejection of various kinds of solute particles such as salts and inert organics by restricting back diffusion of these solute particles from the membrane surface to bulk of the solution [12]. In this case, filtration of larger molecular weight (> 100 g/gmol) inert organic solutes was controlled by steric exclusion, and colloid fouling had little influence on their rejection. This study also showed the inability of the RO membrane to prolonged rejection of hormones as adsorbed hormones diffuse through the membrane matrix to the permeate side, resulting in gradual loss of rejection. This hormone breakthrough is accelerated significantly in the presence of colloidal fouling.

Compressible gel foulants. Membrane fouling is often caused by the formation of gels at the film surface. The extent of compressible gel formation depends on the membrane/feed combination under consideration, as well as the operating regime of the membrane processing system, for example, transient versus steady-state operation, recovery, and so on.

Compressible gel formation often occurs when molecules with very low diffusion coefficient are present. The most common species of this include: humic substances, bioslimes, phenols, pesticides (and other industrial compounds), and macromolecules (proteins, carbohydrates, cheesy whey, greases, oils, surfactants, and tannins).

These compounds block the membrane pores as they leave the bulk solution. In the case of proteins and other macromolecules, diffusion rates are extremely low: once these molecules enter the boundary layer of the membrane, they tend to stay there. Charged species, such as surfactants, have additional fouling potential because they possess some hydrophobic properties. Since most membranes are partially charged, an oppositely charged surfactant is attracted to the membrane surface, changing the barrier layer so that the water flux of the membrane is greatly reduced. As for the mechanism of fouling by humic substances, there is no definitive work that relates the concentration of humic substances in the feed solution and the rate of fouling.

Incompressible gel foulants. Incompressible gel formation is related closely to precipitation at the membrane surface. Typically the most common compounds that form gels are CaSO4, CaCO3, Mg(OH)2, SrSO4, Fe(OH)3, and BaSO4. Most other metal hydroxides have some tendency for gel formation also. Amorphous silica is one of the worst foulants of any type because it is very difficult to remove from the membrane once gel formation occurs. The maximum nonfouling concentration of silica to be fed to a RO module is 10 parts per million (ppm) for the crystalline form, and 120 ppm for the amorphous state. Other sparingly soluble minerals have varying propensities for fouling membranes. Certain operating conditions, such as high recovery and pH, can greatly aggravate the fouling process.

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