Fouling ultrafiltration water treatment

Solute adsorption often involves hydrophobic interactions—hydrophobic membranes have a high tendency to foul in water treatments. However, many hydrophobic membranes remain the most useful media for ultrafiltration due to their superior performance in terms of mechanical, chemical and thermal stability. 

Membrane filters are made in a wide variety of pore sizes (Fig. 1). The effective pore size for membranes vary, and membranes can be used in reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF). RO membranes are widely used in water treatment to remove ionic contaminations from the water. These membranes have an extreme small pore size and, therefore, require excellent pretreatment steps to reduce any fouling or scaling of the membrane, which would reduce the service lifetime. RO membranes are used by extensive pressures on the upstream side of the filter membrane to force the liquids through the pores. 

Kim C.H., Hosomi M., Murakami A., Okada M. (1994a), Effects of clay on the fouling by organic substances in potable water treatment by ultrafiltration. Water Science Technology, 30, 9, 159-168. 

Du, J.R., Peldszus, S., Huck, P.M. and Feng, X. 2009. Modification of polyfvinylidene fluoride) ultrafiltration membranes with poly( vinyl alcohol) for fouling control in drinking water treatment 43 4559—4568. 

By considering forthcoming new environmental regulations, nanofiltration and ultrafiltration using hydrophilic membranes have become even more important for applications in water treatment such as oil-water emulsions. Conventional membranes are effective in the removal of oily microemulsions from water, but they often suffer from low flux due to limited permeability and surface fouling [66, 67]. Membrane surface hydrophilicity is widely accepted as a dominant factor that... 

Microfiltration and ultrafiltration (MF/UF) membranes are flexible water treatment tools that can be used in a number of process configurations to meet advanced effluent treatment objectives. MF/UF membranes, when used by themselves, are limited to the removal of particulate and colloidal contaminants however, they can be combined with biological or chemical treatment to remove dissolved contaminants. Furthermore, they represent the ideal pretreatment to reverse osmosis by addressing their main weakness, fouling by particulate materials. 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

Another example of using ultrafiltration for wastewater treatment and resource recovery is the separation of oil-water emulsions generated from metal machining, oil field wastes, and enhanced oil recovery effluents. Hydrophilic membranes such as cellulose acetate are preferred because they are effective barriers to oil droplets and are less prone to fouling. The UF permeate readily meets direct discharge standards. The oil-rich stream can be processed to reclaim the oil, or disposed at reduced transportation cost because of its reduced volume. 

Membrane pretreatment includes microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF). Microfiltration and UF membrane processes can remove microbes and algae. However, the pores of MF and UF membranes are too large to remove the smaller, low-molecular weight organics that provide nutrients for microbes. As a result, MF and UF can remove microbes in the source water, but any microbes that are introduced downstream of these membranes will have nutrients to metabolize. Therefore, chlorination along with MF and UF is often recommended to minimize the potential for microbial fouling of RO membranes. The MF or UF membranes used should be chlorine resistant to tolerate chlorine treatment. It is suggested that chlorine be fed prior to the MF or UF membrane and then after the membrane (into the clearwell), with dechlorination just prior to the RO membranes. See Chapter 16.1 for additional discussion about MF and UF membranes for RO pretreatment. 

Srijaroonrat P, Julien E, and Aurelle Y. Unstable secondary oil/water emulsion treatment using ultrafiltration Fouling control by backflushing. J. Membr. Sci. 1999 159 11-20. 

Ultrafiltration can adequately produce disinfected water directly from strrface water for different applications. MF can also be used for disinfection, although not all viruses are removed. However, direct membrane filtration is limited by fouling, which, during constant-flux filtration, leads to a continuous increase in transmembrane pressure. In addition, UF and MF membrane treatment alone cannot effectively and consistently remove organic material, measured as total organic carbon (TOC), and THM (tri-halo-methane) precursors, measured as chloroform formation potential (Berube et al., 2002). 

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