Fouling microfiltration water treatment

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. 

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... 

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. 

Various techniques can be used to reduce the loading of suspended solids, organics and microbes in feed water. These include physical processes such as media filtration, cartridge microfiltration and chemical treatments. Chemical addition enhances the filter-ability of the solids such as the addition of coagulants (Table 2.2). Foulants and their control strategies are addressed in Table 2.8. Since any traces of soHds and organics get removed in the first membrane modules in RO and NF systems, these materials typically foul the first stages of an RO/NF system (Table 2.9). Once deposited on the membranes. 

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