Plugging in membrane processes is generally caused by finely dispersed or suspended solids. TFC membranes basically contain two distinct distributions of pore sizes: tight ''polymer network pores'' and wider ''polymer aggregate pores'' [3,6]. The PSD curves and the effective number of pores in the membrane surface indicate plugging of the tight network pores and even their disappearance during fouling ofRO membranes [6]. Fouling also results in the shift of the aggregate pores toward larger values, resulting in noticeable reduction in salt rejection. Iron, organic and inorganic colloids, and humic substances are among the major foulants that plug membranes. Humic acids are troublesome because they can react with chlorine and chloride to form haloforms, that is, CHCl3. Some colloids, including silica, tend to be more troublesome than humic substances because they are very difficult to remove from the feed. Fouling can be worsened by allowing particles that are larger than one-fifth the size of the water channel into the RO module. The cut-off size for colloids is generally 40 A, although the larger supracolloids can form in the size range of 0.45—2 pm. The behavior of the supracolloids is controlled by composition, while the more common foulants tend to be more heavily influenced by small size and high surface area. Most colloids tend to be hydrophobic.

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