Deposition of foulants

Waterside problems that lead to decreases in efficiency and material deterioration can be caused by a variety of mechanisms, such as electrochemical corrosion and deposition of foulants. These problems can be exacerbated by low flow, poor operational practice, process contamination, or specific stresses. It is also important to try to determine cause and effect relationships in order to provide a logical and practical water treatment solution. Such a solution will usually involve some form of cleaning, plus a combined engineering and chemical action plan. Inspection may be made easier by the use of a Boroscope or similar optical/video recording device. The color, texture, and quantity of all deposits should be noted, measurements of pits taken, and microbiological contaminants analyzed. It may be useful to conduct biocide efficiency tests on bacterial slimes. The period when a heat exchanger is open for inspection may be an opportune time for the permanent installation of ports for corrosion-monitoring probes. 

Apart from solute-solute interactions, the deposition of foulants on the membrane can alter rejection. Rejection can increase due to a lower porosity of the fouling layer or pore constriction, or decrease due to a higher concentration in the boundary layer (concentration polarisation effect). 

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... 

Fouling Fouling affec ts MF as it affects all membrane processes. One difference is that the fouling effect caused by deposition of a foulant in the pores or on the surface of the membrane can be confounded by a rearrangement or compression of the sohds cake which may form on the membrane surface. Also, the high, open space found in tortuous-pore membranes makes them slower to foiil and harder to clean. 

The primary difference between these types of metal wastage and oxygen corrosion is that these are all indirect forms of attack, induced by surface shielding (areas of metal surface under deposits or foulants, or cracks and gaps in the metal that are close to a shielding surface). 

As part of the pitting process, tuberculation tends to develop especially when flow rates are low (as they often are in HW heating boilers). The pitting corrosion initiator may be shielding rust or deposits and foulants swept along to the area of lower flow. 

The rate of metal wastage of this indirect form of corrosion may be increased by the presence of other direct corrosion influences in the deposit or foulant. Also (and similar to crevice corrosion), there may be general oxygen corrosion occurring at the same time or perhaps acting as an initiator to the under-deposit corrosion process. 

The use of electric powered steam generators, steam-to-steam heat exchangers, or reboilers to provide steam to steamtables, humidifiers, autoclaves, and for direct injection into the process. Where this type of approach is employed, it generally is necessary to provide a high quality water source to avoid rapid internal deposition of crystalline scales and foulants. 

Any sedimentary deposit or foulant that fails to form a crystalline scale. Often the result of supersaturation or the binding of biological or other organic material with dust, sand, or other mineral deposits. Also, sludge is not always deposited at point of origin and can additionally bake onto heat transfer surfaces. 

Here, Cf is the mean concentration of foulant in the film whose thickness at any time t is 6f, and m and m are, respectively, the mass of foulant deposited and removed per unit area per unit time. Assuming the deposition rate to be independent of the foulant film thickness and the removal rate to be linearly dependent on the film thickness, we may write a first-order rate equation for the film growth. From Eq. (1) this equation is given by... 

Sulfates are also involved in depassivation, especially with deposits or foulants. Sulfates can be reduced by selective microbial actions (sulfate-reducing organisms) that are almost always present in biofilms, producing deep pits of corroded steel. 

Table 4.4 Commonly Found Mineral Components of Deposits and Foulants...

Periodic analysis of makeup and cooling water samples, and possibly other water sources, or samples of scale or other deposits and foulants. 

To compensate for fouling (RF and CEOP) it is necessary to increase transmembrane pressure (Figure 6.1(b)). The constant flux strategy has important implications. Firstly, if J > Jcrit of foulant species, that species will continue to deposit. Secondly, as CP is (exponentially) flux-driven (see Equation 6.6 in Table 6.1) it will rise due to CEOP in a self-accelerating fashion. This is in contrast to a fixed-pressure strategy where the flux declines, net convection of foulant drops and CEOP become self-limiting. 

Membrane fouling involves the deposition of suspended solids, including bacteria, on the membrane or components within the membrane module. These foulants form a layer on the surface of the membrane that becomes an additional barrier for water to flow through to the permeate side of the membrane. Hence, if the feed pressure is held constant, the permeate flow will decrease. 

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