Membrane Process Operation

The MF unit was operated at 0.6 bar with the feed tank continuously fed with surface water direcdy from the bottom of the dam. Concentrate and excess permeate, were both recycled to the feed tank, generating a continuous overflow. Filtration cycles were of 4 to 10 minutes and samples were taken immediately after backwash in order to have a comparable water quality with lowest losses of organic material due to filter cake buildup. 

The RO unit was operated at 13 bar, permeate was discharged and concentrate recycled into the feed tank for further concentration. The final volume of a batch was limited by the inner volume of the unit with about 15 L usually obtained, including concentrate washed out of the unit at the end of cycle. The concentration in this final liquid changed depending on the volume of the batch treated at that time. 

These volumes varied from 300 L to 1500 L per batch. The feed tank was cooled to 18 2 C with a cooling coil supplied with fresh raw water. 

Once 5000 L of surface water was attained, all previously concentrated batches were added into the feed tank for further concentration. All batches were stored in a refrigerator. 

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Figure 20-48 shows Wijmans s plot [Wijmans et al.,/. Membr. Sci., 109, 135 (1996)] along with regions where different membrane processes operate (Baker, Membrane Technology and Applications, 2d ed., Wiley, 2004, p. 177). For RO and UF applications, Sj , < 1, and c > Cl,. This may cause precipitation, fouling, or product denatura-tion. For gas separation and pervaporation, Sj , >1 and c < ci. MF is not shown since other transport mechanisms besides Brownian diffusion are at work. 

Membrane processes operate on the basis of the following mechanisms ... 

A major deficiency of conventional treatment systems is their inability to make use of a single-process separation for aU the dissolved constituents on a molecular or ionic level. Membrane processes operate at ambient temperature and offer one-step separation for all the dissolved constiments on molecular or ionic level without any need for further chemical addition. Table 29.2 gives the application of different membrane processes for diverse contaminants. 

Membrane processes operate in two basic modes. In Figure 9.3(a), the permeate stream is solely the components of the feed stream that transport across the membrane. Figure 9.3(b) illustrates the case where a sweep stream is introduced on the permeate side to collect the permeate. The sweep stream can operate cocurrent or countercurrent to the feed stream. One limiting case is when the streams on both sides of the membranes are perfectly mixed and there is no axial variation in solute concentration. These basic modes are then incorporated into various geometric configurations (Figure 9.4). 

Most membrane processes operate by means of cross-flow filtration, in which only part of the fluid passes through the membrane as filtrate (or, more correctly, permeate, since some membrane processes operate by permeation rather than filtration) the retained part, the concentrate or retentate, conseqnently becomes more concentrated in particulate or solute species. Membrane systems are frequently operated in a closed loop, with the retentate recycled, and final concentrate is taken from the loop in proportion to the added feed suspension. Whereas microfiltration utilizes both through-flow and cross-flow filtration, cross-flow is the nsnal mode for the other membrane filtration processes, and has thereby grown to its present level of importance. 

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