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Retentate (concentrate)

Membrane berglass reinforced plastic tube or a synthetic fiber tube and then inserted into a supporting, perforated stainless steel tube. These tubes are usually 12.5 mm (0.5 in.) to 25 mm (1 in.) in internal diameter and 0.6 to 6.4 m (2 to 20 ft) in length. A number of these tubes are then placed inside of a PVC or stainless steel sleeve or shroud to form a module. This type of membrane module provides a good control of the concentration polarization and membrane fouling. The system can usually tolerate large quantities of suspended solid matter in the feed and can be cleaned by foam swabs without dismantling the equipment. However, it has a lower membrane surface per module volume than other types of membrane module design.

A typical plate-and-frame membrane module is shown in Figure 2. It is similar to the conventional filter press. The membranes, porous support plates, and spacers with curved ribs forming the feed flow channels are clamped together and stacked between two end plates in a vertical or horizontal frame. Permeate is led out through fine, stainless steel tubes from the edge of the plates. All plate-and-frame membrane modules provide a larger membrane surface per unit volume than tubular membrane modules, but the control of concentration polarization is more difficult. Handling of solutions with suspended solids may cause plugging of the flow channel.

Figure 3 shows a spiral-wound membrane module. Two flat membrane sheets, separated by a porous membrane support, are placed together with their active sides facing away from each other. The three sides are then sealed together and the fourth side is attached to a perforated center stainless tube. A cover leaf and a spacer screen are then placed on each side of the membrane and the whole assembly is rolled around the center tube and placed inside a PVC or stainless steel housing fitted with manifolds to form a spiral-wound module. The feed is pumped into one end of the module and flows in parallel to the axis of the module. The permeate flows radially through both the membrane and porous membrane support and finally into the perforations of the center tube. The retentate or concentrate then flows out from the other end of the membrane module. The membrane surface per unit volume is higher than the plate-and-frame type, but the control of concentration polarization effects is more difficult. Severe membrane fouling may occur even with moderate concentrations of suspended solid materials. Thus, the spiral-wound membrane module is usually used in RO applications, and its use in UF is limited (9).

The hollow-fiber geometry is a newer approach to the membrane module design (Fig. 4). The membrane is a self-supporting tube with the active surface on the inside of the tube. Each hollow fiber has a fairly uniform bore and is bonded on each end in an epoxy tube sheet and is housed in a clear polysulfone or translucent PVC shell to form a membrane cartridge. The feed enters the inside of a group of hollow-fiber tubes, and the permeate passes through the tube and enters the shell side of the chamber for collection. Hollow-fiber membrane modules provide the largest membrane surface per unit volume of all membrane module design. However, the small fiber tubes are highly susceptible to fouling and plugging. Pretreatment or prefiltering of the

Retentate (concentrate)

Permeate

Retentate (concentrate)

Permeate

Feed C

Figure 2. A plate-and-frame membrane module.

Figure 3. A spiral-wound membrane module.

Support plate sandwiched by two membranes

Spacer

Figure 2. A plate-and-frame membrane module.

Support plate sandwiched by two membranes

Spacer

Cover leaf

Permeate

Spacer screen

Figure 3. A spiral-wound membrane module.

Retentate (concentrate)

Porous membrane support

Membrane

Cover leaf

Retentate (concentrate)

Permeate

Spacer screen

Porous membrane support

Membrane

Retentate (concentrate)

Retentate (concentrate)

Retentate

Permeate

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