Process Description

UF and RO can be conducted either as a single-pass, batch, feed-and-bleed, or multistage recycle system (5). In singlepass systems, the feed solution is brought to contact with the membrane module only once (Fig. 5). This limits the attainable concentration of the retentate. Batch systems return the retentate continuously to the feed tank for recycling through the membrane module (Fig. 6). Because the permeate is collected in another tank, the retentate is concentrated continuously until the desirable retentate concentration is reached. Feed-and-bleed systems are essentially a combination of the batch and single-pass operations (Fig. 7). Initially, the retentate is totally recycled similar to a batch system until the final retentate concentration is reached. At this point, a portion of the retentate

Retentate (concentrate)

Feed

Feed tank

Membrane module

Feed pump

Figure 5. A schematic diagram of single-pass UF or RO system. Retentate (concentrate)

Feed tank

Membrane module

Feed pump

Figure 6. A schematic diagram of batch UF or RO system.

is bled from the system, and fresh feed is added to the recycle loop. The quantity of feed into the loop is controlled at a rate equal to the permeate flowing out of the loop plus the amount of the retentate being taken out of the system. Multistage operations are achieved by linking two or more feed-and-bleed stages together (Fig. 8). The stages are operated in series with respect to the retentate/feed flow but in parallel for permeate flow. Each stage operates at a constant feed concentration, which increases from the first to

Figure 7. A schematic diagram of feed-and-bleed UF or RO system.

Figure 8. A schematic diagram of multistage recycle UF or RO system (only three stages are shown).

the last stage. The final stage operates at the desired retentate concentration. The advantage of the multistage system is that the retentate can be concentrated to high levels without the long residence time that occurs in a batch system, which can lead to potential microbial problems (12).

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