continuous bleed-and-feed ultrafiltration: It operates in a multipass mode, as shown in Figure

14.28, called single-stage feed and bleed.

This is achieved by recycling, at steady state, a large fraction of the retentate. In effect, feed to the membrane is the sum of fresh feed and recycle retentate.

The bleed is that portion of the retentate that is not recycled, but is withdrawn as product retentate. At startup the entire retentate is recycled until the desired retentate concentration is achieved, at which time bleed is initiated.

The single-pass mode is usually unsuitable for ultrafiltration because the main product is the concentrate rather than the permeate (as in reverse osmosis), and high yields of permeate are required in order to adequately concentrate solutes in the retentate.

A disadvantage of the feed-and-bleed mode, however, is that with the high recycle ratio, the concentration of solutes on the retentate side is at its highest value, resulting, as shown in Figure 14.26, in the lowest flux, with a resulting membrane area larger than that required for the batch mode.

To counter this, the feed-and bleed mode is most often staged as shown for four stages in Figure 14.29, where the retentate (bleed) from each stage is sent to the next stage, while the permeates from the stages are collected into a final composite permeate.

Solute concentrations increase incrementally as the retentates pass through the system. The final and highest concentration is only present in the final stage.

As a result, retentate concentrations are lower and higher fluxes are achieved, compared to a single-stage, bleed-and-feed system, for all but the final stage.

This leads to a smaller total membrane area. In practice,three to four feed-and-bleed stages are usually optimal.