This process pumps a fixed volume of feed solution from a reservoir through a membrane stack and back to the reservoir until the required salt removal is achieved (see Figure 7.37.6). The power requirement for such a batch process depends on the degree of recirculation and mixing.
The advantages of this process include the following: (1) changes in feed-water salinity or temperature and changes in membrane properties only modify the production rate but not the effluent quality; and (2) optimum velocity is independent of the production rate.
This process has the following disadvantages: (1) Higher power is needed; (2) recirculation reservoirs are required; (3) membranes do not operate at one equilibrium point; (4) current density through membrane also varies; and (5) more piping and control hardware are needed.
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