Page 53 - Membranes for Industrial Wastewater Recovery and Re-Use

Membrane technology 33

numbers: membranes generally operate at fluxes between 10 and 1000 LMH.
The flux relates directly to the driving force (Section 2.2.2) and the total
resistance offered by the membrane and the interfacial region adjacent to it.

Conversion
In membrane processes there are three possible streams: a feed, a retentate and a
permeate stream. The retentate stream is unpermeated product. If there is no
retentate stream then operation is termed dead-end or full-flow (Fig. 2.14a).
Such operation is normally restricted to either low-solids water, as for cartridge
filtration of boiler feedwater or ultrafiltration for apyrogenic pure water
production, or cyclic operation with frequent backwashing, such for most
microfiltration and ultrafiltration membrane plant for municipal water
treatment. For waters having a significant solids loading and/or membranes of
limited permeability (dense membranes), it is not desirable to try and convert all
of the feed to permeate product in a single passage through a module. In such
cases, cross-flow operation is employed (Fig. 2.14b) whereby some of the
feedwater is collected as a concentrate (or retentate) stream. This expedites
the removal of accumulated materials from the membrane-solution interfacial
region provided by the scouring action of the retentate flowing over the
membrane surface.
The combination of the flux and the total membrane area determine the
conversion or recovery of the process. The conversion, normally expressed as a
percentage 0, is the amount of the feed that is recovered as permeate. Thus, for
a concentration C and flow Q in feed, retentate and permeate (Fig. 2.1 5), a simple
mass balance dictates that:

where % recovery or conversion is given by:

and the subscripts P and R refer to permeate and retentate, respectively.

filter cake membrane
membrane
or septum concentrate

support
permeate
(a) (b)

Figure 2.14 (a) Dead-endand (b) cross-jlowfiltration