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Membrane Systems Planning and Design 363
4.4.2. Suspension Mode
In membrane filtration systems that operate in suspension mode, a scouring force using
water and/or air is applied parallel (i.e., tangential) to the membrane surface during the
production of filtrate in a continuous or intermittent manner, as illustrated in Fig. 8.4.
The objective of operating in this mode is to minimize the accumulation of contaminants at
the membrane surface or boundary layer, thus reducing fouling. As shown in Eq. (18), the
VCF quantifies the increase in the feed side concentration of suspended solids relative to that
of the influent feed stream that occurs in a suspension mode of operation.
The three most common suspension mode hydraulic configurations are the PFR model, the
crossflow model, and the continuous stirred tank reactor (CSTR) model. Systems operating
under a crossflow hydraulic configuration may be further categorized as either small volume
or large volume systems, since the volume affects the manner in which suspended solids are
concentrated in such a system. Depending on the particular hydraulic configuration, the VCF
may vary temporally or spatially. In a PFR, the VCF increases in the direction of feed flow as
a function of position in the system. Examples of such systems include spiral-wound NF/RO
systems and vacuum-driven MF/UF systems submerged in tanks with large length-to-width
ratios. In a CSTR or crossflow reactor, the VCF increases with time over the course of a
filtration cycle.
The objective of crossflow filtration is to maintain a high scour velocity across the
membrane surface to minimize particle deposition and membrane fouling. Crossflow membrane
processes operate in an unsteady-state manner, in which suspended solids accumulate
on the feed side of the membrane over the course of a filtration cycle. Thus, in crossflow
systems, the VCF varies as a function of time. At the end of each filtration cycle, the
membrane unit is backwashed to remove the accumulated solids. Crossflow filtration has
traditionally been used in conjunction with inside-out hollow-fiber membrane processes to
increase the scouring velocity in the fiber lumen in order to minimize fouling.
The CSTR (also known as feed-and-bleed in some applications) hydraulic configuration is
similar to that of a crossflow system in that the particulate matter is held in suspension and
increases in concentration on the feed side of the membrane as a function of time. However,
the CSTR incorporates a continuous concentrate waste stream (also referred to as the reject or
Fig. 8.4. Conceptual illustration of suspension mode operation.