Membrane and Desalination Technologies - TCE Moodle Website

362 N.K. Shammas and L.K. Wang
systems that may not necessarily be well described by mathematical modeling. In addition to
both the modeling and experimental approaches, there may be some cases in which a
manufacturer of a proprietary membrane filtration system has developed a system-specific
method for determining the VCF.
4.4.1. Deposition Mode
Membrane filtration systems operating in deposition mode utilize no concentrate stream
such that there is only one influent (i.e., the feed) and one effluent (i.e., the filtrate) stream.
These systems are also commonly call “dead-end” or “direct” filtration systems and are
analogous to conventional granular media filters in terms of hydraulic configuration. In the
deposition mode of operation, contaminants suspended in the feed stream accumulate on
the membrane surface and are held in place by hydraulic forces acting perpendicular to the
membrane, forming a cake layer, as illustrated in Fig. 8.3.
In a deposition mode hydraulic configuration, the concentration of suspended material on
the feed side of the membrane (Cm) is assumed to be equivalent to the concentration of
influent feed stream (Cf), independent of time or position in the membrane system, as the
suspended contaminants are removed from the process stream and deposited in the accumulated
cake layer. Therefore, all systems operating in deposition mode have a VCF equal to
one. MCF and most hollow-fiber MF and UF systems operate in deposition mode. Typically,
the accumulated solids are removed from MF/UF systems by backwashing, while most MCF
systems simply operate until the accumulated solids reduce the flow and/or TMP to an
unacceptable level, at which point the membrane cartridge is replaced.
Some MF/UF systems utilize a periodic “back-pulse” – a short interval of reverse flow
(which may include air and/or the addition of small doses of oxidants) designed to dislodge
particles from the membrane surface without removing these solids from the system. This
process re-suspends particles, effectively concentrating the suspended solids in the feed near
the membrane surface and increasing the potential for pathogens or other particulate to pass
through an integrity breach and contaminate the filtrate. Consequently, systems that do not
utilize a concentrate stream, but still practice back-pulsing may be more appropriately and
conservatively modeled as operating in suspension mode.
Fig. 8.3. Conceptual illustration of deposition mode operation.