Drugs and the pharmaceutical sciences

Media and Buffer Filtration Implications 445
cartridges are so constructed that the more open ends of the “V” shaped pores of the
membrane are directed upstream. This enables them to accommodate larger deposits in
their more open regions. The result is a commonly larger total throughput than a
symmetric membrane structure. Another side effect which asymmetric membranes
showed is a higher flow rate, which might be a result of a lower resistivity.
Another design improvement is the membrane combination within the filter
element. A coarser prefilter membrane layer in front of the final filter membrane, the socalled
heterogeneous double-layer membrane, has a distinctly higher total throughput due
to the fractionate retention of the contaminants (Fig. 6). The pre-filter membrane, for
example 0.45 mm rated, creates a protective layer over the 0.2 mm rated final filter
membrane. A large load of contaminants are removed first before they reach the final
filter membrane. This fact is similar to the utilization of pre-filter combination in front of
the final filter cartridge, except the pre-filter membrane of a heterogeneous double layer
construct is contained within the filter cartridge element. This factor also avoids the need
of two separate filter housings, if one wants to use separate retention ratings within the
filtration process. Certainly the pre-filter layer within a heterogeneous filter design adds
to flow resistance, therefore the flow rate of a heterogeneous filter design is lower than its
single layer counterpart, but higher than a homogeneous (0.2/0.2 mm) filter cartridge.
Flow, however, is not the main performance criterion within media filtration. High
throughputs are sought.
Further parameters to improve the total throughput within a filter cartridge design
are the choices of prefilter fleeces and the effective filtration area (EFA), that is, the pleat
design of the membrane within a filter element. Unique pleat designs can create a higher
filtration area within the filter cartridge and therefore higher total throughputs
(Soelkner et al., 1998; Cardona et al., 2004). Thinner pre-filter fleeces allow higher
pleat amounts within a filter element. However, if these are too thin, the flow channel
between the pleats potentially may be too tight and a complete filtration area utilization
might not be given. A thicker fleece creates a wider flow channel and also protects the
next filter layer. Nevertheless, thicker fleece constructs restrict pleat density and also
might press into the membrane. A well administered balance of construction criteria is
FIGURE 6 Total throughput differences of 0.2 mm rated filters with different membrane
combinations.