THE EFFECT OF SUSPENDED PARTICLES
When dealing with a "clean" liquid (i.e., a liquid devoid of suspended particles), the flow rate and the resulting throughput,
per unit of time, at the applied differential pressure is straightforward to measure, based on the selected EFA. (Temperature
must be kept constant, particularly because the liquid's viscosity is its reciprocal, and any increase in either factor will
lead directly to a proportional rise in throughput.)
If the liquid contains suspended particulates, however, the filter's porosity will be decreased by particle retention, so
additional EFA may be required to compensate for the filter area that is blocked or clogged by the particulates.4 In such cases, it will be necessary to conduct experiments to assess the EFA needed for batch processing.
Alternatively, it may be possible to restore lost throughput by using higher inlet pressures rather than increasing EFA. If
filtration is already in progress, that would provide a more manageable alternative. The effectiveness of such a measure would
depend, however, on the total suspended solids (TSS) in the solution. Higher differential pressures could increase the pressure
drop by compacting any filter cake that may have formed. In contrast, a larger EFA would be less likely to produce such compaction
under the same conditions, as there would be little or no filter cake build-up. High differential pressure also commonly leads
to high flow rates, which can cause elevated fouling or adsorptive effects. Those effects, in turn, can lead to losses in
product yield or the need for larger EFA.
Figure 2. Coomassie-blue stained 47-mm filter discs
If the preparation presented for filtration is relatively free from suspended particles, flat discs of even smaller diameters
may suffice for filtration sizing tests because the EFA available for liquid permeation would not be diminished by particle deposits. An example of such an application
would be filter sizing for deionized waters. These liquids contain so little suspended matter that the flow decline data can
be secured fairly quickly from tests using a small filter. The fewer the solids in the suspension, the less demanding the
Nevertheless, a sizeable inaccuracy inheres to the extrapolation of results from a 47-mm disc's EFA of 1.49 in2 to the EFA expected from a 6 ft2 (or 3,864 in2) cartridge. At best, the results indicate only hypothetical, non-committal values; hence the large margins allowed for error.
An assessment method that requires an EFA overdesign of as much as 1 or 1.5 times the extrapolated value does not merit endorsement.
It may well be that the extreme safety margins reported are exaggerations of the arithmetical uncertainties, reflecting the
experimenter's strong fear of having to interrupt the filtration mid-stream to install new filters to allow processing of
the batch to be completed. Indeed, the aseptic replacement of a filter is a risk-prone operation, so its avoidance is strongly