The Proper Use of 47-mm Flat Disc Filters in Filter Sizing Studies - Why staining is crucial in flow decay studies. - BioPharm International


The Proper Use of 47-mm Flat Disc Filters in Filter Sizing Studies
Why staining is crucial in flow decay studies.

BioPharm International
Volume 22, Issue 9


The function of the 47-mm flat disc filter is to serve as the model for the filter to be used in batch processing. Unfortunately, filterability trials that use 47-mm flat discs can only roughly indicate which filter combination might be optimal. To perform reliably in its pilot role, the model filter should be as similar as possible in all its structural details to the production filter. Moreover, it should be tested under the conditions and in a manner as similar as possible to those of the production operation.

Batch filtration will most likely involve pleated filter cartridges. The details of pleated cartridge construction, however, are substantially different from those of flat filters. The measurement of the effective filtration area of flat disc filters is straightforward, whereas that of pleated filters is appreciably more complex. As a result, flow and retention data will not extrapolate well from flat stock to pleated filter cartridges, and EFA forecasts based on flat stock may overestimate the EFA available from pleated filter constructions. This would lead to a need for mid-process change-outs.

The differences in the EFAs of flat disc and pleated filters can result from the pleat-pack constructions and density of pleated filters. These discrepancies may derive from the flow-attenuating influences of the cartridge's support and drainage layers. In pleated filters, the entire filter area may not be available for the filtration function; a portion may serve to satisfy structural or mechanical requirements. How much of the remainder is available to the EFA function depends on how the pleating operation is conducted.

The details of the pleating operation are beyond the scope of this article.9 We can note, however, that the very nature of the pleat numbers, heights, and degrees of tightness affect the flow and retention properties of the cartridges composed of them.

The flow-attenuating influences of the cartridge's support and drainage layers, and the retention-modifying effect of pleat construction features cannot dependably be assessed from pilot studies using flat stock. Therefore, if pleated filter cartridges will be used in batch processing, indicator trials using 47-mm flat disc filters must be followed up by verification trials using miniaturized pleated filter devices.7,8 Using pleated filters in these verification trials will mean that the test results will be much more relevant. Pleated filter cartridges are available on the market in sizes as small as two inches in length.

When even more reliable EFA forecasts for batch processing are needed, assurance trials should be conducted using larger-scale cartridges. Conducting tests using filters with a larger EFA, even to the extent of using full 10-inch pleated cartridges, would proportionately increase the reliability of the extrapolated value.


As noted, the total area of filter material allocated to the pleating process is not necessarily converted to EFA usage. The uncertainty regarding the actual EFA of the pleated cartridge can, however, be resolved. A far more accurate prognostication based on the pleated model would then follow.

The exact amount of filter area used in constructing the pleated model should be known. Were it all converted to the EFA function, its extent could be calculated from flow rate studies as a function of applied differential pressures using Newtonian (clean) fluids at constant (ambient) temperature. Reverse osmosis product water could serve as the test fluid. The ideal flow rate would be the same as the flow rate if the filter matter in toto were used as EFA. That value could be obtained from the filter manufacturer, according to the filter type used, along with its porosity and thickness. The extent to which the expected flow is not realized would quantify the portion of the filter material not available as EFA as a result of pleating. The model filter's EFA would then be known with certainty, and its extrapolation could then be made with confidence.

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