17 DEC 2003
sively high pressure can cause structural damage to the filter. It is, therefore, important
to know the minimum pressure to maintain flow through the filter and evenly distribute
solids across it, as well as the maximum operating pressure that can damage the filter.
As the filter plugs, high differential pressures across it are realized, causing particle or
pore deformation. Some vendors can provide systems with differential pressure alarms
to warn of impending filter failure.
5-3.3.3 Fluid Compatibility. As previously discussed, filter compatibility with the
waste stream must be determined from available literature and from experience. In ad-
dition to the literature review, a chemical compatibility test can be done by immersing
the cartridge filter in the fluid for at least 48 hours, or according to the manufacturer's
recommendation. At the end of the selected soak period, observe the filter for changes
in color, structural integrity, swelling, softening, deformation, and any other changes in it
and its hardware's physical appearance. Also observe changes in the fluid, including
changes in color, clarity, and viscosity.
5-3.3.4 Filter Efficiency. Filter efficiency means the percentage of particles of a spe-
cific size that will be removed from the waste stream. Suppliers of cartridge filters use
different test methods for rating a filter's performance. In many cases these test meth-
ods cannot be correlated. Filter ratings include nominal filtration rating, absolute filtration
rating, beta ratio, and filtration ratio.
5-220.127.116.11 Nominal filtration rating represents some percentage of removal of particles of
a given size or larger. The percentage test method, and thus the rating, varies from
manufacturer to manufacturer. It is not typically reproducible from cartridge to cartridge
and should not be relied on.
5-18.104.22.168 Absolute filtration rating is the diameter of the largest spherical particle that
will pass through the filter under test conditions specified by the vendor that are some-
times performed in air environments. While an absolute filter rating will, for all practical
purposes, tell what particles will definitely be removed, there will also be many much
small particles that will be removed as well. As a result, relying on the absolute filter
rating can lead to over-designing a filter system and removing particles that do not af-
fect the ultimate downstream processes of effluent requirements.
5-22.214.171.124 Different filter elements can be best compared not on an absolute rating or
some nominal rating devised by each manufacturer, but instead by the Beta ratio de-
rived from empirical testing. A filter's Beta ratio is the most useful means of expressing
filter efficiency over its service life. The Beta ratio (x) is the total number of particles in
an influent waste stream greater than a specified size (x) divided by the number of par-
ticles in the treated effluent waste stream of the same size or larger. Thus, the removal
efficiency for a certain particle size (x) can be described as:
( β x - 1)100