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Aquamira Water Treatment Terminology
Understanding Terminology in Regards to Filter Ratings
Aquamira Technologies, Inc.
As we review advertising literature, packaging, product claims, blog comments and other
information that circulates in regards to filter performance, we are often intrigued by the
misunderstanding and misuse of important terms used to describe filter performance. It is
no wonder that many consumers are confused and bewildered when trying to make
informed decisions in regards to personal filtration devices. The purpose of this article is
to clear up some of the confusion about filter ratings and claims and provide valuable
information to the consumer that will aid in making an informed decision as it relates to
the selection of a personal filtration device.
By understanding the correct meaning of filtration terminology, mechanisms, and claims,
a consumer can pull out important information that will lead to making an informed
decision. I have divided this article into three sections to address each of these three
The most misrepresented or confusing terms are pore size, porosity and removal ratings.
Pore size: correctly refers to the actual opening size of the pores (holes) in a membrane
filter. This may be reported in a minimum pore size (smallest measurable hole),
maximum pore size (largest measurable hole, which is the most meaningful
classification), or a pore size range. (example: 5-10µm) In many cases the actual pore
size rating of a given filter may be much larger than the removal rating of a given filter.
Removal Rating: refers to the statistical probability of the filter’s ability to remove a
certain size particle when challenged under controlled conditions. This should not be
confused with the actual pore size of a filter. There are two types of ratings: nominal
and absolute. These terms are misused to a great extent in filter claims and marketing
literature which can mislead the consumer. A nominal rating is attached to filters that can be shown under controlled conditions to
remove an acceptable statistical amount of particles of a certain size, even though the
actual pores or openings of the filter may be much larger than the particles being
removed. The typical way that a nominal filter rating is reported would be in the form of
a statement such as: “Removes >99.9% of particles 3µm (micron) or larger.” This means
that under field conditions, a user can be confident that the filter will remove greater than
99.9% of pathogenic organisms larger than 3 µm. This information is obtained by
challenging filters with test waters containing suspensions of 3µm spheres. A similar
cryptosporidium or giardia claim would require a challenge using live cryptosporidium or
giardia organisms. Nominal ratings are usually applied to depth filters (see definition
An absolute rating can only be applied to a filter that the end user can actually determine
the size of the largest pore (see above). Such a filter can be integrity tested using a nondestructive test method and the data can be used to determine the actual size of the largest
pore. Absolute ratings can only be applied to membrane filters (see definition below)
due to the requirement of a definable pore. If a filter manufacturer applies an absolute
rating to a filter, they should be able to provide the user with a non-destructive test
protocol that will allow the user to verify the absolute rating.
Porosity: refers to the ratio of open space in a filter matrix to the amount of volume
taken by the filter media itself. Typically, a filter with high porosity will have a more
open structure, and therefore, higher flow with lower pressure drops. High porosity does
not necessarily mean that the filter will remove particles better than a low porosity filter.
In order to apply these concepts and definitions correctly, the consumer needs to
understand the mechanics of the filter in relation to the claims that are associated with the
filter. Most filters can be categorized into two main groups, commonly referred to as
membrane filters and depth filters.
Membrane filters are very thin and are usually cast or extruded in a variety of
proprietary processes. Membrane filters retain over 90% of the particles to be removed
on the surface of the filter due to the fact that the well-defined holes or pores are smaller
than the particle being retained. Membrane filters tend to have lower porosity than
typical depth filters requiring higher operating pressures and they typically achieve lower
flow rates. During the casting and curing processes pores are formed that are uniform in
size, shape and length. A bubble point or diffusional flow test can be performed to
confirm the actual opening size of the largest pore in the filter being tested. This type of
test confirms the pore size without exposing the filter to the actual particle or organism
and is called a non-destructive test. Membrane filters can be integrity tested multiple
times, and in most cases, are tested prior to, and after use to verify that the filter
maintained integrity during the entire time it was in use. Note the two examples of membrane filters below: On the left is a low porosity filter
with very well defined pores; On the right is a high porosity filter with a more open, and
yet still well-defined pore structure. Both filters are 0.2µm absolute rated, and in both
cases the largest pore diameter can be verified using a non-destructive integrity test.
Membrane Filter (Low Porosity) Membrane filter (High Porosity)
Depth filters rely on a torturous path to capture particles within the matrix or depth of
the filter. Simply put, particles are caught within the depth of the filter as they come in
contact with obstructions. There is rarely a uniform, defined pore structure in a depth
filter and in many configurations such as fibrous filters there are no pores at all. Even
though there may not be defined pores, depth filters can be performance rated based on
challenge testing. In these tests, the filter is challenged with pre-set quantity of defined
size particles or organisms. This type of testing renders the filter unusable and is referred
to as destructive testing. Manufacturers will perform these tests on a representative
sample of each filter batch. Since every filter cannot be tested and verified individually,
a “nominal” rating is associated with depth filters. Depth filters can be produced using
several methods. The most common types are fiber filters and sintered filters (see
illustrations below). Fiber filters are either spun or woven into a cloth or felt. Sintered
filters such as ceramic, metal or porous plastic filters are formed by fusing particles
together under heat and pressure. The spaces between the particles form the flow path or
pores of the filter. Filter aids, such as activated charcoal, may be added to improve the
filtration capability of the filter. Many filter aids use attractive forces such as chemical or
electro-differential (Zeta) to pull particles out of the water stream and hold them to the
surface of the filter aid. This process is called adsorption.
Fiber type depth filter Sintered depth filter
Flow Rate: refers to the flow of material (usually water) through a filter at a defined
pressure difference between the upstream side of the filter and the downstream side.
Flow rate is always reported in units of volume over time at a specified pressure drop
(example: liters/min. @ 3 psi.). As the filter begins to plug, the flow rate will decrease
due to the openings in the filter being filled with captured contaminants. The filter is
considered exhausted when the flow rate through the filter decreases to a point that it is
too difficult to use. The graph below illustrates a typical flow rate curve showing a
decrease in flow as the time and volume of throughput increases.
Through Put Test Results
0 5 10 15 20 25 30 35 40
ml/min @ 3psi **
Flow RatePressure Drop: or differential pressure is the difference in pressure from the upstream
side of the filter to the down stream side of the filter. Pressure drop is reported using
the Greek symbol “D” (delta) followed by the letter “P” and is stated “delta-p”.
Throughput or Filter Life: is the expected amount of fluid (usually water) that a user
should be able to pass through the filter prior to the filter plugging. This number is
usually reported in gallons or liters and is set by the manufacturer based on test criteria.
The throughput of the filter is determined by testing the actual amount of water that can
pass through the filter at a specified Dp prior to the filter falling below the minimum
specified flow rate. A filter manufacturer should be able to provide test data that
indicates that the filter will perform within the product specification throughout the entire
rated life of the filter. This assures the user that the filter will always provide the
published amount of water.
If filter aids such as activated charcoal, zeolite, or alumina are incorporated into a filter,
the throughput may be determined by the capability of the filter aid to remove specific
contaminants, rather than the total amount of water that can be put through the filter.
Testing can be done to determine that the adsorptive capability of the filter aid is
effective throughout the entire filter life.
Multiple stages or the addition of chemical disinfectants may be incorporated by filter
manufacturers to achieve the required performance specification. It is important to
understand the mechanics of each stage of the filtration process in order to determine the
filters effectiveness. In the case of chemical disinfectants, minimum contact times based
on the temperature, turbidity, and organic load of the feed water are required to achieve
adequate kill rates to meet the required specification. The consumer must also be aware
of the filter’s capability to remove the chemical disinfectant from the water stream prior
Filter manufacturers should always be able to provide relevant test results to verify
published claims. For example, if a throughput claim is 50 gallons of water, the
manufacturer should be able to provide test results showing the filter was able to meet or
exceed this requirement. If the filter incorporates a filter aid, the manufacturer should be
able to show that the adsorptive capability of the filter aid was still viable at the rated
throughput. If a manufacturer claims the filter removes cryptosporidium, there should
be a corresponding cryptosporidium test. This applies for bacteria and virus as well. If a
pore size claim is made, testing acceptance data and procedures should be made available
that would provide the user with the ability to integrity test the filter to verify the pore
size claim. If a filter manufacturer cannot provide this information, be very wary of an
“absolute” claim. Remember, as a consumer you have the right to information that will
support the claims of a manufacturer. Don’t be afraid to ask. Filter manufacturers use a statistical method for rating a filter’s capability to remove
particles or organisms called a “Log Removal Rating” (LRV). In order to determine the
validity of a manufacturer’s claim, the consumer must understand how LRVs work, and
how they are reported. LRVs are reported using a percent removal number based on the
initial challenge. This number is commonly reported using “9s”. (example 99.9%) Each
nine (9) reported, indicates the challenge level as well as the filters removal capability.
For example, a filter that reports four “9s” (99.99%) indicates that the filter was
challenged with at least 10,000 particles or organisms for every ml (milliliter) of water
that was tested, and that no (0) particles or organisms were detected downstream. A
milliliter of water is about the size of a sugar cube.
An understanding of industry standards for LRVs is critical in determining if a filter is
qualified for the intended use of the purchaser. Industry standards are set by ANSI
(American National Standards Institute), NSF (National Sanitary Foundation) and
USEPA (United States Environmental Protection Agency). For example a company may
report a 99.99% removal of bacteria, and to an uninformed consumer this may look very
impressive, but when compared to the industry standard for bacteria removal of
≥99.9999%, an informed consumer would realize that the filter falls way short of the
mark. In fact when tested at >1,000,000 particles or organisms/ml required to achieve a
99.9999% rating, a 99.99% removal rating indicates that 10,000 particles/ml water got
through the filter. Not very comforting or confidence inspiring to the informed
Industry standards are as follows:
Protozoan Cysts: ≥99.9% removal
Bacteria: ≥99.9999% removal
Virus: ≥99.99% removal.
A consumer should also be aware of built in disclaimers found in marketing language or
claims. Many of these disclaimers sound very impressive, but when correctly
understood, may help a consumer understand limitations of the filter. For example
statements such as, “Up To” or “Virtually” are used to allow acceptance of filters that fall
short of the required specification. Up to means “anything below” is ok. Virtually means
“almost”. A consumer should always look for the “Greater Than or Equal To” (≥),
language or symbol in relation to claims. This will assure that the filter will always
perform equal to or better than the stated specification.
Understanding these very important terms, mechanics, and claims can mean the
difference between making a good decision about a filter purchase vs. putting your health
at risk. Remember, that as a consumer, you have the right to know and understand the
validity of any claim associated with a filter. Don’t be afraid to ask.