The other day I sat through a Mountain Travel and Rescue Clinic on Water Treatment in the Backcountry. I listened to the presenter talk about micron ratings, pore sizes and how an effective filter needs to have a specific micron rating. When I began to question some of the information presented, the lecturer admitted that he did not know much about filters and micron ratings and that he had pulled much of the information from the Internet. Luckily for him and the rest of the class, we here at McNett know our water filters! Our Aquamira water treatment brand is the preferred water treatment brand for thru-hikers along the AT trail. That said, we would like to set the record straight on filter ratings and claims.
As we review advertising literature, packaging, product claims, blog comments and other information that circulates in regards to filter performance, we are often amazed 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. We’ll begin with some basic terminology:
Pore size: 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 measurablehole, which is the most meaningfulclassification), or a pore size range. (example: 5-10μm).
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.
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. Nominal ratings are usually applied to depth filters and only apply a degree of filtration.
Example: a nominally rated filter will typically filter anywhere between 50% and 95% of particles of a stated size, with 80% efficiency being the norm. For example, a 5-micron nominally rated filter will remove approximately 80% of contaminants 5 microns or larger. As you can see a nominal rating is not very valuable to a consumer that needs to be sure that greater than 99.9% of contaminants are being removed.
Absolute rating: can only be applied to a filter that the end user can actually determine the size of the largest pore. Such a filter can be integrity tested using a non-destructive 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.
Now that we have some terminology in place, we can better 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 pores are smaller than the particle being retained. Membrane filters tend to have lower flow rates than typical depth filters requiring higher operating pressures.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.
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.
Aquamira water filters are depth filters, so we can now start to see why a micron rating system does not necessarily indicate our filter’s actual ability to remove protozoa, bacteria and cysts. In addition, our filters utilize activated charcoal, which also improves the filtration capability of the filter through 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.
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 only a “nominal” rating is associated with depth filters.
In closing, we do not use a micron rating system, because as we have learned a micron rating does not necessarily describe a filter’s actual ability to remove contaminants. Filters can remove contaminants via the torturous path and adsorption, which are independent of micron ratings. In addition, because we are using a depth filter, we would only be able to provide a nominal rating, despite the fact that our filters have been tested and certified by Biological Consulting Services in North Florida (accredited in accordance with NELAC) to remove greater than 99.9% of Cryptosporidium and Giardia.
A water filter does not necessarily require a specific micron rating in order to gauge its effectiveness. We encourage consumers to 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 the true abilities of the filter.