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Water filters and purifiers

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How to evaluate a water microfilter or purifier

To date, no independent research group, including government agencies or universities has performed a comprehensive scientific analysis and comparison of the microfilters and purifiers that are currently available. Because of this knowledge vacuum, the best that purchasers can do in choosing a device is to apply the knowledge of the risks and the EPA testing protocol to manufacturers’ literature and come to their own conclusions.

While we recommend our preferred treatment method in the next section, we won’t recommend a specific product, however, we can recommend a procedure for selecting a treatment device. While reading these recommendations, remember to keep in mind the EPA log reduction standards and the weaknesses in the EPA tests.

  • Assess your needs in terms of the volume of water you will need to pump, size and weight of devices and the cost and availability of devices and replacement filters.
  • After selecting devices that might meet your needs assess manufacturers’ promotional material and examine them carefully for claims and evidence. Be very skeptical and watch for extraordinary requirements such as slow pumping rates, wait times, the need to pass water through the device several times and other possible red flags that are often tucked away in fine print and footnotes. Ask for EPA registration numbers and results from EPA lab tests. If manufacturers will not send you lab results you should be suspicious.
  • Determine the cost effectiveness of products by looking at the initial upfront cost and the cost of replacement filter elements. Be skeptical of manufacturer claims about filter life if the conditions under which the lifespan is determined are unknown or unrealistic. Remember that the more suspended silt and vegetation in the water, the faster a filter will plug. Consider whether the filter can be cleaned or back flushed if it clogs or whether you need to replace it. If you have to replace filter cartridges, find out how much they cost, consider whether you will be able to find them while travelling and consider the environmental cost of throw away filter cartridges versus cleanable filter elements.
  • Once you have narrowed your search down to a few products that seem to fulfill your needs if possible examine the devices in a store. Look for signs of quality construction and compare warranties. Obvious weaknesses in design are guaranteed to cause headaches at the worst possible times. If parts look flimsy in the store, image how they will look broken in pieces 30 km down the trail. Consider usability and field maintainability.
  • Research the manufacturer. Find out how long they have been in business and their reputation in the water treatment business. Ask for references to find out who uses their products and what the users think of the products. For example, a company that supplies the International Red Cross or United Nations workers certainly has more credibility than a company that cannot, or will not, give you a reference.

Standards for portable microfilters and purifiers

The greatest challenge in selecting a treatment device is evaluating the varied and sometimes outrageous claims made by manufacturers about the effectiveness of their products.

In Canada and Europe there are currently no guidelines or regulations by which to evaluate the effectiveness of water treatment devices. In the US, the EPA creates the regulations. In order to receive an EPA registration number, manufactures must meet or exceed the standards set out in the 1986 (revised 1987) EPA publication ‘Guide Standard and Protocol for Testing Microbiological Water Purifiers.’ While this manual is currently the only regulation that protects consumers, it has come under heavy criticism.

The ‘Guide Standard and Protocol’

The purpose of the ‘Guide Standard and Protocol’ is to establish microbiological challenges and testing procedures that devices must pass in order to be certified as ‘microbiological water purifiers.’ Microbiological water purifiers are devices that make water safe to drink by removing, killing or inactivating all microorganisms that cause disease.

The ‘Guide Standard and Protocol’ requires that devices be tested under specific laboratory conditions with microbiologically contaminated ‘worst case water.’ This test water is made in the lab by adding ‘challenge organisms,’ which are actual pathogens or bacteria, viruses and protozoa with the same size, toughness and general characteristics of disease causing organisms.


To receive certification, three identical units must pass specific tests over a predetermined time period. During the tests, water laced with specific challenge organisms is run through the devices. Currently the challenge organism for bacteria is the usually non-pathogenic Klebsiella terrigena. For viruses the challenge organisms are poliovirus and rotavirus. For protozoa the challenge organism is Giardia lamblia.

The quantity of each challenge organism in test water is based upon so called ‘worst-case’ water out in the environment. In order to pass, no more than ten percent of the two water samples from the three test units can fall below the removal tolerances. These tolerances are expressed as log reductions or percentages. For example, the minimum required reduction for bacteria is six logs or 99.9999% removal. This means that in one litre of water laced with 108 bacteria, only 100 living organisms are allowed to make it through the treatment device. The following chart outlines the EPA microbiological challenge for certifying microfilters and purifiers.

US EPA ‘Guide Standard and Protocol’ Microorganism Challenge
Organism Test Water Minimum reduction
to pass
Log reduction Percentage
Klebsiella terrigena 108 / litre 6 99.9999
Poliovirus type 1 107 / litre 4 99.99
Rotavirus 107 / litre 4 99.99
Giardia lamblia 106 / litre 3 99.9

Note that devices that are designed to strain out bacteria or protozoa, such as ceramic microfilters, can receive credit for passing the bacteria and protozoa challenges for which they were designed, but they are not certified as ‘purifiers’ because they do not inactivate viruses.

Non-Microbiological Parameters

The ‘Guide Standard and Protocol’ also includes ‘worst case’ parameters for non-microbiological factors including water pH, total organic carbon (TOC) such as decaying vegetation, turbidity (suspended particles), total dissolved solids (TDS) such as calcium and other naturally occurring minerals and elements and temperature.

Standards for these parameters are included because they can greatly affect how well devices work. For example, total organic carbon reduces the effectiveness of chemical treatments such as iodine which some purifiers rely on to deactivate viruses and bacteria.

Weaknesses of the ‘Guide Standard and Protocol’

While the “Guide Standard and Protocol” was designed to protect consumers and establish basic benchmarks for manufacturers, in the last decade the criteria have come under heavy criticism by some manufacturers and independent testers.

  • The ‘Guide Standard and Protocol’ does not establish standards or testing requirements for the removal of dissolved inorganic compounds and elements such as heavy metals and other potentially harmful substances. Nor does it require the removal of dissolved organics such as pesticides, herbicides, fertilizer and bacterial toxins. Currently no device that the EPA certifies as a ‘water purifier’ is required to protect the user from non-microbiological contaminants such as elements, chemicals, and heavy metals.
  • Some critics suggest that the ‘Guide Standard and Protocol’ allows devices to pass in a dubious manner. For example, ‘worst case water’ can be passed through a device several times, water is allowed to sit for 20 minutes or longer so that chemical additives from iodine cartridges can inactivate viruses and bacteria and the device can be intentionally pumped at slower than maximum rate. The danger is that a device can be certified even though it relies on the user to assess the level of microbiological contamination in the water they are treating, which is impossible. The user then has to take special measures such as passing water through the device several times, allowing sit time between pass-throughs, or slow pumping. The major problem is that devices that require special measures to pass the tests, can be certified as ‘purifiers’ along with superior devices that pass the tests without any extraordinary measures.
  • Some devices may be certified as ‘purifiers’ without having been tested because the ‘Guide Standard and Protocol’ allows for products with the same basic technology, construction, and operation as a tested unit to be passed and certified without testing.
  • Critics have suggested that non-microbiological parameters, in particular turbidity and total organic carbon, are too low and are not typical of conditions in mountain streams or low-lying bogs and fields. This is important because high turbidity and total organic carbon concentrations can significantly reduce the effectiveness of iodine, which is often used in devices certified as ‘purifiers.’ The danger is that devices may pass the EPA lab tests but then fail in field conditions where the turbidity or total organic carbon is much higher than the test parameters.
  • There is no testing requirement for halogen treatment products such as water purification tablets. Currently manufacturers are free to say anything they want about the effectiveness of their products, especially against resilient organisms such as Giardia and Cryptosporidium, without providing a shred of laboratory evidence to support their claims

Improving the ‘Guide Standard and Protocol’

Some critics of the ‘Guide Standard and Protocol’ have suggested changes that will raise the bar for manufacturers. These changes will better protect thirsty hikers standing on the edge of a stagnant pond wondering if their treatment devices are adequate.

  • Devices must pass the microbiological tests after passing water through only once, at maximum possible flow and pressure rates, and without a waiting period. These changes will ensure that users do not have the impossible task of assessing the level of microbiological contamination in the water.
  • Non-microbiological parameters, particularly turbidity and total organic carbon, should be adjusted to reflect more arduous “worst case” scenarios.
  • Devices should be tested with the exact accessories that consumers get when they buy the devices.
  • Devices that have a filter element that is worn down during cleaning, such as in ceramic filters, should be tested at the minimum filter thickness to simulate ‘worst case’ conditions.
  • Guidelines and testing criteria should be established for inorganic and organic chemicals and other non-microbiological contaminants, including heavy metals. A separate label, distinct from “microbiological water purifier,” should be created for devices that pass these tests.
  • Testing criteria should be created to test the effectiveness of stand-alone halogen treatment products. Manufacturers of these products should have to substantiate claims with lab results from standardized tests.
  • Only specific independent laboratories should be permitted to do testing, rather than labs chosen by manufacturers, and lab test results should be made public.

What this Means for You

The world of device manufacturing and testing is a maze of government regulation or non-regulation, manufacturer claims, marketing and scientific debate. In recent years, some manufactures have even threatened multi-million dollar lawsuits against other manufacturers and independent testers who have threatened to release private lab reports that show competitors’ devices failing to meet EPA standards after being certified by the EPA.

For the average traveller or trekker who just wants to purchase a reliable treatment device the situation is a unclear at best and potentially dangerous at worst. Each manufacturer markets its product as the best available with all of the expected hyperbole. Yet the problems with the ‘Guide Standard and Protocol’ indicate that manufacturer claims must be viewed with great skepticism and should not be accepted as definitive proof that a product works properly.

Reverse Osmosis

There are numerous electric and human-powered reverse osmosis devices on the market. Provided they are functioning properly, these devices work by forcing water at very high pressures through semipermeable membranes. The membranes remove dissolved ions, molecules and solids as well as all larger particles including suspended impurities and microorganisms.

Reverse osmosis units are the only devices capable of making virtually any water drinkable, including seawater. Owing to their power demands, cost, size and weight, reverse osmosis devices are limited to recreational vehicles and boats. Some smaller human-powered units are useful for seakayaks and small sail and powerboats but even they are too large and heavy for general travel and recreation use.

Granular Activated Carbon (GAC)

Granular activated carbon (GAC) has been used for centuries as an adsorbent and it is currently the only proven and realistic way to remove some, but not all, chemical contaminants from recreational drinking water.

GAC has a large number of highly reactive free valences that remove impurities by bonding with them. As a result, GAC filters or elements can remove halogens (iodine and chlorine), some fertilizers, pesticides and and other organic compounds as well as lead at concentrations normally found in groundwater.

However, GAC can only adsorb some chemicals and impurities and only until all binding sites are occupied. When the binding sites are full, impurities will pass through the filter unadsorbed or will bind to the GAC and release chemicals held by weaker bonds. Unfortunately, the only indication that a GAC element is fully saturated is a foul taste or odour in the treated water. GAC elements do not make seawater drinkable.

Some microfilters have primary or secondary elements made entirely of GAC. It is important to note that while GAC elements may remove some bacteria and viruses through adsorption, GAC is not antimicrobial and bacteria can colonize the GAC element and slough off into water coming out of the device.

GAC filters should not be solely relied upon for water treatment. However, when GAC elements are incorporated into a microfilter or purifier with primary filter elements made of other materials, they are unequalled for removing organic and inorganic chemicals, halogens and impurities that cause bad smells or tastes.

Contaminants Removed by GAC Elements in Microfilters

Elements and Inorganic Chemicals
Level of Removal
Cadmium Some removal
Chromium Some removal
Lead Some removal
Mercury Some removal
Nitrate Little removal
Organic Chemicals
Contaminant Level of Removal
Arsenic Little removal
Bacterial toxins Fair to good removal
Benzene Fair to good removal
Chlorine disinfection by-product Good removal
Cyanide Little removal
Ethylbenzene Fair to good remova
Flouride Good removal
Iodine disinfection by-product Good removal
Pesticides* Fair to good remova
Toluene Fair to good removal
Taste and odor compounds Good removal

*Includes: DDT, Aldrin, Dieldrin, Heptachlor, Heptachlorepoxide, 2,4-D, 2,4,5-T, Chlordane, Lindane, Methoxychlor, Benzene hexachloride, Triazine, Hexachlorobenzene, Endrin, Toxaphene and Aldicarb.

What is a water filter versus a purifier?

There are two main types of devices designed to make water safe for drinking for travallers.

Hand-held microfilters (sometimes simply called ‘filters’) physically remove organisms from water by straining them out while purifiers combine chemical treatment elements with microfilters in one device.

There are many units on the market, but for our purposes we will only consider compact lightweight devices that employ a pump, gravity feed, squeeze bottle or straw to force water through them. These devices are designed for single person use or small groups.

Note that only microfilters or purifiers with granular activated carbon (GAC) elements can remove some chemical contaminants however no hand-held device can make seawater drinkable.

Water microfilters

Microfilters work by physically straining out bacteria, protozoa and helminth eggs and larvae with one or more filter elements as water passes through the device.

Most filter elements are ceramic, glass fibre or composite depth filters that trap microorganisms in a maze of tiny passageways and dead-ends. You can usually clean ceramic filter elements by abrading the surface with a scrub pad after it begins to clog with silt, vegetative debris and organisms. Generally, non-ceramic filters cannot be cleaned by scrubbing but some can be back-flushed. Non-cleanable primary or back-up filter elements made of paper, metal, or glass fibre have to be discarded when they clog.

Filter Pore Sizes

Filter pore size, usually expressed in micrometres (microns), ranges from 6 microns down to 0.2 microns. Manufacturer literature often lists either ‘nominal’ or ‘absolute’ pore sizes.

Absolute pore size is the maximum size of any particle that can pass through a filter tested with microscopic beads. This means that a filter with an absolute pore size of one micron will allow nothing larger than one micron to pass through it.

Nominal pore size refers to a reduction level for particles, usually above 90%. This means that a filter with a nominal pore size of one micron will allow up to 10% of particles larger than one micron in diameter to pass through the filter.

It is important that you assess and compare water filters only on their absolute pore size and not their nominal pore size.

In addition, while pore sizes are tested with rigid microscopic beads, microorganisms are flexible, so you should leave a margin of error when selecting a filter for a particular task. For example, if you are selecting a water filter for Cryptosporidium oocysts, which are as small as three microns it is preferable to choose a filter with an absolute pore size in the one or two micron range, or smaller, to ensure that no oocysts are forced through the filter under the pressure of pumping.

While microfilters are very effective against protozoa, helminth eggs and larvae, and even tiny bacteria, no filter element has pores small enough to physically remove viruses unless the viruses are attached to larger particles or clumped together.

The smallest microfilter pore size currently available is in the 0.2 micron range while viruses can be as small as 27 nm (0.027 microns), which is only 1/7 the diameter of the pores.

Microfilter Absolute Pore Sizes for Types of Pathogens
Pathogen Organism Size in microns Maximum Absolute Pore size in microns
Viruses 0.027–0.05 Too small to filter with hand-held devices
Bacteria 0.2–5 0.2
Protozoa 2–20 1
Worm eggs and larvae 20–150 5–6

Water purifiers

On the outside, most water purifiers and microfilters are indistinguishable and share many common features.

Most purifiers include a filter element to remove protozoa, and helminth eggs and larvae and occasionally bacteria. The big difference is that purifiers have an additional antimicrobial feature, usually in the form of an iodine impregnated resin, that is supposed to kill bacteria and viruses as they pass through it.

Some units use electrostatically-charged elements to attract microorganisms to the resin surfaces. The idea is that the device strains out large halogen-resistant pathogens, such as protozoa and helminth eggs and larvae, with perhaps a one micron filter element and then the purification element kills or inactivates bacteria and viruses as they pass through the impregnated resin.

There is great debate about how well iodine resins work, given the short time period that bacteria and viruses are in contact with them, and the known need for sufficient contact time.

In some purifiers, tiny amounts of iodine are released into the water. This residual halogen continues to disinfect water until it is drank and adds to the effectiveness of the purifier, particularly if the water sits for 30 minutes or more before being consumed.

Some devices incorporate a granular activated carbon filter after the iodine element to remove the iodine, other chemicals and offensive tastes from the treated water. The problem with this scenario is that the iodine is removed almost immediately after being added to the water, which reduces contact time to almost nothing.

To Purify or not to Purify?

The limited amount of independent test data on purifiers generally agrees that they are very effective treatment devices when used according to manufacturer guidelines.

The best recommendation for purchasing a purifier is to look for one that combines a filter element with pores one micron or smaller to strain out protozoa and helminth eggs and larvae and an iodine resin to kill bacteria and inactivate viruses.

Since it is impossible to determine the microbiological contamination levels in recreational water without a lab, after pumping, let water sit for 20–30 minutes to allow the iodine to do its job.

Granular activated carbon attachments or elements should not be used if they remove iodine from the water immediately after it is added. They can be reattached and the water run through a second time to remove the iodine after the water has sat for an appropriate amount of time.