The U.S. Environmental Protection Agency (EPA) has defined three general categories of filters for treating home drinking water: water filters, bacteriostatic water filters and water purifiers.
1) Water filters, generally comprised of activated carbon (AC), are intended to remove rust; sediment; organic compounds that impart a taste, odor or color; chlorine and some other contaminants. They are not designed remove or destroy bacteria unless they are labeled water purifiers.
2) Bacteriostatic water filters: These filters, also comprised of AC, generally remove the same contaminants as water filters, but they contain an additional chemical agent, such as silver ions, that hinders the growth of bacteria trapped within the filter itself. (Bacteriostatic means the ability to inhibit the further growth of bacteria.)
3) Water purifiers are designed to treat raw water to make it suitable for human consumption. They must kill or remove essentially all bacteria, protozoa and protozoan cysts that the label or instructions claim to remove.
Water purifiers are further subdivided as pesticidal devices and pesticides. As used here, the words pesticide and pesticidal shouldn't be confused with chemicals used in agriculture or households to control weeds, insects, molds or bacteria. Here the words mean an agent that destroys a pest. In this case the pest is bacteria that cause disease.
a) pesticidal devices purify water by physical or mechanical means, such as filtration, heating, etc.
b) pesticides purify water through the use of antimicrobial agents (such as iodine) contained in the product.

Mechanical Water Filters
A mechanical water filter removes sediment (particulate) by a mechanical process based on physical size of the particulate. It can remove inorganic contaminants, such as heavy metals, if they are in the particulate form and not dissolved in the water and if the particulate is large enough for the filter medium to retain.
Most heavy metals found in drinking water, particularly lead, rarely occur there naturally. They come most likely from the water distribution system or from brass fittings, faucets or household plumbing that is copper with lead solder.
Mechanical filters include depth filters and surface filters. A depth filter consists of an array of filter material, with openings of decreasing size between the material. Sediment is trapped throughout the depth. It is rated according to the approximate particle size (in microns) that it will retain (I micron is approximately 0.00004 inches). This rating is not an absolute. An unknown percentage of particles larger than the rating will pass through, while some particles smaller than the rating will be retained.
You would choose a depth filter when particulate loads are high (i.e., when there are visible and settleable particles) or when you need to filter out a large amount of particulates without clogging the filter.
Surface filters have an advantage over depth filters because the size of the retained particle can be defined more precisely, but they clog more readily than depth filters. Usually they are preceded by a depth filter.

How Activated Carbon Structure Works
The majority of water filters purchased today contain activated carbon or charcoal (AC). Some devices contain AC but make no claim that it is present. The only way the consumer can determine its presence may be to break open the devices, thus destroying their effectiveness.
AC has an immense surface area. A single gram can have a total surface area of more than 1,000 sq. ft. AC is extremely adsorptive (able to attract minute particles so they stick to the surface). It can effectively remove organic compounds, chlorine and dissolved radon. Carbon filters will not remove bacteria, calcium and magnesium (hard water), fluorides, nitrates, chlorides and many other inorganic chemicals. Only a very specific type of AC can effectively adsorb heavy metals.
The molecules that are removed sink into the AC pores and eventually stick to the internal surfaces (see Figure 1).

Activated Carbon Filter Types
Four types of AC filters are marketed as home treatment devices (see Figures 2A to 2D).
1) Faucet filters:
•These slip over the mouth of the water faucet. Two basic designs are the bypass and the no-bypass:
•Bypass: has a valve that allows you filter only the water used for cooking and drinking (prolongs the life of the filter).
•no-bypass: filters all the water flowing through the faucet.
2) Pour-throughs:
•These are the simplest and most portable. They require no installation at all. You simply hold the filter over a container and pour in tap water.
3) In-line or stationary:
•Tapped into the cold-water pipe, these filter all the water flowing through the pipe.
4) Line bypass:
•These are installed by cutting into the water line beneath the sink. A separate faucet attached to the sink delivers filtered water for drinking and cooking, but unfiltered water can still be drawn from the regular faucet.

Factors Affecting Activated Carbon Fitter Performance
The following factors seem to affect the performance of AC filters, and consumers should investigate them before choosing an AC filter:
1)water contact time with AC
2)iodine number
3)particle size of the AC
4)manufacturer's recommended water volume treatment capacity
5)tests and ratings of independent organizations

Contact Time.
This is the time it takes water to flow through the device. Contact times can vary from one second to two minutes. The longer the contact time, the more chance for the chemicals to stick to the AC.

Iodine number.
One measure of AC's capacity to remove organics is the iodine number. This is the amount of iodine in milligrams, adsorbed by one gram of AC at a standard set of conditions. The higher the iodine number, the more adsorptive the AC. Such a number is rarely reported in the advertising literature or instructions or on box labels of AC devices.

Particle Size.
The smaller the particle size, the more outside surface is available for compounds to enter the AC, resulting in a higher removal rate of organic contaminants.

Recommended Capacity.
Some manufacturers of AC water treatment devices give a recommended water treatment capacity in gallons. When the rated capacity is exceeded, they recommend replacing the AC.
Most devices on the market do not indicate how much water has passed through the filter during use. A consumer can estimate the number of days a filter will last before needing replacement. Assume that each person uses one gallon of water each day for drinking and one to three gallons a day for cooking. To illustrate, if a household of four people used one gallon per person per day, four gallons of water will need to be treated. At this rate of use, an AC treatment device with a 200-gallon capacity will last about 50 days (200 gal./4 gal. a day).

Table 1. Performance Factors and Removal Efficiency for Selected AC.

	Manufacturer's  rated capacity  (gallons)	Amount  of carbon  (grams)	Iodine  number  of carbon	Contact  time  (seconds)	Average %  Removal  of THM	Average %  Removal  of NPTOC	Average %  Removal of  Halogenated  Hydrocarbons
Line by-pass
Culligan SG-2	4,000	1,708	980	39	89	28	99
Aquacell Bacteriostatic	2,000	417	867	13	86	23	97
Aqualux CB-2	2,000	1,150	966	35	98	23	99
Everpure QC4-THM	1,000	765	1,010	43	99	55	99
Seagull IV	1,600	300	434	15	70	41	97
Faucet-mounted
Hurley Town & Country	4,000	895	913	36	69	31	97
Aqua Guard ACT31	500	51	1,275	3	43	12
Instapure FI-C	200	27		1.6	24	11
Stationary
AMF Cuno-IM	3,000	395	870	3.6	34	7
Pour-through
Filbrook	1,000	97	788	44	40	14	94

Source: GSRI Study for EPA, 1984. THM: trihalomethanes. NPTOC: Non-purgeable total organic carbon.
For more detailed information see Extension publication IP-6.
 

Table 2. Activated Carbon Filters - Cost Comparisons.

Manufacturer	Model	Cost	Replacement  Filter Cost  (each)	Filter Rated  Capacity (gal.)	Chloroform  Removal (%)
High Volume Filters
Ametek	CCF-201	$158	$20 (2 req'd)	1500	>97
Amway	E-9230	276	69	5000	>97
Culligan	Supergard THM	349	37	1000	90
Cuno	Aqua Pure AP-CRF	155	15	450	90
Everpure	H200	298	90	750	>97
Filterite	CF-10	85	8	750	90
Kinetico	MAC	275	32	500	90
NSA	Bacteriostatic 50C	179		5000	80
Omni	UC-2	99	20 (2 req'd)		80
Faucet-Mounted
Cuno	PP01105	30	6	735	60
Pollenex	WP90K	22	5	200	30
Pour-Through
Brita		30	8	35	65
Innova		7	5	30	45
Glacier Pure		13	5	100	40

Source: Consumer Reports (Jan. 1990) and National Sanitation Foundation.

Independent Performance Ratings:
Several independent consumer testing organizations have rated the performance of AC filters. These reports are the source for the information shown at Tables 1 and 2.

Validation of Performance Claims
The National Sanitation Foundation (NSF) validates manufacturers' claims if they voluntarily submit their units for testing and if their devices meet NSF standards for the specific compound the manufacturer claims to remove.
The NSF tests treatment devices for two separate factors: 1) chemicals that affect only the aesthetics (i.e., taste, odor, color and appearance) of drinking water and 2) hazardous chemicals. The outflow from these units must meet the EPA Secondary Drinking Water Regulations while processing the water up to the device's rated capacity. The devices must be periodically tested to certify that they continue to meet claims. Those devices meeting certain standards are allowed to display the NSF Mark on the device, literature and advertising.
NSF also tests and certifies treatment devices that claim to reduce hazardous chemicals in drinking water for the rated capacity, i.e. those chemicals that exceed the EPA Primary Drinking Water Standards or those chemicals that are suspected to cause illness but for which there is no EPA standard.
The NSF requires that the manufacturers of tested equipment provide a means (possibly an indicator or warning) to alert the consumer when the unit is not performing properly. These may be on the device (e.g., an automatic shut-off, a reduction in flow, an alarm) or in a separate test kit provided to the consumer.
To be approved by NSF, AC filter units that do not provide a means to alert the consumer must remove the contaminants for twice the rated capacity. This gives a safety factor to the consumer.
NSF also evaluates bacteriostatic devices. It tests 1) whether the bacterial population is any greater in the water coming out of the device than in the water coming in and 2) whether the active bacteriostatic agent or its degradation product exceeds EPA's Primary Drinking Water Regulations or those of any other federal regulatory agency for chemicals not regulated by EPA.

Radon Gas
Scientists know that AC can remove 99 percent of radon gas dissolved in water, but they have not yet established efficiency rates for radon removal for commercially available drinking water treatment devices.

Bacterial Growth
AC units have several drawbacks. Because AC deactivates it, chlorine cannot disinfect bacteria present in the AC. However, if water is treated to eliminate disease-causing bacteria before it reaches the device, these bacteria do not grow and multiply on the AC.
But non-pathogenic bacteria, in particular heterotrophic plate count (HPC) bacteria, will grow. The health effects of high counts of HPC bacteria are not clear. We take in millions of bacteria a day, normally with no ill effects, and a healthy person is generally not at risk. But there may be a potential health risk for those who a more vulnerable, such as the aged, the very young or the sick whose immune systems are weaker. Certain HPC bacteria are known to be 41 opportunistic" and may take advantage of these weaknesses and cause illness.
A high bacterial count can occur overnight when water does not pass through an AC filter. The first water drawn from the filter that day may be cloudy with bacteria. Flushing the filter at full flow for 30 seconds reduces the HPC bacteria counts to 1/7 the initial numbers, and as the AC filter is used during normal household activity for four hours, the HPC bacteria are reduced by 1/25. Still, several studies indicate that the HPC bacteria count is higher in water coming out of the device than in water coming in.
One promoted solution may be a bacteriostatic filter. The AC in bacteriostatic filters is impregnated with silver to prevent HPC bacterial accumulations. The silver is a disinfectant, and when released or leached from the AC in small quantities, it interacts with the bacteria in the filter and reduces their ability to multiply. The silver, a heavy metal, should be released in small enough quantities so as not to exceed the toxic limits set forth by the EPA Primary Drinking Water Regulations.
Studies have indicated that silver-impregnated AC made little difference in reducing HPC bacteria except for the first month of use when bacterial counts were lower than AC without silver.
The best recommendation for preventing high HPC bacteria counts is to replace the AC filter at least as often as the manufacturer recommends or even more frequently. If the manufacturer makes no recommendation, replace the AC at least every three months even if the manufacturer's recommended treatment capacity is not exceeded. Otherwise, the owner should follow the manufacturer's recommendations for changing the AC.

When an AC Filter Is No Longer Effective
Another disadvantage of an AC filter is that the only way to be sure the filter has reduced the contaminants of concern is to test the water coming out of the filter which is expensive unless the manufacturer provides a testing procedure. You will be aware of the loss of effectiveness because of an "off' taste, odor or color in the water, but many hazardous chemicals cannot be detected in this way.

Recommendations
•Use AC filters to treat water only for drinking and cooking unless radon removal is required.
•Use AC filters on water that is disinfected before it reaches the filter.
•Use AC filters on cold water only.
•Replace the filter: - if signs of sediment appear in treated water. - if taste, odor or color changes. This can mean that the AC is no longer effectively removing the compounds. - when the flow is noticeably reduced
•Filter the water at the slowest possible rate tolerable to increase contact time.
•Flush filters for 30 seconds when first used each day. Flush the filter for two or three minutes if not used for several days.
•Change the AC filter:
- As frequently as the manufacturer recommends -- preferably more often. If there is no recommendation, change the filter every three months even if the water treatment capacity is not exceeded.
- When treatment capacity is reached. Estimate use at one gallon of water per person per day for drinking and two gallons per person per day for drinking and cooking.
•Select AC filters whose claims are independently validated by a nationally recognized independent testing laboratory.
•Silver-impregnated filters reduce bacteria on filters for up to four weeks, then give similar results as other AC filters.

This material is based on work supported by the U.S. Department of Agriculture, Extension Service, under special project number 89 -FWQI-1-9156.
Trade names are used for comparison purposes only. No endorsement is intended, nor is criticism implied of similar products not named.