Using Activated Carbon Filters to Treat Home Drinking Water (2024)

EPA Drinking Water Treatment Device Categories
The U.S. Environmental Protection Agency(EPA) has defined three general categories of filters for treating homedrinking water: water filters, bacteriostatic water filters and water purifiers.
1.Water filters, generally comprisedof activated carbon (AC), are intended to remove rust; sediment; organiccompounds that impart taste, odor or color; chlorine and some other contaminants.They make no claims for pesticidal (antimicrobial) activity, and they arenot designed to remove or destroy bacteria unless they are labeled waterpurifiers.
2.Bacteriostatic water filters,also comprised of AC, generally remove the same contaminants as water filters,but they are impregnated with an additional chemical agent, such as silverions, that is intended to hinder the growth of bacteria trapped withinthe filter itself. (Bacteriostatic means the ability to inhibit the furthergrowth of bacteria.) The label can then state "inhibits bacterial growthwithin the filter medium."
3.Water purifiers are designedto treat raw water of unknown microbial quality to make it suitable forhuman consumption. They must kill or remove essentially all bacteria, protozoaand protozoan cysts that the label or instructions claim to remove.
Water purifiers are further subdividedas pesticidal devices and pesticides. Consumers should not confuse thewords pesticide and pesticidal with chemicals used in agriculture or householdsto control weeds, insects, molds or bacteria. Here the words mean an agentthat destroys a pest. In this case the pest is human pathogen bacteria,protozoa and protozoan cysts.
(a)pesticidal devices purifywater by physical or mechanical means, such as filtration, heating, etc.No antimicrobial chemical agent is involved.
(b)pesticides purify water throughthe use of antimicrobial agents (such as iodine) contained in the product.

Registering Treatment Devices
If a manufacturer claims that a unitwill inhibit or reduce the growth of microorganisms, or kill or removepathogenic organisms, and the unit contains a chemically active ingredientto promote the inhibition, the unit and the manufacturer are required toregister with the EPA before the devices can be legally offered for sale.But if a unit does not contain a chemically active ingredient, then onlythe manufacturer must be registered. If a manufacturer makes no claimsthat the unit will inhibit or reduce microorganism growth, then neitherthe unit nor the manufacturer must be registered. The EPA registrationdoes not imply any EPA approval of the unit nor its effectiveness for themanufacturer's stated purpose.

The registration means:
•The manufacturer claims that the unithas some sort of pesticidal property.
•Under normal use the pesticidal agentwill not leach out of the unit in concentrations which would be harmfulto humans.

The registration does not mean:
•The unit is in any way endorsed orapproved by EPA as a water treatment device.
•The unit is in any way superior orinferior to any other unit.

Mechanical Water Filters
A mechanical water filter removes particulatesby a mechanical process based on the physical size of the particulate.It can remove inorganic contaminants, such as heavy metals, if these inorganicsor metals are in the particulate form and not dissolved in the water andif the particulate size is large enough for the filter medium to retain.It has been found that the total amount of some heavy metals, e.g. lead(Pb), copper (Cu) and cadmium (Cd) that are found in drinking water fromtaps in the household, is in both the dissolved and the particulate forms.The amount that is in the particulate form can vary from nearly zero toalmost 100% of the total heavy metal present, and this undissolved partdepends on the water pH, hardness, total dissolved solids, temperature,other inorganic and organic chemicals that are present and their concentrations.
Most heavy metals found in drinkingwater, particularly lead, rarely occur there naturally. They come mostlikely from the water distribution system or from brass fittings, faucetsor household plumbing that is copper with lead solder.
Mechanical filters include depth filtersand surface filters. A depth filter consists of an array of fibrous, granularor sintered material that is wound, pressed or bonded together, with openingsof decreasing size.
A consumer would choose a depth filterwhen particulate loads are high (i.e., when there are visible and settleableparticles) or when there is a need to filter out a large amount of particulateswithout clogging the filter.
Surface filters trap sediment at orvery near the material surface. Included in this category are filters madeof membrane, pressed fiber and ceramic-coated or resin-bonded filters.They function like a screen, and precision openings in the filter can bemanufactured. A specific opening size (e.g., 0.3 microns) can be made tofilter out bacteria, protozoa, spores and cysts. These will remain trappedon the surface of the filter. These filters cannot screen out viral particlesbecause a virus can be as small as 0.01 microns.
Surface filters have an advantage overdepth filters because the size of the retained particle can be definedmore precisely, but they clog more readily than depth filters. Usuallythey are preceded by a depth filter.

Activated Carbon Water Filters
The majority of water filters purchasedtoday contain activated carbon or charcoal (AC). The AC may be powder,granules, solid block, paper membrane or wound spool made of carbon-impregnatedcotton cord or foam. Some devices contain AC but make no claim that itis present. Literature for some devices claim to remove odor and tastewithout mentioning AC. The only way the consumer can determine its presencemay be to break open the device, thus destroying its effectiveness.

Activated Carbon Types
AC is a form of carbon that is modifiedby a carefully controlled oxidation process to develop a porous carbonstructure with a large surface area. Some of the raw materials from whichAC is made are coal, bones, wood, nut shells, peat, lignite, residue frompetroleum processes or other organic materials. The oxidation process canproduce two distinct types of AC:
•L-carbon (L-AC) which is formed byoxidation at 300° - 400°C (570° - 750°F) with air or anoxidation chemical and
•H-carbon (H-AC) that is produced ina cooking process at 800° -1000°C (1470° - 1830°F) andcooled in an inert atmosphere.
L-AC has the ability to adsorb dissolved alkaline heavy metal ionssuch as Pb⁺², Cu⁺², Cd⁺², Hg⁺²depending on certain operational parameters such as pH and total dissolvedsolids. L-AC has acidic surface characteristics that interact with thebasic metals. L-AC can be regenerated using a strong acid to remove theadsorbed metal ions in a process similar to regeneration of ion exchangeresins used in the water softening process by the use of salt (NaCl) oran acid.
One comprehensive study has shown thatlead can be reduced to less than 1 ppb for typical water passing througha household plumbing system using AC and mechanical filtration. Much researchhas been conducted using AC to reduce heavy metals, but these studies havenot documented reductions of heavy metals to the maximum health level concentrationsthat are typically a few ppb.
The EPA has set heavy metal maximumconcentrations which are listed in Table 1 (effective November 1989).
NOTE: Many of these concentrations are subject to review, and new concentrationlevels may be established. Updated concentrations can be obtained throughyour local health department, public water system or your local CooperativeExtension office.

Table 1. Drinking Water Standards for Heavy Metals (EPA, 1989)

H-AC has alkaline surfaces that do noteffectively attract the alkaline heavy metal ions in solution in the water.The surface characteristics of H-AC make them more efficient absorbersof organic chemicals, particularly those that are hydrophobic, i.e., thosechemicals that have very low solubility in water. The ability of H-AC toadsorb heavy metal ions from solutions seems to be increased with acid-washedH-AC which neutralizes the alkalinity of this AC while not significantlyreducing the ability to adsorb organic chemicals. Heavy metal ions thatare complexed with synthetic or natural organic compounds, i.e., chelatedions, have shown to be effectively removed from water by unmodified H-ACdue to AC's ability to adsorb the organic chelating compound.

How Activated Carbon Structure Works to Filter Water
A lattice of internal microscopic passagesformed during the oxidation process gives AC an immense surface area. Asingle gram of AC can have a total surface area of more than 1,000 sq.ft. AC is extremely adsorptive. It can effectively remove organic compounds,chlorine and dissolved radon. Carbon filters will not remove bacteria,calcium and magnesium (hard water), fluorides, nitrates, chlorides andmany other inorganic chemicals. Heavy metals can be adsorbed onto AC byonly a very specific type AC.
The molecules that are removed diffuseinto the AC pores and eventually stick to the internal surfaces (see Figure1). All compounds are not adsorbed onto the AC surface equally. Smallermolecules will diffuse deeper into AC and can adsorb on more surface areathan large molecules because of the size of the pores. Organic chemicalswhich are the least soluble in water (high molecular weight, low polarity,less ionic) have greater adsorption onto the AC.
AC's effectiveness to remove organiccompounds decreases with increased temperature and AC absorptivity is reduced.Particulate and bacteria growing on the AC may clog the pores.
The number and kind of compounds inthe water will affect AC's ability to remove compounds. A compound witha higher affinity for adsorption on AC may displace a compound alreadyadsorbed. When an AC filter is nearly saturated with compounds, those compoundswith a low affinity for AC may not be adsorbed at all.
A parallel can be seen in the watersoftening process using ion exchange resins. The resins are recharged bypassing a high concentration of sodium (Na⁺) ions over the resinbed replacing the magnesium (Mg²⁺) and calcium (Ca²⁺)ions in the resins because the Na+ has a higher affinity when it is atvery high concentration. When Na⁺ is at a low concentrationin hard raw water, the Mg²⁺ and Ca²⁺ ions have ahigher affinity on the exchange resin and take the place of the Na⁺ion on the resin.
The organic material used and how itis processed to AC (see Activated Carbon Types) affect both its abilityto adsorb chemicals and its total removal capacity.

Activated Carbon Filter Types
Four types of AC filters are marketedas home treatment devices (seeFigure2A & 2B, ).
1.Faucet filters: These slip over themouth of the water faucet. Two basic designs are the bypass and the no-bypass:
*bypass:has a valve that allows you to filter only the water used for cooking anddrinking (prolongs the life of the filter).
*no-bypass:filters all the water flowing through the faucet.
2.Pour-throughs: These are the simplestand most portable. They require no installation at all. The user simplyholds the filter over a container and pours in tap water.
3.In-line or stationary: Tapped intothe cold-water pipe, these filter all the water flowing through the pipe.
4.Line bypass: These are installedby cutting into the water line beneath the sink. A separate faucet attachedto the sink delivers filtered water for drinking and cooking, but unfilteredwater can still be drawn from the regular faucet.

Factors Affecting AC Filter Performance
The following factors seem to affectthe performance of AC filters, and consumers should investigate them beforechoosing an AC filter:
1.water contact time with AC
2.iodine number
3.particle size of the AC
4. manufacturer's recommended watervolume treatment capacity
5.tests and ratings of independentorganizations

Contact Time. This is the time it takeswater to flow through the device. Contact times can vary from one secondto two minutes. The longer the contact time, the more chance for the chemicalsto diffuse into the AC to be adsorbed.
The more AC in a device seems to indicatemore treatment capability if the flow rate of water (e.g., gallons perminute) is the same. Table 2 lists the contact times for a few home ACfilters.
Iodine number. One measure of AC'scapacity to remove organics is the iodine number. This is the amount ofiodine, in milligrams, adsorbed by one gram of AC at a standard set ofconditions. The higher the iodine number, the more adsorptive the AC. Itis rare to see such a number reported in the advertising literature orinstructions or on box labels of AC devices. Table 2 gives the iodine numberfor a limited number of AC filters.
Particle Size. The smaller the particlesize, the more outside surface is available for compounds to enter theinternal porous matrix of the AC, resulting in a higher removal rate oforganic contaminants. Therefore, powdered AC and block AC, made from compressedpowdered AC, would be more effective than granulated AC if the AC had thesame iodine number, AC amount and contact time.
Recommended Capacity. Some manufacturersof AC water treatment devices give a recommended water treatment capacityin gallons. When the rated capacity is exceeded, they recommend replacingthe AC.
Most devices on the market do not indicatehow much water has passed through the filter during use. A consumer canestimate the number of days a filter will last before needing replacement.Assume that each person uses one gallon of water each day for drinkingand one to three gallons a day for cooking. For a household of four peoplewho would use one gallon per person per day, four gallons of water willneed to be treated daily. At this rate of use, an AC treatment device witha 200-gallon capacity will last approximately 50 days (200 gal./4 gal.per day).

Table 2. Performance Factors and Removal Efficiency for SelectedAC.

Performance of Activated Carbon Filters
Performance of AC filters has beenreported by Consumer Reports (Jan. 1990 and Nov. 1983), Rodale's PracticalHomeowner (Jan. 1987), EPA from results of tests conducted by Gulf SouthResearch Institute (GSRI) in J. American Waterworks Assoc. (April 1984)and National Sanitation Foundation (NSF) (address: 3475 Plymouth Road,P.O. Box 1468, Ann Arbor, MI 48106, telephone number: 313-769-8010).
The results summarized below are fora limited number of AC devices to illustrate their performance differences.Table 2 lists a number of AC treatment devices from the GSRI study, andTable 3 summarizes AC filters tested by Consumer Reports and NSF. GSRItested for the removal of these organic compounds:
•trihalomethanes (THMs): chloroform,bromoform, dichlorobromomethane, and dibromochloromethane) which are primarilybyproducts of chlorination disinfection of drinking wafer
•NPTOC (nonpurgeable total organiccarbon) which is predominated by larger molecules whose origins are naturalorganics that can cause "off" taste, odor and color in water and
•halogenated hydrocarbons (carbon tetrachloride,trichloroethylene, tetrachloroethylene, trichloroethlane, dichlorobenzene,hexachlorobenzene and chlordane) whose origins are industrial solvents.

Table 2 summarizes the percent removalof each of these three categories during the manufacturer's rated filterlife. Figures 3 and 4 illustrate the removal efficiency of THMs and NPTOCsas the filter processes water. In all examples the removal efficiency decreasesas increased volume of water is processed. Substantial differences do occur.Polycyclic aromatic hydrocarbons (PAHs) have been removed by AC at efficienciescomparable to PCBs and halogenated hydrocarbons.
Consumer Reports (Jan. 1990) statesthat high treatment capacity AC filters (>1,000 gal. capacity) are moreeffective than other type AC filters, such as faucet-mounted or pour-throughtypes (see Figures 2A and 2B) when chloroform (a THM) removal was tested.
Rodale Press analyzed AC treatmentdevices and published results for chlorine and halogenated organics (72of EPA's 129 priority pollutants, the tested organics not defined). Theseresults are listed in Figures 5 and 6 for the rated filter life capacity.The percentage removal of halogenated organics was the total removed anddoes not differentiate between specific chemicals. No chemical-specificremoval percentages were listed.

Table 3. Activated Carbon Filters -- Cost Comparisons.

(Figure 3)

(Figure 4)

(Figure 5)

(Figure 6)

Validation of Performance Claims
The National Sanitation Foundation(NSF) validates manufacturers' claims if they voluntarily submit theirunits for testing and if their devices meet NSF standards for the specificcompound the manufacturer claims to remove.
The NSF tests treatment devices undertwo separate standards (#42 and #43): 1 ) chemicals that affect only theaesthetics of drinking water (i.e., taste, odor, color and appearance)and 2) hazardous chemicals.
The devices tested for aesthetics arechallenged with a standard prepared water (chemical components exceededthe recommended concentrations of EPA's Secondary Drinking Water Standards)to substantiate claims. The effluent from these units must meet the EPASecondary Drinking Water Regulations while processing the water up to thedevice's rated capacity. The devices must be periodically tested to certifythat they continue to meet claims.
NSF also established Standard #43 forassessing and certifying drinking water treatment devices that claim toreduce hazardous chemicals in drinking water (i.e., those chemicals thatexceed the EPA Primary Drinking Water Standards or those chemicals thatare suspected to cause illness but for which there is no EPA standard).
The NSF requires that the manufacturersof tested equipment provide a means (possibly an indicator or warning)to alert the consumer when the unit is not performing properly. These maybe on the device (e.g., an automatic shut-off, a reduction in flow, analarm) or in a separate test kit provided to the consumer. If these arenot provided, then the AC filter must meet the removal efficiency of Standard#43 for twice the rated filter capacity. This gives a safety factor tothe consumer.
NSF also evaluates under Standard #42bacteriostatic devices designed to limit the passage and/or growth of heterotrophicbacteria. It requires that the bacterial population is no greater in theeffluent from the device than in the influent. NSF tests whether the activebacteriostatic agent or its degradation product in all effluent samplesexceeds the EPA Primary Drinking Water Regulations or those of any otherfederal regulatory agency for chemicals not regulated by EPA.
Devices meeting NSF's standards areallowed to display the NSF Mark (see Figure 7)on the device, literature and advertising. Twice a year NSF publishes alist of those devices currently meeting their standards.

Radon Gas Removal
Scientists know that AC can remove99 percent of radon gas dissolved in water, but they have not yet establishedefficiency rates for radon removal for commercially available drinkingwater treatment devices.

Bacterial Growth on Activated Carbon
AC units have several drawbacks. BecauseAC deactivates it, chlorine cannot disinfect bacteria present in the AC.However, if water is pretreated to eliminate pathogenic bacteria beforeit reaches the device, these bacteria do not grow and multiply on the AC.
But non-pathogenic bacteria, in particularheterotrophic plate count (HPC) bacteria, will grow. The health effectsof high counts of HPC bacteria are not clear. We take in millions of bacteriaa day, normally with no ill effects, and a healthy person is generallynot at risk. But there may be a potential health risk for those who aremore vulnerable, such as the aged, the very young or the sick whose immunesystems are weaker. Certain HPC bacteria are known to be "opportunistic"and may take advantage of these weaknesses and cause illness.
A high bacterial count can occur whenwater does not pass through an AC filter after it has not been used overnight.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 bacteriacounts to 1/7 the initial numbers, and as the AC filter is used duringnormal household activity for four hours, the HPC bacteria are reducedby 1/25. Still, several studies indicate that the HPC bacteria count ishigher in effluent than in influent.
One promoted solution may be a bacteriostaticfilter. The AC in bacteriostatic filters is impregnated with silver toprevent HPC bacterial accumulations. The silver is a disinfectant, andwhen released or leached from the AC in small quantities, it interactswith the bacteria in the filter and reduces their ability to multiply.The silver, a heavy metal, should be released in small enough quantitiesso as not to exceed the toxic limits set forth by the EPA Primary DrinkingWater Regulations.
Studies by the GSRI for the EPA haveindicated that silver-impregnated AC made little difference when comparedto untreated AC in terms of HPC bacteria growing on the AC or in totalcounts found in the effluent water. The only advantage noted in severalstudies of silver-impregnated AC was that in the first month of use, thebacterial counts were lower than AC without silver.
The best recommendation for preventinghigh HPC bacteria counts is to replace the AC filter periodically at leastas often as the manufacturer recommends or even more frequently. If themanufacturer makes no recommendation, replace the AC at least every sixmonths (maybe even every three months) even if the manufacturer's recommendedtreatment capacity is not exceeded. Otherwise, the owner should adherestrictly to the manufacturer's recommendations for changing the filter'sAC.

When An AC Filter Is No Longer Effective
Another disadvantage of an AC filteris that the only way to be sure the filter has reduced the contaminantsof concern is to test the water coming out of the filter unless the manufacturerprovides a testing procedure. The consumer will be aware of the loss ofeffectiveness because of an "off" taste, odor or color in the water. Ifthe contaminant affects only the aesthetics of the water, then the filter'sineffectiveness does not pose a health risk. But many hazardous chemicalscannot be detected by taste, odor or color.

Recommendations to the Consumer
•Use AC filters to treat water onlyfor drinking and cooking unless radon removal is required.
•Use AC filters on water that is disinfectedbefore it reaches the filter.
•Use AC filters on cold water only.
•Replace the filter:

•Filter the water at the slowest possiblerate tolerable to increase contact time.
•Flush filters for 30 seconds whenfirst used each day. Flush the filter for two or three minutes if not usedfor several days.
•Change the AC filter:

•Select AC fillers whose claims areindependently validated by a nationally recognized independent testinglaboratory.
•Silver-impregnated filters reducebacteria on filters for up to four weeks, then give similar results asother AC filters.

This material is based on work supportedby the U.S. Department of Agriculture, Extension Service, under specialproject number 89-FWQI-1-9156.
Trade names are used for comparisonpurposes only. No endorsem*nt is intended, nor is criticism implied ofsimilar products not named.

Water Quality Terms
Activated carbon or activated charcoal(AC) -- Particles or granules of carbon produced by carbonization ofcellulosic or other organic matter in limited or no air. These particlespossess a very porous structure that has highly adsorptive properties toremove some organic and inorganic contaminants and certain dissolved gasesfrom water.
Aesthetic quality -- The qualityof water as sensed by sight, taste and smell. These quality standards areusually referred to in drinking water quality standards as secondary orother contaminants that do not have a direct health impact.
Adsorption -- The process bywhich a gas, vapor, dissolved material or a minute particle adheres tothe surface of a solid.
Bacteriostatic -- The abilityto inhibit the further growth of bacteria.
Contamination -- Any introductioninto water of microorganisms or chemicals in a concentration that makeswater unfit for its intended use.
Chelated ion -- An ion, usuallya metal, that is in close combination with an inorganic or organic compoundthat keeps the ion dissolved and prevents it from exhibiting its usualproperties, e.g., forming a solid that settles.
Cooking process -- Carbonizationprocess of organic material in the absence of air at high temperature.
Contact time -- The time ittakes for water to flow through a treatment device.
Deactivate -- Reduction in theability of a solid surface to adsorb chemicals. An action causing pathogenicorganisms to lose their ability to cause disease.
Disinfection -- The removalor destruction of infectious or pathogenic microorganisms (bacteria, virusor protozoa).
Drinking water treatment unit (DWTU)-- A device used to improve the quality of water for its effects on aestheticsand human health and to make it suitable for drinking.
Effluent -- The water that flowsout of a DWTU.
Hardness -- A measure of theminerals, predominantly calcium and magnesium, dissolved in water thataffect its soap neutralizing characteristics and the formation of scaleon pipes and in boilers.
Heavy metal(s) -- One or moreof the following metals whose density is greater than 5 gm/cc: cadmium(Cd), lead (Pb), mercury (Hg), copper (Cu), silver (Ag), zinc (Zn), chromium(Cr), barium (Ba), arsenic (As), selenium (Se).
Heterotrophic bacteria -- Bacteriathat thrive only on organic matter for energy and growth.
Hydrophobic -- Lacking the affinityfor, repelling or failing to be absorbed by water.
Influent -- The water enteringa DWTU.
Inorganic compound -- A substancethat does not contain carbon (except as carbonates, cyanates, cyanides,or carbide).
Maximum contaminant level (MCL)-- A standard that is the highest allowable concentration of a contaminantin drinking water. This standard is set as a result of scientific studiesof contaminant effects on health or aesthetics.
Mechanical filter -- A devicethat removes particulates, sediments or colloidal material from water byphysical size as the water passes through a medium made of a screen, fibrous,granular or sintered material that is wound, pressed or bonded together.
Micron -- A unit of measurethat equals 0.000039 inches (abbreviated as 1 m) is a micrometer (1 x 10.6meter).
Microorganism -- A microscopicorganism, including bacteria, protozoa, yeasts, viruses and algae.
Nonpurgeable total organic carbon(NPTOC) -- Usually relates to large organic molecules whose originsare natural and that impart taste, odor and color to water. These compoundsare not in general related to health risks.
Pathogen -- Any microorganismwhich may cause a disease.
Pesticide -- An agent that destroysa pest. In water, a pest generally refers to human pathogenic bacteria,protozoa or protozoan cysts or viruses.
pH -- The strength of the acidor base present measured on a scale of 0 to 14 with a pH of 0 to 7 beingan acid, pH of 7 being neutral and a pH of 7 to 14 being a base.
Point of entry treatment (POE)-- Treatment of water at the entry point to a home or business.
Point of use treatment (POU)-- Treatment of water at the point of use, such as a kitchen tap.
Polychlorinated biphenyls (PCBs)-- A group of man-made chemicals made up of two benzene rings bonded together(biphenyl) with one or more hydrogen atoms replaced by chlorine. PCBs havebeen used in electrical equipment, hydraulic fluid, inks, paints, adhesives,fire retardants and heat transfer fluids. They have been banned for mostuses since 1979.
Polycyclic aromatic hydrocarbons(PAHs) -- Also known as polynuclear aromatic hydrocarbons (PNAs) formedduring incomplete combustion of fuels (coal and petroleum products) andcellulosic material (wood, paper, tobacco). They are multiringed hydrocarboncompounds (aromatics) that share two or more carbon atoms by two or morerings. Many compounds in this group are carcinogenic.
ppb -- Parts per billion. Thenumber of weight or volume units of a minor constituent present with onebillion units of a major constituent of a mixture.
ppm -- Parts per million. Thenumber of weight or volume units of a minor constituent present with onemillion units of the major constituent of a mixture.
Primary drinking water standards-- Standards for maximum contaminant limits (MCL) of pollutants in drinkingwater that affect human health. These standards are set by the EPA to bemet by public water systems.
Public water system -- Any systemowned by any person for the provision to the public of piped water forhuman consumption if the system has at least 15 service connections orserves regularly an average of at least 25 individuals daily at least 60days a year.
Purification -- The removalof objectionable matter from water by natural or artificial methods.
Radon -- A radioactive gas thatis a natural radioactive decay product of uranium.
Regenerate -- A process usedto restore the adsorption activity of a substance.
Secondary drinking water standards-- Standards for maximum contaminant levels (MCL) in drinking water thataffect aesthetics (taste, odor, color and corrosivity) but do not posea health risk. These standards are encouraged by EPA but not enforced exceptif alternate water sources have comparable costs and have lower secondaryconcentrations.
Sediment -- Suspended solidparticles that settle from water.
Solubility -- The extent towhich one substance will dissolve in another substance.
Trihalomethanes (THMs) -- Agroup of chemical organic substances that contain halogen elements (i.e.,chlorine, fluorine, bromine, etc.) attached to three positions on a methanemolecule. These compounds are derived from many sources and are toxic whenfound in more than trace amounts. THMs are a by-product of the chlorinatedprocess to disinfect water when organic compounds are present.
Validate -- To confirm or verifythat claims made are correct based on a standard or standard method.

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