Water Filter Glossary: 100+ Terms Explained
Updated July 2026
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Before investing in a water filtration system, understand the terminology manufacturers, certifications, and plumbers use. This comprehensive glossary covers every essential term from basic chemistry to advanced membrane technology.
A Top ↑
Activated Carbon
A highly porous form of carbon processed to have an enormous surface area (typically 500-1,500 m² per gram) available for adsorption of contaminants. Activated carbon is produced by heating carbonaceous materials (coconut shells, coal, wood) in the absence of oxygen, then activating with steam or chemicals to create a network of micropores. It is the most widely used water filtration media, effective at removing chlorine, VOCs, pesticides, and organic compounds through adsorption. The two primary forms used in filtration are granular activated carbon (GAC) and carbon block.
Activated Alumina
A granular, porous form of aluminum oxide (Al₂O₃) specifically used for adsorbing fluoride, arsenic, and selenium from water. Activated alumina has a surface area of 200-300 m²/g and works best at pH 5.5-6.0 for fluoride removal, with capacity typically 1-5 mg fluoride per gram of media. It requires periodic regeneration with sodium hydroxide or replacement when exhausted.
Adsorption
The physical process by which molecules adhere to the surface of a solid material (adsorbent) through van der Waals forces or chemical bonding. In water filtration, adsorption is the primary mechanism by which activated carbon captures contaminants. Unlike absorption (where a substance penetrates into the volume of another), adsorption occurs only at surfaces. Adsorption capacity decreases as the adsorbent becomes saturated, requiring periodic replacement or regeneration.
Aeration
A water treatment process that brings water into contact with air to remove dissolved gases (hydrogen sulfide, methane, carbon dioxide) and volatile organic compounds (VOCs). Aeration methods include spray nozzles, cascading trays, diffused bubble systems, and packed tower aerators. Aeration is particularly effective for removing radon (achieving 80-99% reduction) and reducing water hardness caused by dissolved CO₂.
Alkalinity
A measure of water's capacity to neutralize acids, primarily due to the presence of bicarbonate (HCO₃⁻), carbonate (CO₃²⁻), and hydroxide (OH⁻) ions. Alkalinity is expressed as mg/L CaCO₃ (calcium carbonate equivalent). Water with alkalinity below 75 mg/L is considered low and may be corrosive to pipes; water above 200 mg/L may cause scale formation and bitter taste. Alkalinity acts as a buffer, stabilizing pH.
Anion
A negatively charged ion. Common anions in water include chloride (Cl⁻), sulfate (SO₄²⁻), nitrate (NO₃⁻), bicarbonate (HCO₃⁻), and fluoride (F⁻). Anion exchange resins in water softeners and deionization systems target these negatively charged contaminants, swapping them for chloride or hydroxide ions.
AOP (Advanced Oxidation Process)
Water treatment processes that generate highly reactive hydroxyl radicals (OH•) capable of destroying virtually all organic contaminants, including pharmaceuticals, pesticides, and disinfection byproduct precursors. Common AOP combinations include UV + hydrogen peroxide, UV + ozone, ozone + hydrogen peroxide, and photocatalysis with titanium dioxide. AOPs achieve 90-99%+ contaminant destruction but are energy-intensive and may produce oxidation byproducts.
B Top ↑
Backwash
A cleaning cycle in which water flows upward through a granular filter bed (activated carbon, sand, greensand) at a flow rate sufficient to expand and fluidize the media, dislodging trapped particles and flushing them to drain. Backwash typically runs 5-15 minutes at 8-12 GPM per square foot of bed area. Without regular backwash, filter media becomes clogged, channeling occurs, and filtration performance degrades. Most whole-house filters require backwash every 3-7 days.
Bacteria
Single-celled microorganisms ranging from 0.2 to 10 microns in size. Pathogenic bacteria in drinking water include E. coli, Salmonella, Shigella, Campylobacter, and Vibrio cholerae. The EPA's Surface Water Treatment Rule requires public water systems to maintain detectable disinfectant residual and achieve 99.9% (3-log) removal/inactivation of Giardia and 99.99% (4-log) removal/inactivation of viruses. Bacteria are removed by microfiltration (0.1 micron), UV disinfection, chlorination, and reverse osmosis.
Brine Tank
A separate tank in an ion exchange water softener that holds sodium chloride (salt) or potassium chloride in a concentrated solution. During the regeneration cycle, brine (typically 10-12% salt concentration) is drawn from the tank and passed through the resin bed to displace accumulated calcium and magnesium ions, restoring the resin's softening capacity. A typical residential softener uses 40-80 pounds of salt per regeneration cycle, occurring every 3-14 days depending on water hardness and usage.
Bypass Valve
A plumbing fitting that allows water to circumvent a filter, softener, or other treatment device when maintenance is needed or the unit fails. Whole-house systems should always be installed with a bypass valve to ensure uninterrupted water supply during filter changes, repairs, or backwash cycles. Bypass valves typically feature three positions: "Filter" (normal operation), "Bypass" (water goes around unit), and "Off" (stops all water flow for servicing).
Biological Filtration
A treatment process that uses beneficial microorganisms growing on filter media to biodegrade organic contaminants. Biofilms on sand, activated carbon, or engineered media break down naturally occurring organic matter (NOM), reducing disinfection byproduct formation potential and removing taste- and odor-causing compounds like geosmin and MIB (2-methylisoborneol). Biological activated carbon (BAC) filters combine adsorption with biodegradation, extending carbon bed life by 2-4x compared to adsorption alone.
C Top ↑
Carbon Block
A solid, compressed form of activated carbon manufactured by bonding fine carbon particles (typically 5-50 microns) with a thermoplastic binder and extruding into cylindrical cartridges. Carbon block filters offer superior performance to granular activated carbon (GAC) due to longer contact time, more uniform flow distribution, and ability to incorporate additional adsorbents (zeolite, KDF) and sub-micron particle filtration. Common ratings include 0.5, 1, 5, and 10 microns.
Catalytic Carbon
A specially processed activated carbon with enhanced catalytic properties for removing chloramines and hydrogen sulfide. Standard activated carbon has limited effectiveness against chloramines (monochloramine, dichloramine) - the combination of chlorine and ammonia used by an increasing number of water utilities. Catalytic carbon contains surface modifications that accelerate the breakdown of chloramines into chloride, nitrogen, and ammonia without rapidly exhausting the carbon capacity. Removal rates of 90%+ are achievable with proper contact time.
Cross-Flow Filtration
A membrane filtration configuration in which feed water flows parallel to the membrane surface rather than directly at it. A portion of the water permeates through the membrane as "permeate" or "product water," while the remaining concentrated water ("concentrate" or "reject") continues flowing across the membrane, sweeping away rejected contaminants. Cross-flow is used in reverse osmosis, nanofiltration, and ultrafiltration systems. The cross-flow velocity (typically 1-4 ft/s for RO) prevents excessive fouling and concentration polarization at the membrane surface.
Cyst
A dormant, resistant form of protozoan parasites including Giardia lamblia (size: 8-14 microns) and Cryptosporidium parvum (size: 4-6 microns). Cysts are surrounded by a tough outer wall that makes them highly resistant to chlorine disinfection - Cryptosporidium can survive in chlorinated water for days. The EPA requires 3-log (99.9%) removal of Giardia and 4-log (99.99%) removal of viruses. Cysts are removed by microfiltration (0.1-0.2 micron), ultrafiltration, reverse osmosis, and UV disinfection at doses of 10 mJ/cm² (for Giardia) or 3 mJ/cm² (for Cryptosporidium).
CT Value
A measure of disinfection efficacy calculated as the product of disinfectant Concentration (C, in mg/L) and contact Time (T, in minutes). The EPA publishes CT tables specifying the minimum CT value required to achieve specific log inactivation of pathogens with different disinfectants. For example, achieving 3-log Giardia inactivation with free chlorine at 20°C and pH 7.0 requires a CT of 54 mg·min/L. Higher CT values are needed at lower temperatures and higher pH. UV disinfection uses a different metric: dose in mJ/cm².
D Top ↑
Deionization (DI)
A water purification process using ion exchange resins to remove dissolved mineral salts, producing water with very low electrical conductivity. Mixed-bed deionizers contain both cation exchange resins (H+ form, removing positive ions) and anion exchange resins (OH- form, removing negative ions), replacing all dissolved ions with H+ and OH- which combine to form pure water. DI is used in laboratory settings, aquariums, and industrial applications. DI resin has finite capacity and must be regenerated or replaced when exhausted, indicated by increasing conductivity readings.
Disinfection Byproduct (DBP)
Chemical compounds formed when disinfectants (chlorine, chloramine, ozone, chlorine dioxide) react with naturally occurring organic matter (NOM) in water. The two major classes are trihalomethanes (THMs - chloroform, bromodichloromethane, dibromochloromethane, bromoform) and haloacetic acids (HAAs - monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, etc.). The EPA regulates TTHMs at 80 ppb and HAA5 at 60 ppb. DBPs are associated with increased cancer risk and reproductive effects. Reducing NOM through activated carbon or coagulation before chlorination minimizes DBP formation.
Distillation
A water purification process that heats water to boiling, captures the steam, and condenses it back to liquid in a separate chamber, leaving dissolved contaminants, minerals, and most microorganisms behind. Distillation provides near-complete removal of bacteria, viruses, heavy metals, minerals, and organic compounds with boiling points higher than water. Distillers typically produce 3-6 gallons per day, consume approximately 3 kWh per gallon, and do not remove volatile organic compounds (VOCs) with boiling points below 100°C unless equipped with a carbon post-filter.
TDS (Total Dissolved Solids)
The combined concentration of all inorganic and organic substances dissolved in water, measured as the weight of residue remaining after evaporating a filtered water sample. TDS is commonly measured with a conductivity meter and expressed in mg/L or parts per million (ppm). The EPA's secondary (non-enforceable) standard for TDS is 500 mg/L. TDS comprises calcium, magnesium, sodium, potassium, bicarbonate, chloride, sulfate, and silica. High TDS causes scaling, bitter taste, and reduced soap lathering. TDS is NOT a measure of water safety - some healthy minerals contribute to TDS.
E Top ↑
Endocrine Disruptor
A chemical that interferes with the body's endocrine (hormone) system by mimicking, blocking, or altering hormone production and signaling. Common endocrine-disrupting contaminants in water include atrazine, ethinyl estradiol (birth control hormones), bisphenol A (BPA), phthalates, perchlorate, and certain PFAS compounds. Health effects associated with endocrine disruption include reproductive disorders, developmental abnormalities, thyroid dysfunction, and increased cancer risk. Endocrine disruptors are biologically active at extremely low concentrations - often parts per trillion (ppt) levels.
Electrodeionization (EDI)
A continuous, chemical-free deionization process that combines ion exchange resins with ion-selective membranes and an electrical current. The electric field drives ions through the resins and membranes into concentrate streams, continuously regenerating the resin without chemical addition. EDI produces ultrapure water (resistivity >15 MΩ·cm) and is used in pharmaceutical manufacturing, power generation, and semiconductor fabrication. EDI modules are typically installed as a polishing step after reverse osmosis.
F Top ↑
Flocculant
A chemical added to water to promote the aggregation (flocculation) of small suspended particles into larger clusters (flocs) that settle more readily. Common flocculants include aluminum sulfate (alum), ferric chloride, and polyacrylamide polymers. In municipal water treatment, coagulation-flocculation is the primary method for removing turbidity, organic matter, and microorganisms. Residential applications include pool clarifiers and emergency water treatment. Flocculants do not remove dissolved contaminants like dissolved metals or salts.
Flow Rate
The volume of water passing through a system per unit time, typically expressed in gallons per minute (GPM) for whole-house systems or gallons per day (GPD) for reverse osmosis membranes. Flow rate directly impacts filtration effectiveness - insufficient contact time with filter media reduces contaminant removal. A whole-house carbon filter requires 2-3 minutes of empty bed contact time (EBCT) for effective chlorine removal. RO systems list "rated" flow at specific conditions (60 PSI, 77°F); actual household production is typically 30-50% lower due to colder water and lower pressure.
Fluoride
The fluoride ion (F⁻) is added to municipal water supplies at 0.7 mg/L (the current PHS recommendation) to prevent dental caries. Naturally occurring fluoride in groundwater can reach 10+ mg/L in areas with volcanic or fluorine-rich geology. The EPA MCL for fluoride is 4.0 mg/L (to prevent skeletal fluorosis) with a secondary standard of 2.0 mg/L (to prevent dental fluorosis). Fluoride is removed by activated alumina (85-95%), reverse osmosis (85-92%), distillation, and bone char carbon. Standard activated carbon does NOT effectively remove fluoride.
Fouling
The gradual degradation of membrane or filter performance due to accumulation of contaminants on the surface or within the pore structure. Types of fouling include particulate/suspended solids fouling, organic fouling (biofilms, natural organic matter), colloidal fouling (silica, iron, aluminum), scaling (calcium carbonate, calcium sulfate), and biological fouling (microbial growth). Fouling increases pressure drop, reduces flow rate, and decreases salt rejection in RO systems. Pretreatment (sediment filters, carbon filters, water softeners) is essential to minimize fouling and extend membrane life.
G Top ↑
GPG (Grains Per Gallon)
A unit of water hardness measurement where 1 grain = 1/7,000 pound. One GPG equals 17.1 mg/L (ppm) of hardness as CaCO₃. Water hardness classification: soft (0-3.5 GPG), moderately hard (3.5-7.0 GPG), hard (7.0-10.5 GPG), and very hard (10.5+ GPG). Water softeners are sized based on grains of hardness to be removed per day, with typical residential units rated at 24,000-80,000 grain capacity. A family of four using 300 gallons per day with 10 GPG water requires removal of 12,000 grains daily.
GPM (Gallons Per Minute)
The standard unit for measuring water flow rate in plumbing and filtration systems. A typical residential water supply delivers 5-10 GPM at 40-60 PSI. Whole-house filters specify minimum and maximum GPM ratings - exceeding the maximum reduces contact time and filtration effectiveness; falling below the minimum may prevent backwash filters from cleaning properly. Showerheads use 1.5-2.5 GPM (federal standard), kitchen faucets 1.5-2.2 GPM, and washing machines 3-5 GPM.
Granular Activated Carbon (GAC)
Loose activated carbon particles (typically 12x40, 20x50, or 8x30 mesh size) used in filter beds, cartridges, and contactors. GAC provides high flow rates and large contaminant capacity but has shorter contact time than carbon block, resulting in lower removal efficiency for some contaminants. GAC is ideal for whole-house chlorine removal, taste/odor improvement, and VOC reduction. GAC beds should be backwashed regularly and replaced every 1-3 years depending on usage and contaminant load.
Greensand
A naturally occurring mineral (glauconite) coated with manganese dioxide, used as a filtration media for removing iron, manganese, and hydrogen sulfide from water. Greensand filters operate through both oxidation (converting soluble Fe²+ to insoluble Fe³+) and physical filtration. The media requires periodic regeneration with potassium permanganate (KMnO₄) to restore oxidizing capacity. Effective for iron levels up to 10 mg/L, manganese up to 2 mg/L, and hydrogen sulfide up to 5 mg/L.
H Top ↑
Hardness
The concentration of calcium (Ca²+) and magnesium (Mg²+) ions in water, expressed as mg/L or GPG of calcium carbonate equivalent. Hard water causes soap scum, reduced detergent effectiveness, scale buildup in pipes and appliances, and increased energy consumption in water heaters (1/16 inch of scale increases heating costs by ~15%). Hardness does not pose a health risk - the World Health Organization states "there does not appear to be any convincing evidence that water hardness causes adverse health effects in humans." Softening is primarily a comfort and equipment protection measure.
HAAs (Haloacetic Acids)
A group of disinfection byproducts formed when chlorine reacts with natural organic matter. The five regulated HAAs (HAA5) are monochloroacetic acid (MCAA), dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), monobromoacetic acid (MBAA), and dibromoacetic acid (DBAA). The EPA MCL for HAA5 is 60 ppb. DCAA and TCAA are classified as possible human carcinogens (Group 2B). Long-term exposure above the MCL may cause liver, kidney, or nervous system problems. Activated carbon and reverse osmosis effectively remove HAAs.
Head Loss
The reduction in water pressure (energy loss) as water flows through a filter, pipe, or fitting, measured in PSI or feet of head. Head loss increases as filter media becomes clogged or fouled. A clean whole-house sediment filter might create 2-3 PSI of head loss, while a heavily loaded filter could create 15+ PSI of loss, reducing flow throughout the house. Monitoring head loss with pressure gauges before and after filters indicates when replacement is needed.
Hollow Fiber Membrane
A membrane configuration consisting of hundreds or thousands of tiny tubes (fibers) with porous walls, typically 0.5-2 mm in diameter. Water flows either inside-out or outside-in through the fiber walls, with contaminants rejected at the surface. Hollow fiber modules pack enormous membrane surface area into a compact housing. They are used in ultrafiltration (0.01-0.1 micron pore size) and some microfiltration (0.1-0.2 micron) systems. They can be backwashed, giving them longer operational life than spiral-wound membranes.
I Top ↑
Ion Exchange
A chemical process in which unwanted ions in water are exchanged for desirable ions attached to a solid resin material. Water softeners use cation exchange resins where calcium and magnesium ions are exchanged for sodium or potassium ions. Anion exchange resins remove nitrate, sulfate, arsenic, and fluoride. Deionization systems use both cation and anion resins to produce mineral-free water. Ion exchange is highly effective for targeted contaminant removal but does not remove non-ionic contaminants like bacteria, VOCs, or sediment.
Inorganic Contaminant
Mineral-based contaminants that do not contain carbon-hydrogen bonds, including metals (lead, arsenic, mercury, iron, manganese), salts (sodium, chloride, sulfate, nitrate), and minerals (asbestos, fluoride). Inorganic contaminants enter water through natural geological deposits, mining, industrial discharge, agricultural runoff, and plumbing corrosion. The EPA regulates approximately 25 inorganic contaminants with enforceable MCLs. Inorganics are generally removed by reverse osmosis, ion exchange, distillation, and specific media (activated alumina for fluoride/arsenic, KDF for heavy metals).
Iodine Number
A standardized test measuring the adsorption capacity of activated carbon, expressed as milligrams of iodine adsorbed per gram of carbon. The iodine number correlates with the surface area available for adsorbing small molecules. Higher iodine numbers indicate greater micropore volume and better performance for removing small contaminants like chlorine and VOCs. Premium activated carbons have iodine numbers of 1,000-1,200 mg/g. Coconut shell carbon typically has higher iodine numbers (1,000-1,100) than coal-based carbon (800-950).
K Top ↑
KDF (Kinetic Degradation Fluxion)
A high-purity copper-zinc alloy filtration media that removes contaminants through redox (oxidation-reduction) reactions. KDF converts free chlorine to chloride, reduces heavy metals (lead, mercury, chromium, copper) by plating them onto the media surface, and inhibits bacterial growth within the filter. KDF is available in two forms: KDF 55 (granular, for chlorine removal) and KDF 85 (fine, for iron and hydrogen sulfide). KDF is often combined with activated carbon to extend carbon life and provide bacterial control. KDF media can last 6-8 years under normal conditions.
L Top ↑
Langelier Saturation Index (LSI)
A calculated index used to predict water's tendency to either dissolve or precipitate calcium carbonate scale. LSI is calculated from pH, temperature, calcium hardness, total alkalinity, and TDS. An LSI of 0 indicates balanced water; positive values (>0.5) predict scaling; negative values (<-0.5) predict corrosive water that may dissolve pipe material. LSI is important for preventing scale in RO membranes, water heaters, and boilers. Antiscalant chemicals are added to RO feedwater when LSI is positive to prevent calcium carbonate precipitation on the membrane.
M Top ↑
Membrane
A thin, semi-permeable barrier that selectively allows water molecules to pass while rejecting dissolved and suspended contaminants. Membrane filtration is classified by pore size: microfiltration (MF, 0.1-1.0 micron), ultrafiltration (UF, 0.01-0.1 micron), nanofiltration (NF, 0.001-0.01 micron), and reverse osmosis (RO, <0.001 micron). Membranes are manufactured from polyamide thin-film composites (TFC), cellulose acetate, polysulfone, or ceramic materials. Proper pretreatment, pressure, temperature, and pH are critical for membrane performance and longevity.
Micron (Micrometer)
A unit of length equal to one-millionth of a meter (0.000039 inches). In water filtration, micron ratings indicate the smallest particle size a filter can remove. For reference: human hair = 50-100 microns, pollen = 10-30 microns, red blood cell = 7 microns, bacteria = 0.2-10 microns, virus = 0.02-0.3 microns, molecule (water) = 0.0003 microns. Sediment filters typically range from 1-50 microns; cyst-rated filters must be 1 micron absolute or smaller; reverse osmosis membranes reject contaminants at the molecular level (approximately 0.0001 microns).
MTBE (Methyl Tertiary Butyl Ether)
A volatile organic compound formerly added to gasoline as an oxygenate to reduce air pollution. MTBE has contaminated groundwater across the United States through leaking underground storage tanks and spills. It is highly soluble in water, moves rapidly through aquifers, and causes unpleasant taste and odor at very low concentrations (5-15 ppb). The EPA has established a drinking water advisory of 20-40 ppb. MTBE is classified as a possible human carcinogen. It is effectively removed by granular activated carbon (GAC) and air stripping.
N Top ↑
NSF International
An independent, non-profit organization that develops standards and certifies products for public health and safety. NSF/ANSI drinking water treatment standards include: Standard 42 (aesthetic effects - chlorine, taste, odor), Standard 44 (cation exchange water softeners), Standard 53 (health effects - cysts, lead, VOCs, pesticides), Standard 55 (UV microbiological systems), Standard 58 (reverse osmosis), Standard 62 (distillation), and Standard 401 (emerging contaminants including pharmaceuticals). Products displaying the NSF mark have been independently researched and audited for material safety, structural integrity, and contaminant reduction claims.
Nanofiltration (NF)
A membrane separation process with pore sizes between ultrafiltration and reverse osmosis (approximately 0.001 microns or 1 nanometer). Nanofiltration membranes typically reject 50-90% of multivalent ions (calcium, magnesium, sulfate) while allowing most monovalent ions (sodium, chloride) to pass. NF achieves 90-95% removal of organic molecules larger than 200 Daltons, making it effective for removing pesticides, VOCs, and color while partially softening water. NF operates at lower pressures (50-150 PSI) than RO and produces less wastewater (typically 15-25% rejection).
Nitrate (NO₃⁻)
A nitrogen-containing compound that is the most widespread chemical contaminant in groundwater globally. Nitrate enters water from agricultural fertilizers, livestock manure, septic systems, and sewage. The EPA MCL for nitrate is 10 mg/L as nitrogen (45 mg/L as nitrate), established to prevent methemoglobinemia ("blue baby syndrome") in infants under 6 months. Nitrate is removed by reverse osmosis (85-95%), ion exchange (90-95%), and distillation. Activated carbon and standard filtration do NOT remove nitrate. Private wells in agricultural areas should be tested annually for nitrate.
O Top ↑
Organic Contaminant
Carbon-based chemical compounds that enter water from natural sources (decomposing vegetation), industrial processes, agriculture, and consumer products. Organic contaminants include volatile organic compounds (VOCs like benzene, TCE, PCE), pesticides (atrazine, 2,4-D), pharmaceuticals (ibuprofen, acetaminophen), disinfection byproducts (THMs, HAAs), and per- and polyfluoroalkyl substances (PFAS). Organic contaminants are primarily removed by activated carbon adsorption, reverse osmosis, advanced oxidation, and air stripping.
Oxidation
A chemical reaction in which a contaminant loses electrons, typically transforming it from a dissolved, problematic form to an insoluble or less harmful form. In water treatment, oxidation is used to convert ferrous iron (Fe²+, clear and soluble) to ferric iron (Fe³+, insoluble and filterable), manganous manganese (Mn²+) to manganic manganese (Mn&sup4;+), and hydrogen sulfide (H₂S) to elemental sulfur. Common oxidants include chlorine, potassium permanganate, ozone, hydrogen peroxide, and air (aeration).
Ozonation
A water treatment process using ozone (O₃), a powerful oxidant formed by passing dry air or oxygen through a high-voltage electrical discharge. Ozone is one of the most potent disinfectants available, achieving 99.99% inactivation of bacteria, viruses, and protozoan cysts at contact times of 1-10 minutes. Ozone also oxidizes iron, manganese, hydrogen sulfide, and many organic compounds. Unlike chlorine, ozone does not form THMs or chloramines. However, ozonation can produce bromate (a probable human carcinogen) when bromide is present in source water. Ozone is unstable and must be generated on-site.
P Top ↑
Permeate
The purified water that has passed through a membrane in reverse osmosis, nanofiltration, or ultrafiltration systems. Permeate is also called "product water" or "filtrate." The quality of permeate is measured by total dissolved solids (TDS) or conductivity - lower TDS indicates better membrane rejection. Residential RO systems typically produce permeate with TDS reduced by 90-98% from the feedwater. The ratio of permeate to feedwater is called "recovery rate," typically 15-25% for residential RO systems.
PPB (Parts Per Billion)
A unit of concentration equivalent to micrograms per liter (μg/L). One ppb represents one part of contaminant per billion parts of water - approximately one drop in a 10,000-gallon swimming pool. Most regulated contaminants have MCLs expressed in ppb: atrazine (3 ppb), lead (15 ppb action level), benzene (5 ppb). PPB is used for contaminants present at very low concentrations. Activated carbon can reduce many VOCs from hundreds of ppb to below detection limits.
PPM (Parts Per Million)
A unit of concentration equivalent to milligrams per liter (mg/L). One ppm represents one part of contaminant per million parts of water. PPM is used for higher-concentration parameters: TDS (500 ppm EPA secondary standard), hardness (typically 50-300 ppm), nitrate (10 ppm as N MCL), and fluoride (4 ppm MCL). For most water quality parameters, ppm and mg/L are interchangeable. Water with TDS of 500 ppm contains 500 mg of dissolved solids per liter of water.
PSI (Pounds Per Square Inch)
The standard unit for measuring water pressure. Residential water pressure typically ranges from 40-80 PSI. Municipal supplies usually deliver 50-70 PSI at the street connection; private well pumps are typically set to 40-60 PSI (cut-in/cut-out). Reverse osmosis systems require minimum 40 PSI feed pressure to operate effectively - below this, production drops dramatically and TDS rejection suffers. Whole-house filtration systems have maximum pressure ratings (typically 80-125 PSI) that must not be exceeded. Pressure regulators should be installed when street pressure exceeds 80 PSI.
Post-Filter
A final-stage filter installed after the main treatment process to polish water and remove any residual taste, odor, or contaminants. In reverse osmosis systems, a carbon post-filter (also called "post-carbon" or "polishing filter") removes any VOCs that may have passed through the RO membrane and improves taste. Post-filters typically use coconut shell activated carbon and have a service life of 6-12 months. Some systems also include a sediment post-filter to capture any carbon fines shed from upstream filters.
PFAS (Per- and Polyfluoroalkyl Substances)
A large family of synthetic, fluorinated organic compounds including PFOA, PFOS, PFNA, PFHxS, and GenX. PFAS are highly persistent in the environment ("forever chemicals") due to the strong carbon-fluorine bond. They have been used in non-stick cookware, waterproof fabrics, firefighting foam, and food packaging. The EPA has established health advisory levels of 4 ppt for PFOA and 20 ppt for PFOS, and is moving toward enforceable MCLs. PFAS are linked to thyroid disease, high cholesterol, testicular cancer, and developmental effects. Effective removal requires activated carbon (GAC or carbon block), reverse osmosis, or anion exchange resins.
R Top ↑
Rejection Rate
In membrane filtration, the percentage of a specific contaminant or total dissolved solids that is prevented from passing through the membrane. Rejection rate is calculated as: (Feed TDS - Permeate TDS) / Feed TDS x 100%. New RO membranes typically achieve 95-98% TDS rejection. Rejection rates vary by contaminant: sodium (92-98%), calcium (93-99%), sulfate (96-99%), nitrate (85-95%), arsenic (94-98%). Rejection declines over time due to membrane compaction, fouling, and degradation, with most membranes requiring replacement every 2-5 years.
Remineralization
The process of adding beneficial minerals back to water after purification. Reverse osmosis and distillation produce nearly pure water that can be slightly acidic and lacking in calcium and magnesium. Remineralization filters typically contain calcite (calcium carbonate) and corosex (magnesium oxide) that dissolve into the water, raising pH from 5.5-6.5 to 7.0-8.5 and adding 10-30 mg/L of calcium and magnesium. Some systems use alkaline/calcium cartridges or pH+ inline filters. Remineralization is primarily for taste improvement and pH balance, not health necessity.
Resin
The synthetic polymer beads used in ion exchange systems. Cation exchange resins (water softeners) are typically cross-linked polystyrene with sulfonic acid functional groups that exchange sodium/potassium ions for calcium and magnesium. Anion exchange resins have quaternary ammonium groups that remove nitrate, sulfate, and other negative ions. Resin beads are typically 0.3-1.2 mm in diameter. Cross-linkage percentage (usually 6-10%) determines durability and chemical resistance - higher cross-linkage provides longer life but lower exchange capacity.
RO (Reverse Osmosis)
A membrane separation process that uses pressure to force water through a semi-permeable membrane while rejecting dissolved salts, organic molecules, bacteria, viruses, and particulates. RO membranes have pore sizes of approximately 0.0001 microns. Residential RO systems typically operate at 50-80 PSI feed pressure, producing 50-100 gallons per day with 75-85% water rejection (wastewater). RO removes 90-99% of TDS, 95-99% of nitrates, 94-99% of heavy metals, and 85-95% of most organic contaminants. NSF/ANSI Standard 58 certifies RO system performance.
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Sediment
Particulate matter suspended in water including sand, silt, rust particles, clay, and organic debris. Sediment is measured by turbidity (NTU) or by filtration through a 0.45-micron filter. High sediment causes cloudiness, stains fixtures, clogs appliances, and protects bacteria from disinfection. Sediment filters are rated by micron size - 5-micron filters remove most visible particles; 1-micron filters remove fine silt and some cysts. Sediment filters should always be installed as pretreatment before carbon filters and RO membranes to prevent clogging.
Semi-Permeable Membrane
A barrier that allows certain molecules (water) to pass while blocking others (dissolved salts, contaminants) based on size, charge, and chemical properties. RO membranes use thin-film composite (TFC) construction with a polyamide active layer 0.2 microns thick on a polysulfone support layer. The polyamide layer contains pores at the angstrom scale (approximately 10 angstroms or 0.1 nanometers), large enough for water molecules (2.75 angstroms) but small enough to reject hydrated sodium ions (7.2 angstroms) and most organic molecules.
Softener (Water Softener)
An ion exchange system that removes hardness minerals (calcium and magnesium) by exchanging them for sodium or potassium ions. A typical residential softener consists of a mineral tank containing cation exchange resin, a brine tank for salt storage, and a control valve that manages service, backwash, brine draw, and rinse cycles. Softeners are sized by grain capacity (24,000-80,000 grains) and regeneration frequency. Water softeners do not remove contaminants like lead, bacteria, or chemicals - they address only hardness. Softened water contains elevated sodium levels (approximately 12.5 mg/L added per GPG of hardness removed).
Sub-Micron Filtration
Filtration capable of removing particles smaller than 1 micron. Sub-micron filters include 0.5-micron carbon blocks (removes cysts and fine sediment), 0.1-micron absolute filters (removes bacteria), 0.01-micron ultrafilters (removes viruses), and 0.0001-micron RO membranes (removes dissolved salts). Sub-micron ratings may be expressed as "nominal" (meaning the filter removes a percentage of particles at that size) or "absolute" (meaning the filter removes 99.9%+ of particles at that size). NSF 53 certified cyst-rated filters must be 1 micron absolute or tighter.
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THMs (Trihalomethanes)
A group of four disinfection byproducts formed when chlorine reacts with natural organic matter in water: chloroform (CHCl₃), bromodichloromethane (BDCM), dibromochloromethane (DBCM), and bromoform (CHBr₃). The EPA MCL for total trihalomethanes (TTHM) is 80 ppb. Chloroform and BDCM are classified as probable human carcinogens (Group 2B). TTHM formation increases with higher chlorine doses, longer contact time, higher temperatures, and higher organic matter content. Activated carbon, reverse osmosis, and distillation effectively remove THMs. Switching to chloramine disinfection reduces THM formation but may increase HAA formation.
Throughput
The total volume of water that can be processed by a filter before replacement is required, expressed in gallons. Throughput ratings are specified by manufacturers under standard test conditions and assume average influent water quality. Actual throughput varies based on incoming contaminant levels, flow rates, and water chemistry. A sediment filter rated for 20,000 gallons may last 6 months in average conditions but only 3 months in high-sediment areas. Carbon filters for chlorine removal are typically rated at 100,000-600,000 gallons at standard flow rates. Always follow manufacturer replacement intervals even if throughput has not been reached.
Turbidity
A measure of water's cloudiness or haziness caused by suspended particles that scatter light. Turbidity is measured in Nephelometric Turbidity Units (NTU) using a turbidimeter. The EPA requires public water systems to maintain turbidity below 1.0 NTU (with 95% of samples below 0.3 NTU) because turbidity can shield microorganisms from disinfection and interfere with chlorine effectiveness. High turbidity indicates inadequate filtration or source water problems. Residential turbidity removal requires sediment filtration (5-20 micron) followed by fine filtration (1-5 micron) or ultrafiltration.
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Ultrafiltration (UF)
A membrane filtration process with pore sizes of 0.01-0.1 microns, capable of removing bacteria, protozoan cysts, viruses (with some models), colloids, and macromolecules while allowing dissolved salts and small molecules to pass. Unlike RO, ultrafiltration does not remove TDS, hardness, or dissolved ions. UF operates at lower pressure (10-30 PSI) than RO, produces no wastewater, and preserves beneficial minerals. NSF/ANSI Standard 53 certifies UF systems for cyst and bacteria removal. UF is ideal for microbiologically unsafe water with acceptable mineral content.
UV (Ultraviolet) Disinfection
A water treatment method using ultraviolet light at 254 nanometers wavelength to inactivate microorganisms by damaging their DNA, preventing reproduction. UV is highly effective against bacteria (99.99%+ inactivation), viruses (99.99%), and protozoan cysts including Cryptosporidium and Giardia (99.9%). UV systems are classified by NSF/ANSI Standard 55: Class A systems (40 mJ/cm² minimum dose) treat microbiologically unsafe water; Class B systems (16 mJ/cm²) are for supplemental treatment of already disinfected water. UV does not remove chemical contaminants, sediment, or dissolved substances. Water must be pre-filtered to <5 NTU turbidity for UV effectiveness.
Upstream
The direction from which water flows - the source side of a filter, valve, or treatment device. The upstream connection receives influent (untreated) water from the supply. Proper installation requires identifying upstream and downstream ports to ensure water flows through filters and treatment media in the correct direction. Arrow indicators on filter housings show proper flow direction. Installing a filter backwards prevents proper sealing, may damage the filter media, and can allow bypass of unfiltered water.
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VOC (Volatile Organic Compound)
Organic chemicals that readily evaporate at room temperature, including industrial solvents (TCE, PCE, benzene, toluene), fuel components (MTBE, BTEX), and disinfection byproducts (THMs). VOCs enter water through industrial discharge, leaking underground storage tanks, and atmospheric deposition. Many VOCs are carcinogenic or cause liver, kidney, and nervous system damage. The EPA regulates 21 individual VOCs with MCLs ranging from 5 ppb (benzene) to 700 ppb (xylenes). Activated carbon (GAC and carbon block) is the primary treatment method, achieving 90-99%+ removal of most VOCs. NSF/ANSI Standard 53 certifies VOC reduction.
Valve
A mechanical device that controls water flow, pressure, or direction in a plumbing system. Key valve types in water filtration include: ball valves (quarter-turn on/off), gate valves (gradual flow control), check valves (one-direction flow prevention), pressure-reducing valves (regulate incoming pressure), bypass valves (divert flow around treatment equipment), and solenoid valves (electrically operated, used in RO systems and softeners). Whole-house filtration systems should include isolation ball valves before and after the unit for maintenance and emergencies.
Virus
Submicroscopic infectious agents ranging from 0.02 to 0.3 microns, significantly smaller than bacteria. Waterborne viruses of concern include norovirus, rotavirus, hepatitis A, adenovirus, and enteroviruses. The EPA requires 4-log (99.99%) virus inactivation in public water supplies. Viruses are inactivated by UV disinfection (dose of 10-40 mJ/cm²), chlorination, ozonation, and removed by reverse osmosis and ultrafiltration (0.01-micron tight UF membranes). Standard microfiltration and most carbon filters do NOT remove viruses. Point-of-use UV systems are the most common residential virus treatment method.
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Waste Ratio (Rejection Ratio)
The ratio of wastewater (concentrate/reject) to purified water (permeate) produced by reverse osmosis systems. A 4:1 waste ratio means 4 gallons of water go to drain for every 1 gallon of purified water. Standard residential RO systems have waste ratios of 3:1 to 4:1. "Zero waste" systems use a booster pump to recirculate reject water back to the hot water line or a storage tank, but this adds complexity and potential for premature membrane fouling. Permeate pumps can reduce waste ratios to 2:1 or 1:1 by equalizing pressure across the membrane. Some municipalities have considered restricting RO systems due to water waste concerns.
Water Hammer
A pressure surge or shock wave that occurs when water flow is suddenly stopped by a closing valve or running pump, causing pipes to bang or shake. Water hammer generates pressure spikes of 200-1,000 PSI - far exceeding normal operating pressure and capable of damaging pipes, fittings, water heaters, and filtration equipment. Solutions include installing water hammer arrestors (air-filled chambers or piston-type devices that absorb the shock), pressure-reducing valves, and soft-start pump controls. Whole-house water filters should be rated for maximum pressure including potential hammer spikes.
WQA (Water Quality Association)
A trade organization representing the residential water treatment industry. WQA operates a voluntary certification program (Gold Seal) that tests products to NSF/ANSI standards and evaluates manufacturer claims. The WQA Gold Seal indicates that a product has been tested by an accredited third-party laboratory and meets applicable industry standards. WQA also certifies water treatment professionals (Certified Water Specialist, CWS) and publishes consumer education materials. While WQA certification is respected, NSF certification from an independent body provides broader consumer confidence.
Whole-House Filter
A water treatment system installed at the point where water enters a building, treating all water distributed to every faucet, shower, appliance, and fixture. Whole-house systems typically use large-diameter filter housings (10"x4.5" or 20"x4.5") with higher flow rates (5-15 GPM) than point-of-use filters. Common configurations include sediment pre-filters, activated carbon filters, water softeners, UV disinfection units, and iron/manganese filters. Whole-house treatment protects plumbing infrastructure, extends appliance life, and provides filtered water for bathing and laundry in addition to drinking.
This glossary is maintained by Filter Tested and updated regularly. Last reviewed January 2026. For questions about specific terminology or application guidance, consult our detailed filtration guides or contact a certified water treatment professional.
Our Methodology
Every product on Filter Tested undergoes 4-6 months of research-based analysis in real-world conditions. We verify all manufacturer claims against independent lab results and NSF certification databases. Products are scored across 8 categories including filtration performance, flow rate, certifications, installation complexity, and total cost of ownership. Learn more about how we test.