Water filtration has its own vocabulary. Whether you are comparing whole-house filters, evaluating reverse osmosis systems, or reading your water softener manual, you will encounter technical terms that determine what a system actually does. This glossary defines essential terminology from activated carbon and micron ratings to NSF/ANSI 58 and template assisted crystallization.
A
Activated Carbon
A highly porous form of carbon (500–1,500 m²/g), the most common filtration media in residential water treatment. It removes contaminants through adsorption, capturing chlorine, VOCs, pesticides, and compounds causing bad taste and odor. Available as granular activated carbon (GAC) or carbon block. Used in whole-house filters, under-sink systems, and refrigerator filters.
Activated Alumina
A granular aluminum oxide media that adsorbs fluoride, arsenic, and selenium. One of the few proven methods for fluoride removal, used in RO systems and under-sink filters.
Adsorption
The process where contaminants stick to a filter medium's surface. In water filtration, activated carbon uses adsorption to capture chlorine, VOCs, and organic chemicals on its porous surface. Once saturated, carbon filters must be replaced.
C
Catalytic Carbon
An enhanced activated carbon modified to break apart chloramine, a disinfectant many municipal systems use. It accelerates the chemical reaction breaking the chlorine-ammonia bond. Systems like the Aquasana Rhino EQ-1000 use catalytic carbon for chloramine-treated water.
CTO
An acronym for Chlorine, Taste, and Odor—the most common rating category for carbon-based water filters. A CTO-rated filter is designed to reduce free chlorine and the unpleasant tastes and smells it causes, typically certified to NSF/ANSI 42. CTO filters do not address health-related contaminants like lead or cysts unless they carry additional certifications. They improve aesthetics but may not improve safety.
Capacity
The total gallons a filter can process before media becomes exhausted. Exceeding rated capacity causes contaminant breakthrough. When comparing whole-house systems, capacity impacts operating cost and maintenance frequency.
Contaminant
Any physical, chemical, biological, or radiological substance in water that may be harmful or undesirable. The EPA regulates approximately 90 contaminants in public drinking water. Common contaminants include sediment, chlorine, chloramine, lead, cysts, VOCs, PFAS, nitrates, fluoride, arsenic, and hard water minerals. Understanding which contaminants are present in your water is the first step in choosing an appropriate system.
Crosslink
A measure of structural bonding within ion exchange resin beads used in water softeners. Standard softener resin is 8% crosslinked, offering good durability. Higher crosslink percentages (10%+) provide greater resistance to chlorine damage. Lower crosslink resin (6%) is less durable and breaks down faster. Crosslink percentage is an indicator of long-term resin durability when comparing water softeners.
F
Flow Rate
The volume of water passing through a system per minute (GPM). Critical for sizing: too low causes pressure drops at showers. Most homes need 5–7 GPM minimum; larger homes need 10–15 GPM. RO systems use GPD instead.
Fluoride
A naturally occurring mineral added to many municipal water supplies for dental health. Standard activated carbon does not effectively remove fluoride. The two proven residential methods are reverse osmosis (85–95% reduction) and activated alumina media (90%+ depending on contact time). If fluoride removal is a priority, verify the system specifically lists fluoride reduction with supporting data.
G
GAC (Granular Activated Carbon)
Loose carbon granules used as filtration media, as opposed to solid carbon block. GAC allows water to flow through a bed of granular particles, providing good contact time for contaminant adsorption. It is less dense than carbon block—lower pressure drop but less effective at filtering fine particles. GAC is commonly used in whole-house systems and as pre- or post-filters in RO systems.
GPD (Gallons Per Day)
The production capacity measurement for RO systems. A typical under-sink unit is rated 50–75 GPD under ideal conditions. Real-world output is 30–50% lower. Choose a GPD rating exceeding your daily drinking and cooking needs.
Grains per Gallon (GPG)
The standard U.S. unit for water hardness. One GPG equals ~17.1 mg/L. Water is soft (0–3.5 GPG), moderately hard (3.5–7), hard (7–10.5), or very hard (10.5+). Essential for sizing a water softener.
Groundwater
Water stored underground in aquifers, accessed through wells and springs. Groundwater is naturally filtered through soil and rock, often resulting in higher mineral content but lower organic contaminants. However, it can contain agricultural runoff (nitrates), industrial contaminants, and naturally occurring substances like arsenic. See our well water filtration guide for treatment recommendations.
I
Ion Exchange
A chemical process swapping unwanted ions for desirable ones. In water softening, calcium and magnesium are exchanged for sodium or potassium as water flows through resin beads. The resin eventually saturates and must be regenerated with salt brine. The most effective method for producing truly soft water.
Inlet
The water entry point into a filtration system. Proper inlet sizing (typically 3/4" or 1" for whole-house systems) ensures adequate flow rate. The inlet must be connected to the cold water line before any fixture branches. Incorrect inlet plumbing—such as installing on a hot water line—can damage media and void warranties.
K
KDF (Kinetic Degradation Fluxion)
A copper-zinc alloy that removes contaminants through redox reactions. KDF reduces chlorine, heavy metals, hydrogen sulfide, and bacteria growth. Often used alongside activated carbon to extend carbon life. KDF-55 targets chlorine and heavy metals; KDF-85 targets iron and hydrogen sulfide.
M
Membrane
A semi-permeable barrier that lets water pass while blocking contaminants. RO membranes have ~0.0001 micron pores, rejecting dissolved salts and heavy metals. Made from thin-film composite polyamide. Pre-filtration protects membranes from fouling; they typically last 2–5 years.
Micron
A unit of length equal to one-millionth of a meter. In filtration, micron ratings indicate the smallest particle size a filter captures. A 5-micron sediment filter removes sand and rust; a 1-micron filter captures cysts. Human hair is ~50–70 microns; bacteria are 0.3–2 microns. RO membranes at 0.0001 microns filter out virtually all dissolved contaminants.
Municipal Water
Public water supplied and treated by a city or water authority. Municipal water is EPA-regulated and generally safe, though it may contain disinfectant byproducts, lead from aging pipes, and emerging contaminants. Many homeowners install filtration systems to improve taste and remove residual chlorine. See our guide on whether a whole-house filter is worth it.
N
NSF/ANSI
The National Sanitation Foundation and American National Standards Institute—the organizations that jointly develop standards for water treatment products. NSF International is the independent testing body; ANSI accredits the standards. "NSF/ANSI certified" means a product has been independently tested, manufactured under audited conditions, and subject to annual retesting. See our NSF certifications guide for details on each standard.
NSF 42
The NSF/ANSI standard certifying reduction of aesthetic effects: chlorine taste and odor, chloramine, and particulates. It is the entry-level certification. A product certified only to NSF 42 improves taste and smell but has not been tested for health-related contaminants. Many basic pitcher filters and refrigerator filters carry NSF 42. Valuable for what it covers, but not a health protection certification.
NSF 53
The NSF/ANSI standard certifying reduction of health-related contaminants: lead, cysts, VOCs, mercury, asbestos, and certain pesticides. More rigorous than NSF 42, requiring contaminant-by-contaminant testing. The minimum certification for health protection.
NSF 58
The NSF/ANSI standard for reverse osmosis systems. Requires a minimum 75% TDS reduction and tests for contaminants including arsenic, lead, fluoride, and nitrate. Also evaluates recovery rate (purified vs. wastewater) and structural integrity. Any RO system you consider should carry NSF 58 certification to verify performance claims.
Nitrates
A chemical contaminant from agricultural runoff, fertilizer, and septic systems. Nitrates are dangerous for infants under six months, interfering with the blood's ability to carry oxygen. The EPA MCL is 10 mg/L. Standard carbon does not remove nitrates. Proven methods include reverse osmosis, ion exchange, and distillation. If you use well water in an agricultural area, annual nitrate testing is essential.
P
Permeate Pump
A non-electric device on RO systems that improves efficiency by reducing back pressure on the membrane using energy from reject water. Cuts wastewater by up to 80%. The Home Master TMAFC-ERP includes one as standard.
PFAS / PFOA / PFOS
Synthetic "forever chemicals" that do not break down in the environment or human body. Linked to cancer and thyroid disease. Enter water through industrial discharge. Carbon filters and RO systems certified to NSF/ANSI 53 or 58 are the most reliable treatment options.
Point of Entry (POE)
A system installed where water enters your home, treating all water distributed to every tap and appliance. Whole-house filters, softeners, and conditioners are POE systems. Ideal for addressing chlorine, sediment, and hardness throughout the home. Often paired with a POU drinking water system for maximum protection.
Point of Use (POU)
A system installed at a single water outlet, treating only that tap's water. Under-sink RO units, countertop filters, and faucet-mounted filters are POU systems. They typically provide more thorough contaminant removal than POE systems because they process smaller volumes. Most POU systems are less expensive to purchase and operate than POE systems.
PSI (Pounds per Square Inch)
The standard unit of water pressure. Residential water pressure typically ranges from 40 to 80 PSI. RO systems need 40–50 PSI minimum; whole-house filters work best at 45–70 PSI. Low pressure may require a booster pump; pressure above 80 PSI may need a regulator. Check your home's PSI with a pressure gauge before installing any system.
R
Remineralization
The process of adding beneficial minerals back into water after filtration. RO systems remove both contaminants and healthy minerals, producing slightly acidic water. A remineralization stage—typically a cartridge with calcium carbonate—restores minerals, improving taste and raising pH. Systems like the iSpring RCC7AK and Home Master TMAFC-ERP include built-in remineralization.
Reverse Osmosis (RO)
A purification process forcing water through a semi-permeable membrane with ~0.0001 micron pores. Removes up to 99% of dissolved solids, heavy metals, fluoride, and nitrates. Produces purified water and wastewater. Typical under-sink systems have multiple stages. See our RO buying guide.
Regeneration
The process by which a water softener recharges its ion exchange resin using a salt brine solution. The brine replaces accumulated calcium and magnesium ions with fresh sodium ions; displaced hardness minerals are flushed to drain. Regeneration can be timer-based (fixed schedule), meter-based (by usage), or sensor-based. Meter-based regeneration is most efficient. The Fleck 5600SXT uses a digital metered valve to optimize timing and minimize waste.
S
Sediment
Particulate matter suspended in water: sand, rust, silt, clay, and organic debris. Common in well water and aging plumbing. While not typically a health hazard, sediment damages appliances, clogs fixtures, and reduces downstream filter life. Removed using mechanical filters with specified micron ratings—typically 5 microns for general protection. Most whole-house systems include a sediment pre-filter as the first stage.
Salt-Free Conditioner
A device that reduces scale buildup without removing hardness minerals or using salt. Unlike softeners, salt-free conditioners use TAC technology to change the structure of hardness minerals so they remain suspended rather than crystallizing on surfaces. They are maintenance-free, produce no wastewater, and do not add sodium. However, water still tests as hard. See our best salt-free conditioners guide.
Service Flow
The normal operating flow rate of a filtration system under typical conditions, measured in GPM. Exceeding the rated service flow reduces contact time between water and media, lowering contaminant removal effectiveness. When sizing a whole-house filter, ensure the service flow rating meets your household's simultaneous water demands.
Stage
An individual filtration step within a multi-stage system. Each stage targets different contaminants. A typical 3-stage whole-house system might use: Stage 1 (sediment), Stage 2 (carbon), Stage 3 (post-filter). RO systems often have 5–7 stages. More stages do not always mean better filtration—the quality of each stage matters more than the count.
Surface Water
Water from lakes, rivers, reservoirs, and streams. More susceptible to contamination from agricultural runoff and organic matter than groundwater. Municipalities using surface water typically apply more extensive treatment, including higher disinfectant levels. Homes on surface water may experience more taste and odor issues, making whole-house carbon filters particularly beneficial.
T
TAC (Template Assisted Crystallization)
A salt-free conditioning technology converting hardness minerals into microscopic crystals that stay suspended in water. TAC does not remove hardness—it changes mineral behavior. No electricity or salt required. Not suitable for hardness above ~25 GPG. The Aquasana SimplySoft uses TAC.
TDS (Total Dissolved Solids)
A measurement of dissolved substances in water (minerals, salts, metals), measured in PPM. The EPA secondary standard is 500 PPM. RO systems reduce TDS by 85–95%. TDS meters are inexpensive tools for monitoring RO performance, but TDS alone does not indicate safety.
Turbidity
Cloudiness in water from suspended particles. High turbidity indicates sediment, organic matter, or algae. Can shield microorganisms from disinfection. The EPA requires municipal turbidity below 0.3 NTU. Whole-house filters with sediment pre-filters reduce turbidity effectively.
U
Ultrafiltration (UF)
A membrane technology with ~0.01 micron pores that removes bacteria, cysts, and viruses while allowing dissolved minerals to pass. Unlike RO, UF does not remove dissolved solids or heavy metals. It operates at lower pressure, produces no wastewater, and retains beneficial minerals.
UV Purification
A disinfection method using ultraviolet light (254 nm wavelength) to inactivate microorganisms by damaging their DNA. Effective against bacteria, viruses, and cysts without adding chemicals. UV does not remove sediment, chemicals, or dissolved solids—it is purely biological disinfection. Requires pre-filtration (5-micron sediment) for effective light penetration. Commonly used on well water systems. See our UV purification guide.
Upflow
A system design where water flows upward through the filter media bed rather than downward. Upflow prevents channeling—where water finds the path of least resistance, bypassing full filtration. It also reduces pressure drop and extends media life by distributing contact more evenly. Some whole-house carbon filters and salt-free conditioners use upflow design.
W
Waste Ratio
The ratio of wastewater to purified water in an RO system. Typical systems produce 3–4 gallons of waste per gallon purified. Permeate pumps cut waste by up to 80%; some systems achieve 1:1 ratios.
Water Softener
An ion exchange system removing calcium and magnesium, replacing them with sodium or potassium ions. Prevents scale buildup and improves detergent effectiveness. Requires regular salt replenishment. A POE system treating all incoming water. See our best water softeners guide.
Well Water
Water from a private well drilled into underground aquifers. Unlike municipal water, the homeowner is solely responsible for quality and safety. Well water often contains higher sediment, iron, manganese, hardness, and sometimes hydrogen sulfide. It may also have bacterial contamination or nitrates. The EPA recommends annual testing for coliform bacteria, nitrates, pH, and TDS. See our well water filtration guide for treatment approaches.
Frequently Asked Questions
What is the difference between adsorption and absorption in water filtration?
Adsorption is a surface-based process where contaminants stick to the surface of filter media like activated carbon. Absorption is a bulk process where one substance soaks into another throughout its volume. In water filtration, adsorption is the correct term—it explains why carbon filters become exhausted once surface area is saturated.
How do I know which micron rating I need for my water filter?
The right micron rating depends on your target contaminants. A 5-micron sediment filter catches sand and rust. A 1-micron rating removes cysts. For bacteria, use ultrafiltration (0.01 micron). For dissolved contaminants, reverse osmosis (0.0001 micron) is required. Most whole-house systems use a 5-micron pre-filter followed by carbon. Always test your water first.
What is the difference between a Point of Entry (POE) and Point of Use (POU) system?
A Point of Entry (POE) system treats all water entering your home. Whole-house filters and softeners are POE systems. A Point of Use (POU) system treats one tap only—under-sink RO units are POU. POE handles aesthetic issues throughout the home; POU provides thorough contaminant removal for drinking water. Many homes use both.