When it comes to providing clean, safe drinking water for your family, reverse osmosis (RO) systems stand out as one of the most effective filtration technologies available for home use. But with technical terms like “TDS,” “permeate,” and “micron ratings” flying around, making an informed decision about which system to purchase can feel like trying to solve a complex puzzle with missing pieces.
This article is part of our Beginner’s Guide to Reverse Osmosis, a comprehensive introduction designed to help homeowners understand how reverse osmosis works, what systems are available, and how to choose the best solution for their water quality needs. Click here to explore the full guide
Why Understanding RO Terminology Matters
Shopping for a reverse osmosis system without understanding the terminology is like buying a car without knowing what horsepower or fuel efficiency means. You might end up with something that works, but it may not be optimal for your specific needs or budget.
The difference between the right RO system and the wrong one can mean:
- Hundreds of dollars in unnecessary spending
- Wasted water from inefficient systems
- Improper filtration for your local water conditions
- Higher maintenance costs over time
- Shorter system lifespan
Empowering Your Purchasing Decision
This comprehensive glossary serves as your translation guide to the world of reverse osmosis. By understanding these terms, you’ll be able to:
- Compare systems accurately based on their actual capabilities, not just marketing claims
- Ask the right questions when researching different models
- Identify the features that matter most for your household’s specific water challenges
- Maintain your system properly once purchased, extending its life and performance
- Make sense of water quality reports to ensure your system addresses your actual needs
How to Use This Glossary
Whether you’re a complete beginner or have some familiarity with water filtration, this glossary is designed to be your go-to reference. We’ve organized everything alphabetically with an easy-to-navigate index.
Start by familiarizing yourself with the basic components and processes:
- Reverse Osmosis – The fundamental process
- RO Membrane – The heart of the system
- Pre-filters and Post-filters – The supporting cast
- TDS (Total Dissolved Solids) – A key measurement of water quality
- GPD (Gallons Per Day) – Understanding system capacity
As you narrow down your options, dive deeper into specific terms relevant to the systems you’re considering.
Beyond the Terminology
Remember, the perfect RO system isn’t necessarily the most expensive or the one with the most features—it’s the one that best addresses your specific water quality concerns, fits your household’s water usage patterns, and matches your budget for both purchase and ongoing maintenance.
Use this glossary as your foundation, and you’ll be well on your way to putting together all the pieces of the puzzle—resulting in the perfect clean water solution for your family.
Scroll down to explore our comprehensive alphabetical glossary of reverse osmosis terms, or use the index to jump to specific sections.
Ultimate Comprehensive Reverse Osmosis Glossary
Index: A | B | C | D | E | F | G | H | I | L | M | N | O | P | R | S | T | U | V | W
A
Adsorption: A process where dissolved molecules or small particles in water adhere to the surface of a filter medium, like activated carbon, effectively removing them from the water. This differs from absorption, where substances penetrate the volume of the material.
Alkaline Filter: A type of post-filter added to some RO systems designed to increase the pH of the purified water (making it less acidic) and often reintroducing beneficial minerals like calcium and magnesium. (See also: Remineralization)
ANSI (American National Standards Institute): A private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. Often works with NSF on water filter standards.
Arsenic: A toxic heavy metal that can contaminate water sources through natural deposits or human activities. RO systems are generally effective at removing arsenic.
Auto Shut-Off Valve (ASOV): A component in RO systems that automatically stops the flow of feed water into the system once the storage tank is full, conserving water and protecting the system from excessive pressure.
B
Bacteria: Single-celled microorganisms, some of which can be pathogenic (disease-causing). RO membranes can block bacteria, though proper system maintenance and potentially UV sterilization are important for ensuring microbial safety.
Booster Pump: An electric pump used to increase the incoming water pressure (feed water pressure) supplied to the RO system. This is often necessary when source water pressure is below the optimal range (typically < 40-50 PSI) for efficient membrane performance.
Brackish Water: Water that has a higher salinity level than freshwater but lower than seawater. RO is commonly used to desalinate brackish water.
Brine: See Concentrate/Reject Water.
C
Calcium Cartridge: A specific type of remineralization cartridge designed to add calcium minerals back into the purified RO water.
Carbon Block Filter: A water filter made from activated carbon compressed into a solid block. It offers a large surface area for adsorption and fine filtration, effective at removing chlorine, chloramine, VOCs, sediment, and improving taste and odor. Often used as a pre-filter or post-filter.
Cellulose Triacetate (CTA): An alternative membrane material sometimes used in RO systems, more chlorine-resistant than TFC membranes but generally less effective at contaminant removal and requiring more frequent replacement.
Certifications and Standards: Third-party validations (e.g., from NSF International, WQA Gold Seal) indicating that an RO system or its components meet specific performance, safety, and structural integrity requirements.
Check Valve: A valve that allows water to flow in only one direction. In RO systems, it typically prevents water from the storage tank from flowing backward through the membrane.
Chlorine: A chemical commonly used by municipalities to disinfect tap water. While effective for killing microbes, it can damage certain RO membranes (like TFC) and affect taste/odor. Carbon pre-filters are used to remove it.
Chloramine: A disinfectant formed by combining chlorine and ammonia, also used in municipal water treatment. It is more stable than chlorine but can also damage some membranes and requires specific types of carbon filters for effective removal.
Commercial/Industrial RO: Larger scale reverse osmosis systems designed for applications like manufacturing, food and beverage production, power generation, pharmaceuticals, and municipal water treatment.
Concentrate (or Reject Water/Brine): The stream of water that carries away the impurities rejected by the RO membrane. This wastewater flows to the drain.
Contaminants: Any unwanted physical, chemical, biological, or radiological substances found in water.
Contaminant Categories: Broad classifications of impurities RO can address, including dissolved solids, heavy metals (lead, arsenic, mercury), salts, minerals (hardness), microorganisms (bacteria, viruses, cysts), chemicals (chlorine, pesticides, herbicides, VOCs), nitrates, fluoride, etc.
Countertop RO System: A portable RO unit designed to sit on a kitchen counter, often connecting directly to the faucet for easy installation and use.
Cross-flow Filtration: The method used by RO membranes where the feed water flows across (parallel to) the membrane surface. Purified water passes through (permeate), while the remaining water sweeps rejected contaminants away (concentrate).
D
Desalination: The process of removing salts and minerals from saline water (seawater or brackish water) to produce fresh, potable water. RO is a primary technology for desalination.
Diffusion: The natural movement of molecules from an area of higher concentration to an area of lower concentration. Osmosis is a specific type of diffusion involving water across a semi-permeable membrane.
Distillation: A water purification method involving boiling water to create steam, leaving impurities behind, and then condensing the steam back into liquid water. It differs significantly from RO filtration.
Drain Saddle: A plumbing fitting used to connect the RO system’s concentrate (wastewater) line to the sink’s drain pipe, typically installed above the P-trap.
E
Eco-Mode: A feature in some modern RO systems aimed at improving water efficiency, potentially through optimized flushing cycles or permeate pump technology.
Efficiency Ratio: See Recovery Rate.
F
Feed Water: The untreated source water supplied to the RO system for purification.
Feed Water Adapter: A valve or fitting used to connect the RO system’s inlet tubing to the home’s cold water supply line (often under the sink).
Filter Life: The recommended operational period (in months) or volume of water (in gallons) after which a filter cartridge should be replaced to ensure optimal performance and water quality. Varies by filter type, water quality, and usage.
Fittings: Connectors used to join tubing and components within the RO system (e.g., quick-connect fittings, compression fittings).
Flow Restrictor: A small component, often inserted into the concentrate line, that limits the flow of wastewater. This helps maintain pressure against the RO membrane, which is crucial for the reverse osmosis process to occur efficiently. Sized to match the membrane’s GPD rating.
Flush Valve: A valve (manual or automatic) that allows for periodic high-flow rinsing of the RO membrane surface to wash away accumulated contaminants and potentially extend membrane life.
Fluoride: A mineral sometimes added to public water supplies for dental health benefits. RO systems are effective at removing fluoride.
Flux: The rate of water passage through the membrane per unit area, typically measured in gallons per square foot per day (GFD).
Fouling: The accumulation of contaminants (including biological growth, colloidal matter, organics) on the membrane surface, which reduces water flow, decreases efficiency, and can damage the membrane.
G
GAC (Granular Activated Carbon) Filter: A filter containing loose granules of activated carbon. Effective at adsorbing chlorine, VOCs, and improving taste and odor. Often used as a pre-filter or polishing post-filter.
GPD (Gallons Per Day): A unit measuring the maximum volume of purified water (permeate) an RO system can produce in a 24-hour period under ideal conditions (specific pressure, temperature, and TDS level). Actual output may vary.
H
Hardness: Primarily caused by dissolved calcium and magnesium ions in water. RO effectively removes these minerals, thus softening the water.
Heavy Metals: Metals with high density or atomic weight, such as lead, arsenic, mercury, cadmium, and chromium, which can be toxic even at low concentrations. RO is effective at removing most heavy metals.
I
Inline Filter: A self-contained filter cartridge installed directly within the water tubing line. Can contain various media (sediment, carbon, remineralization) and is often used for post-filtration or in compact systems like refrigerator filters.
L
Lead: A toxic heavy metal that can leach into drinking water from old pipes, solder, and fixtures. RO systems are highly effective at removing lead.
M
Membrane Element: The individual spiral-wound or hollow fiber units that contain the RO membrane, designed to fit into pressure vessels in larger systems.
Membrane Types: Different materials used for RO membranes. The most common are TFC/TFM (Thin Film Composite/Material), known for high rejection rates but sensitivity to chlorine, and older CTA (Cellulose Triacetate) membranes, which are chlorine tolerant but less efficient and have lower pH tolerance.
Micron Rating: A measure of the pore size of a filter, indicating the smallest particle size (in microns, one-millionth of a meter) it can capture. A lower micron rating means finer filtration. Sediment filters might be 5 microns, while carbon blocks can be 0.5 microns. The RO membrane itself filters down to approx. 0.0001 microns.
Mineralizer/Remineralization: The process of adding desirable minerals (like calcium and magnesium) back into the purified RO water, often done using a specific post-filter (Alkaline Filter, Calcium Cartridge). Addresses concerns about the slightly acidic nature and lack of minerals in RO water.
N
NSF International: An independent organization that develops public health standards and certification programs to help protect food, water, consumer products, and the environment. NSF certifications on RO systems verify performance claims (e.g., NSF/ANSI Standard 58 for RO systems).
O
Osmosis: The natural tendency of water to move across a semi-permeable membrane from a solution with a lower concentration of dissolved solids to one with a higher concentration, seeking equilibrium.
Osmotic Pressure: The pressure differential that must be applied to a more concentrated solution to prevent the inflow of water from a less concentrated solution across a semi-permeable membrane. In RO, external pressure must exceed this osmotic pressure to reverse the flow.
P
Pathogens: Disease-causing microorganisms, including certain bacteria, viruses, and protozoan cysts (like Giardia, Cryptosporidium). RO membranes are generally an effective barrier against these, but system integrity is crucial.
Permeate: The purified water that has successfully passed through the reverse osmosis membrane.
Permeate Pump: A device used to increase permeate production and efficiency by utilizing the energy from the concentrate water to boost the pressure of the permeate water.
Pesticides/Herbicides: Chemicals used in agriculture that can contaminate water sources. Carbon filters and RO membranes can remove many of these organic compounds.
pH: A measure of the acidity or alkalinity of water on a scale from 0 to 14. 7 is neutral, below 7 is acidic, above 7 is alkaline (or basic). RO water tends to be slightly acidic (pH < 7) because dissolved alkaline minerals are removed.
Post-filter(s): Filters placed after the RO membrane (and usually after the storage tank) to provide a final “polishing” step. Typically carbon-based (GAC or carbon block) to remove any residual tastes or odors, or a remineralization filter.
Pre-filter(s): Filters placed before the RO membrane to protect it from larger particles and chemicals that could cause damage or clogging. Usually includes a sediment filter and one or more carbon filters (to remove chlorine/chloramine).
PSI (Pounds per Square Inch): A unit of pressure measurement. Adequate feed water pressure (typically 40-100 PSI for residential systems without a pump) is crucial for RO system performance.
R
Recovery Rate (or Water Efficiency): The percentage of feed water that becomes purified water (permeate). Calculated as (Permeate Flow Rate / Feed Water Flow Rate) x 100%. Higher recovery rates mean less water is sent to the drain as concentrate. Residential systems might range from 10% to 50% or more with efficiency improvements.
Reject Water: See Concentrate.
Rejection Rate: The percentage of a specific contaminant that is removed by the RO membrane. For example, a 98% TDS rejection rate means the membrane removes 98% of the total dissolved solids from the feed water. Rates vary by contaminant and operating conditions.
Reverse Osmosis (RO): A water purification process that uses pressure to force water molecules through a semi-permeable membrane, separating them from dissolved salts, minerals, contaminants, and microorganisms, which are retained and flushed away.
RO Membrane: The core component of an RO system. A semi-permeable barrier with extremely fine pores (approx. 0.0001 microns) that allows water molecules to pass through while blocking larger dissolved solids, salts, heavy metals, and microorganisms.
S
Sanitization: The process of periodically cleaning and disinfecting an RO system (especially the storage tank, lines, and faucet) to prevent the growth of bacteria or biofilm.
Scaling: The precipitation and crystallization of minerals (particularly calcium and magnesium salts) on the membrane surface, a specific type of fouling that reduces membrane efficiency.
SDI (Silt Density Index): A measurement used to quantify the fouling potential of water by suspended solids, critical for predicting RO membrane performance and lifespan.
Sediment Filter: A pre-filter designed to remove larger suspended particles from water, such as sand, silt, dirt, and rust. Protects downstream filters and the RO membrane from clogging. Rated by micron size.
Semi-Permeable Membrane: A membrane that allows certain molecules (like water) to pass through but blocks others (like dissolved salts and larger contaminants). Essential for both osmosis and reverse osmosis.
Stage: Refers to the arrangement of membrane elements in series or parallel in larger RO systems. Multi-stage systems increase efficiency and recovery rates.
Storage Tank: A container used in most undersink RO systems to hold a reserve of purified water. Because RO produces water slowly, the tank ensures water is available on demand. Most use a pre-pressurized air bladder to push water out to the faucet.
System Types:
- Countertop RO System: Portable, sits on the counter.
- Undersink RO System: Most common residential type, installed under the kitchen sink with a dedicated faucet.
- Whole House RO System: Treats all water entering the home; requires higher capacity, often pre-treatment (like softening), and large storage tanks.
T
TDS (Total Dissolved Solids): The total concentration of all dissolved substances (minerals, salts, metals, cations, anions) in water. Measured in parts per million (ppm) or milligrams per liter (mg/L). RO significantly reduces TDS levels.
TFC/TFM (Thin Film Composite/Material): The most common type of modern RO membrane material, consisting of an ultra-thin polyamide layer on a porous polysulfone support. Highly effective but sensitive to chlorine.
Transmembrane Pressure: The pressure differential across the RO membrane that drives the filtration process, calculated as the average of feed and concentrate pressure minus permeate pressure.
Tubing: The flexible plastic lines (often 1/4″ or 3/8″) used to connect the various components of an RO system.
Turbidity: A measure of the cloudiness or haziness of water caused by suspended particles. Sediment pre-filters are used to reduce turbidity before water reaches the RO membrane.
U
UV (Ultraviolet) Sterilizer: UV light systems sometimes added as a final stage after RO, especially if microbiological contamination is a concern. UV light inactivates bacteria and viruses without adding chemicals.
V
VOCs (Volatile Organic Compounds): A group of carbon-based chemicals that evaporate easily at room temperature. Some can be harmful contaminants in water (e.g., solvents, pesticides). Activated carbon filters are effective at removing many VOCs.
W
WQA (Water Quality Association): A trade organization representing the water treatment industry. Offers educational programs and product certifications (e.g., WQA Gold Seal) indicating systems meet industry standards.
