Testing
Water treatment glossary: 46 terms in one plain sentence each
Quick answer
A grain per gallon equals 17.1 mg/L of hardness, salt efficiency is graded in grains removed per pound of salt, and a micron rating is absolute only if it guarantees 99.9 percent removal at that size. This glossary defines 46 water treatment terms in one plain sentence each.
Water treatment borrows vocabulary from chemistry, plumbing and federal regulation all at once, and a term from one of those worlds gets thrown around in the others without much explanation. This page defines 46 of them in one plain sentence each, grouped by the part of the system they belong to: sizing and softening, filtration, reverse osmosis, disinfection and well water treatment, and the regulatory terms that decide what a limit actually means.
A handful of the terms below name topics that are genuinely not do it yourself territory: iron bacteria, and the regulatory terms tied to lead, nitrate, arsenic and PFAS. Defining a term is not the same as diagnosing your water for it. Where any of these come up, confirming them needs a certified laboratory test and usually a licensed professional or your local health department, not a definition on this page.
On this page
Hardness, softening and sizing terms
Salt efficiency is measured in grains of hardness removed per pound of salt, and NSF/ANSI 44 sets 3,350 grains per pound as its "high efficiency" threshold.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Term | Definition |
|---|---|
| Grain per gallon | A unit of water hardness equal to 17.1 milligrams per liter of hardness expressed as calcium carbonate |
| ppm / mg/L | Two names for the same concentration unit, parts per million and milligrams per liter, used interchangeably for dilute substances dissolved in water |
| Compensated hardness | A softener sizing figure that adds grains of hardness credit for dissolved iron and manganese, since resin exchanges those minerals along with calcium and magnesium |
| Ion exchange | The process a water softener uses, trading calcium and magnesium ions in the water for sodium or potassium ions held on a resin bed |
| Resin | The small plastic beads inside a softener tank that carry the exchange sites where hardness minerals trade places with sodium or potassium |
| Crosslink | The degree of crosslinking, commonly around 8 percent, in the resin's polystyrene structure, which affects its durability and exchange capacity |
| Regeneration | The cycle a softener runs to flush accumulated hardness minerals off the resin and recharge it with a fresh brine solution |
| Backwash | The reverse-flow step of regeneration that lifts and resettles the resin bed to clear trapped sediment before brine is drawn |
| Brine draw | The step in regeneration where concentrated salt solution is pulled from the brine tank through the resin bed to recharge it |
| Salt efficiency | The number of grains of hardness a softener removes per pound of salt used in regeneration, with NSF/ANSI 44 setting 3,350 grains per pound as the minimum for a "high efficiency" rating |
| Nameplate capacity | The advertised grain rating of a softener, reached only at the maximum published salt dose, roughly 15 pounds per cubic foot of resin |
| Service flow | The maximum flow rate a softener or filter can pass while still performing at its rated capacity or reduction claim |
| Peak flow | The highest simultaneous flow rate a house's fixtures can demand at once, the number whole house filtration and softener sizing are built around |
Filtration and sediment terms
An absolute micron rating guarantees 99.9 percent removal at the stated size; a nominal rating carries no standardized guarantee at all.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Term | Definition |
|---|---|
| Micron, nominal vs absolute | An absolute rating means at least 99.9 percent removal at the stated size; a nominal rating is not standardized and generally implies something close to 85 percent removal at a threshold the manufacturer chooses |
| Sediment | Undissolved particulate matter such as sand, silt or rust that a mechanical filter removes by size rather than by chemistry |
| Turbidity | The cloudiness of water caused by suspended particles, measured in nephelometric turbidity units |
| NTU | Nephelometric turbidity unit, the standard unit turbidity is measured in |
| Carbon block | A filter cartridge made of compressed powdered activated carbon, which reduces chlorine taste and odor and can carry a finer, more consistent micron rating than loose granular carbon |
| GAC | Granular activated carbon, loose carbon granules used in a filter bed to adsorb chlorine, taste and odor compounds |
| Catalytic carbon | A specially processed activated carbon that targets hydrogen sulfide and chloramine rather than the taste and odor compounds standard carbon handles |
| KDF | A copper-zinc filter media that uses a redox reaction to reduce chlorine and inhibit bacterial growth on the media bed, usually paired with carbon |
Reverse osmosis terms
Recovery is the share of feed water a reverse osmosis system converts to usable permeate; the rest leaves as concentrate carrying away what the membrane rejected.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Term | Definition |
|---|---|
| Reverse osmosis | A filtration process that forces water through a semipermeable membrane under pressure, separating a purified permeate stream from a concentrated reject stream |
| Permeate | The purified water that has passed through a reverse osmosis membrane |
| Concentrate | The reject stream from a reverse osmosis membrane that carries away the dissolved solids the membrane blocked, usually sent to drain |
| Recovery | The percentage of feed water a reverse osmosis system converts to permeate rather than sending to drain as concentrate |
| Drain ratio | The ratio of concentrate to permeate a reverse osmosis system produces, for example 4 to 1 at 20 percent recovery |
| Membrane | The semipermeable barrier inside a reverse osmosis housing that rejects dissolved solids while passing water molecules |
| TDS | Total dissolved solids, an estimate of everything dissolved in water derived from measuring its electrical conductivity |
Disinfection and well water treatment terms
Iron bacteria is a living colony rather than a dissolved mineral, and is treated with shock chlorination of the well, not a filter.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Term | Definition |
|---|---|
| UV dose | The amount of ultraviolet energy delivered to water, measured in millijoules per square centimeter, that determines whether a system inactivates target organisms |
| mJ/cm2 | Millijoules per square centimeter, the unit ultraviolet dose is measured in |
| UVT | Ultraviolet transmittance, the percentage of UV light that passes through a water sample rather than being absorbed or scattered, which has to stay above a published threshold for a system to deliver its rated dose |
| Ferrous iron | Dissolved, clear water iron that is invisible at the tap and only turns orange after the water sits and the iron oxidizes |
| Ferric iron | Already oxidized iron that is visibly orange or rust colored the moment it leaves the tap |
| Iron bacteria | A living colony that forms slimy orange or rust colored jelly in fixtures and tanks, treated by shock chlorinating the well rather than by filtration |
| Hydrogen sulfide | A dissolved gas that produces a rotten egg odor in well water, commonly treated with an oxidizing filter |
| Oxidizing filter | A filter that converts dissolved iron, manganese or hydrogen sulfide into a solid or gas form that can then be filtered or vented out |
| Greensand | Manganese greensand, an oxidizing filter media regenerated with potassium permanganate, used for iron and manganese removal |
| Birm | An oxidizing filter media for iron and manganese that needs dissolved oxygen present and a pH of 6.8 or higher, and is destroyed by chlorine |
| Calcite | Calcium carbonate media that dissolves slowly into acidic water to raise its pH, adding hardness as a byproduct |
| Corosex | Magnesium oxide media, faster acting than calcite, used in blends to correct a lower starting pH |
| Acid neutralizer | A media tank or chemical feed system that raises the pH of acidic, corrosive water |
Regulatory terms
Lead carries an action level, not a maximum contaminant level, which triggers utility treatment steps rather than describing a safe amount.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Term | Definition |
|---|---|
| MCL | Maximum contaminant level, the enforceable legal limit the EPA sets for a regulated primary drinking water contaminant |
| MCLG | Maximum contaminant level goal, the health based target the EPA sets with no enforcement mechanism attached, sometimes set at zero |
| Action level | A concentration, used for lead and copper, that triggers required utility treatment steps rather than describing a safe amount of the contaminant |
| Secondary standard | An aesthetic guideline the EPA sets for taste, odor or appearance, not an enforceable health limit |
| CCR | Consumer Confidence Report, the annual water quality report a public utility is required to publish for its customers at no charge |
Frequently asked questions
What is the difference between grains per gallon and ppm?
Grains per gallon and ppm (parts per million, equivalent to mg/L) both measure hardness, but on different scales. One grain per gallon equals 17.1 mg/L. Laboratories commonly report in mg/L while softeners are programmed and sized in grains, so converting between the two correctly is a routine but essential step.
What does absolute vs nominal mean for a micron rating?
Absolute means the cartridge is tested to remove at least 99.9 percent of particles at or above the stated size. Nominal is not standardized at all, generally implying roughly 85 percent removal at a threshold each manufacturer chooses independently, which is why two nominal ratings of the same number cannot be assumed equivalent.
What is the difference between an MCL and an action level?
An MCL, maximum contaminant level, is the enforceable legal limit for most regulated contaminants. An action level, used for lead and copper, instead triggers required utility treatment steps once exceeded, rather than describing a safe concentration; lead's health goal, the MCLG, is zero regardless of the action level figure.
What does salt efficiency measure on a water softener?
Grains of hardness removed per pound of salt used during regeneration. NSF/ANSI 44 sets 3,350 grains per pound as the minimum for a "high efficiency" certification, and that rating only holds when the softener is actually operated at the low salt dose the certification was earned at.
What is recovery in a reverse osmosis system?
The percentage of feed water a reverse osmosis system converts into usable permeate rather than sending to drain as concentrate. A conventional tank system runs about 20 percent recovery, a 4 to 1 drain ratio, while more efficient tankless systems can reach 50 percent recovery.
What is a secondary standard, and how is it different from an MCL?
A secondary standard is an aesthetic guideline for taste, odor, staining or appearance, such as the 500 mg/L TDS figure, and it is not legally enforceable. An MCL is an enforceable primary health limit. A utility can exceed a secondary standard without violating the law the way exceeding an MCL would.
Get the complete water treatment shopping list
Every item in the starter, complete and premium systems for both city water and well water, with running totals, in one printable list. It opens with the test kit, because that is genuinely the first purchase. No charge.
One list, no sequence of daily emails, and you can leave at any time.
Researched, not professional advice. This page is compiled from published standards, regulatory limits, manufacturer specifications and owner-review consensus, not hands-on testing. Nothing here diagnoses a water problem or says any product makes water safe to drink. Figures described as a rule of thumb are water treatment trade convention rather than published standards, and they are labeled that way wherever they appear. Test your water before you buy equipment: on city water start with the Consumer Confidence Report your utility publishes each year, and on a private well only a certified laboratory test tells you what you have. Bacteria, nitrate, lead, arsenic and PFAS are not do it yourself topics, they are invisible and tasteless, and they need a laboratory and usually a licensed professional rather than a product. Remember that a certification covers a specific claim under a specific standard, so "NSF certified" with no number attached tells you very little.