Glossary

Every term on this site, defined once, in plain language. This list grows as more of the site gets reviewed — if you hit a term here that isn’t defined yet, tell us and we’ll add it.

A

Adsorption — contaminants sticking to the surface of a material (like activated carbon) rather than being caught in a mesh or dissolved into it. This is how carbon filtration works.

Anion — a negatively charged ion. Nitrate, sulfate, and chloride are anions. Removing them takes anion exchange resin or a mechanism like reverse osmosis that doesn’t care about charge.

Anode rod — a sacrificial metal rod inside a water heater that corrodes on purpose to protect the tank. A degrading rod can throw a metallic or salty taste into hot water only — a water heater issue, not a water quality one.

B

Brine — a concentrated salt solution. In ion exchange, brine is what regenerates a softener’s resin bed; the used brine (carrying the calcium and magnesium it stripped off the resin) is what flushes down the drain during regeneration.

C

Cation — a positively charged ion. Calcium, magnesium, and sodium are cations. A water softener is a cation exchanger.

Catalytic carbon — activated carbon specially treated to react with chloramine, not just adsorb it. Standard carbon barely touches chloramine; catalytic carbon is the version built for it.

Chromatographic dumping (also: “dumping”) — when an ion exchange resin has captured one ion but a more-preferred competing ion displaces it, releasing the first ion back into the water in a concentrated burst. On a standard (non-selective) anion resin treating nitrate, sulfate can displace previously-captured nitrate this way — meaning water coming out can briefly carry more nitrate than what went in. Nitrate-selective resins are built specifically to prevent this.

Concentrate (also: reject water, brine — in the RO context) — the water left over on a reverse osmosis membrane’s high-pressure side, carrying everything the membrane rejected. Flushed to drain.

D

Dissolved — broken down into ions or molecules small enough to be part of the water itself rather than a separate particle floating in it. Dissolved things pass straight through any mechanical filter, no matter how fine. See Water Chemistry 101.

M

MCL (Maximum Contaminant Level) — the legally enforceable limit for a contaminant in a public water system. Doesn’t apply to private wells, which aren’t covered by MCL enforcement at all.

MCLG (Maximum Contaminant Level Goal) — the health-based target regulators would set with no cost or feasibility limits. Aspirational, not enforceable. For some contaminants (lead is the classic case) the MCLG is zero.

Methemoglobinemia (“blue baby syndrome”) — the condition caused by nitrite (converted from nitrate by gut bacteria) binding to hemoglobin and blocking its ability to carry oxygen. The reason nitrate has its own urgent rules for infants. See Nitrate in Drinking Water.

Micron rating — the particle size a filter is built to catch, measured in microns. A coarse (larger-micron) filter catches sand and silt; a fine one reaches down to bacteria and some cysts. Only meaningful for particles — dissolved contaminants ignore micron rating entirely.

N

Nominal vs. absolute (filter rating) — a nominal rating is a rough average (some particles above the stated size still get through); an absolute rating is a tested guarantee. It’s a marketing distinction, not a built-in quality grade — check which one a specific product actually claims.

O

Osmosis — water naturally moving through a semi-permeable membrane from a less-concentrated solution toward a more-concentrated one. Reverse osmosis uses pressure to force water to move the opposite way, leaving dissolved contaminants behind.

P

Permeate — the treated water that passes through a reverse osmosis membrane. What comes out the tap.

pH — a 0–14 scale measuring how acidic or basic water is; 7 is neutral. See Water Chemistry 101 for why it matters even though it isn’t a direct health threshold.

POE / POU (Point of Entry / Point of Use) — POE means whole-house treatment, installed where water enters the home. POU means a single tap, usually the kitchen sink. Some contaminants need POE (hardness, anything affecting every fixture); others are honestly a POU-only concern (most drinking-water-specific contaminants).

ppm / ppb — parts per million and parts per billion. For water, ppm is effectively the same measurement as mg/L. ppb is a thousand times smaller, used for trace-level contaminants like PFAS.

R

Recovery rate — in a reverse osmosis system, the ratio of treated water (permeate) produced to water sent to drain (concentrate). A conventional system might produce 1 gallon of drinking water for every 2–4 gallons wasted; high-efficiency systems improve this significantly.

Regeneration — the process of flushing an exhausted ion exchange resin with a concentrated brine solution to strip off what it captured and reload it with fresh exchangeable ions. What a water softener does automatically on a schedule.

Resin — the tiny polymer beads inside an ion exchange system (like a softener) that actually do the ion-swapping. Has a finite capacity; needs regenerating or replacing once exhausted.

S

Secondary standard — a non-enforceable EPA guideline covering taste, odor, and appearance rather than health. A housekeeping limit, not a safety one.

Selective resin — an ion exchange resin engineered to strongly prefer one specific ion over competing ones, even when the competitor is more abundant. Nitrate-selective resin is the key example — it holds nitrate even in the presence of sulfate, which a standard resin would grab first (and later dump nitrate to make room for).

Suspended — solid particles floating in water rather than dissolved into it (silt, rust flakes, sand). Physically caught by a fine-enough filter. See Water Chemistry 101.

T

TDS (Total Dissolved Solids) — everything dissolved in water, added into one aggregate number, usually measured by conductivity rather than a real lab panel. Tells you how much is dissolved, not what — a high number could be harmless minerals or something worth worrying about. See Water Chemistry 101.

Full list of standards mentioned across the site

NSF/ANSI 44 — softeners (cation exchange). Covers softening efficiency, salt efficiency, structural integrity. Cannot carry a nitrate claim — that’s a different standard entirely.

NSF/ANSI 53 — the standard most often cited for PFAS reduction, when PFAS is specifically named on the certification listing. Also covers carbon and anion-exchange claims for various other contaminants. Being “53-certified” does not by itself mean a product handles PFAS — the specific contaminant has to be named.

NSF/ANSI 58 — reverse osmosis systems. Covers contaminant reduction claims (lead, arsenic, nitrate, PFAS, etc., each tested and listed individually), materials safety, structural integrity.

NSF/ANSI 61 — confirms a material (like resin) won’t leach anything harmful into water. Not a removal or performance claim on its own.

NSF/ANSI 62 — distillation systems.

NSF P473 — an older PFOA/PFOS protocol, retired in 2019 and folded into Standards 53 and 58. If you see a product still citing P473, it’s referencing something that no longer exists.