Water treatment glossary

Hardness is the concentration of calcium and magnesium, reported as mg/L of calcium carbonate. USGS classifies 0–60 mg/L as soft, 61–120 as moderately hard, 121–180 as hard and above 180 as very hard. Grains per gallon (gpg) is the other common unit for the same measurement; softener capacities are marketed in grains, which is why sizing is quoted as 32,000, 48,000 or 96,000 grains.

TDS (total dissolved solids) is everything dissolved in the water, minerals and salts included. It is not a contamination measure by itself. NSF/ANSI 58 covers reverse osmosis systems specifically for reducing TDS along with chemical and particulate contaminants. ppm (parts per million) is numerically equivalent to mg/L for dilute water solutions, which is why lab reports and product literature switch between them freely.

Ion exchange is the mechanism a conventional softener uses: hardness ions swap onto a resin bed in exchange for sodium or potassium. Regeneration is the cycle that flushes accumulated hardness off the resin using brine and restores capacity. NSF/ANSI 44 covers residential cation exchange softeners with manual, automatic or demand-initiated regeneration — demand-initiated units trigger on actual water use rather than a timer. Backwash is the reverse flow that lifts and rinses a media bed to clear trapped particles.

NSF/ANSI 42 is aesthetic effects: chlorine, taste, odor, particulate, iron, manganese, zinc, TDS. NSF/ANSI 53 is health effects, with over 50 contaminant reduction claims including lead, Cryptosporidium, VOCs and chromium. NSF/ANSI 401 covers emerging compounds such as pharmaceutical and pesticide residues, up to 15 contaminants. NSF/ANSI 55 covers UV microbiological systems in Class A and Class B configurations, and NSF/ANSI 62 covers distillation.

How to choose

Know your hardness number, in one unit

Write it down in mg/L and convert consistently. Quotes that mix mg/L and grains are easy to misread in the seller's favor.

Don't treat TDS as a danger score

High TDS can be entirely benign minerals. It is a reason to look at the detailed lab report, not a reason to buy on the spot.

Ask which regeneration type you're buying

Timer-based regeneration wastes salt and water on low-use weeks. Demand-initiated is recognized in NSF/ANSI 44 and is usually worth the difference.

Learn the four standard numbers

42, 53, 58 and 44 will carry you through nearly every residential conversation. Add 55 if UV is on the table.

Watch for terms with no standard behind them

"Conditioning," "structuring" and "catalytic media" are marketing categories, not certified performance claims.