The short answer: every water problem from step 1 maps to a working material — a filtration media in a backwashing tank, or an ion-exchange resin that swaps one ion for another. A softener is just one resin doing one job. Real systems chain several stages, and the order matters: sediment first, iron and sulfur before the softener, chlorine out before the softener on city water, pH fixed early, RO at the sink, UV last. Learn the table and the order rules and you can spec almost any house. This is the field guide from the Roadmap, condensed.
Media: what goes in the tank
Media is the material inside a backwashing filter tank. Different media target different problems. These are the ones you'll actually spec.
For sediment and cloudy water - Multimedia (anthracite / sand / garnet) — sediment, turbidity, suspended particles. Pre-filtration before any other stage. Layered by density; backwashes clean. - Filter-Ag / Micro-Z — fine sediment with low pressure loss. Lightweight, uses less backwash water.
For chlorine, taste, odor and organics (city water) - Granular activated carbon (GAC) — chlorine, taste, odor, VOCs, many organics. Coconut-shell carbon has more micropores. It also protects softener resin from chlorine — which is why it sits before the softener. - Catalytic carbon — chloramine, hydrogen sulfide, chlorine. Surface-modified; far better on chloramine than plain GAC. If the city uses chloramine, this is the carbon. - KDF 55 / KDF 85 — copper-zinc redox media. KDF 55 for chlorine; KDF 85 for iron, H₂S and heavy metals. Bacteriostatic, so it controls bacteria and scale inside a carbon bed and extends the carbon's life. Usually paired with carbon.
For iron, manganese and sulfur (well water) - Birm — iron and manganese on well water that has dissolved oxygen and pH of about 6.8 or higher. Cheap catalyst; needs oxygen; chlorine destroys it. - Manganese greensand / Greensand Plus — iron, manganese, hydrogen sulfide. Broad well-water use; regenerated with potassium permanganate. - Katalox Light — iron, manganese, H₂S, arsenic, some radium. Manganese-dioxide-coated, light, less backwash, wide pH range. Compact, high-capacity systems for high-iron wells. - Pyrolox / Filox (MnO₂) — heavy iron and sulfur loads. Very dense, so it needs high backwash flow — check the well can deliver it.
For pH - Calcite (calcium carbonate) — raises low pH; adds calcium hardness. For acidic water and blue-green fixture staining. Sacrificial: it dissolves and needs topping up. - Corosex (magnesium oxide) — strongly raises very low pH. Usually blended with calcite; alone it can over-correct.
For specific contaminants - Activated alumina — fluoride and arsenic. pH-sensitive; periodic regeneration. - Bone char — natural fluoride reduction, some heavy metals; lower capacity than activated alumina.
Resins: the trading floor
Ion-exchange resins swap unwanted ions for harmless ones. Here's the toolbox:
- Strong acid cation (SAC), sodium form — the water-softener resin. Trades hardness (calcium and magnesium) for sodium, then recharges from a brine tank.
- SAC, hydrogen form — removes all cations, releasing H⁺. The cation stage of demineralization; regenerated with acid.
- Weak acid cation (WAC) — removes hardness tied to alkalinity; dealkalization. Very efficient regeneration.
- Strong base anion (SBA), Type I / II — sulfate, nitrate, chloride, silica, CO₂, TOC, arsenic. The anion stage of DI; nitrate, arsenic and organics removal.
- Weak base anion (WBA) — strong-acid anions below neutral pH; paired with SAC-H for DI; tolerates organics.
- Nitrate-selective anion — prefers nitrate even when sulfate is high. Well-water nitrate; salt-regenerated.
- Tannin / organic scavenger — tannins, color, dissolved organics. The yellow, tea-colored well water. Salt-regenerated.
- Fine-mesh SAC — softening with higher iron tolerance, for wells with some iron present.
- Selective resins (arsenic / uranium / perchlorate) — single-contaminant problems needing a precise fix.
The thing to internalize: a softener is strong acid cation resin in sodium form. Every other resin is a variation on the same trick aimed at a different ion. Once that clicks, the whole table makes sense.
Treatment trains: how they combine
Real systems chain stages in a deliberate order. Get it wrong and you destroy the next stage. The rules that decide the sequence:
- Sediment out first. Particles clog and foul everything downstream.
- Oxidize and remove iron, manganese and sulfur before the softener. Iron fouls softener resin. Air injection, chlorine, or greensand with permanganate.
- Remove chlorine before the softener on city water. Chlorine above about 1 ppm oxidizes and destroys resin beads. Carbon or KDF.
- Fix pH early if the water is acidic. Low pH harms equipment, and some media need a pH range.
- Soften after the water is clean of sediment, iron and chlorine.
- Polish for drinking at the point of use. RO at the sink removes TDS, nitrate, arsenic, fluoride — and soft water going in protects the membrane.
- Disinfect with UV last, on already-clear water.
The trains you'll spec most:
| Water | Train |
|---|---|
| City, chlorinated | Sediment → carbon (or KDF + carbon) → softener → RO at the sink |
| City, chloraminated | Sediment → catalytic carbon → softener → RO at the sink |
| Well: iron + hardness + rotten-egg smell | Sediment → air injection + Katalox/greensand (iron, Mn, H₂S) → softener → carbon polish → UV → RO at the sink |
| Acidic well (low pH) | Calcite/corosex → iron filter if needed → softener → carbon |
Special targets: nitrate → nitrate-selective anion (or RO); fluoride or arsenic → activated alumina or RO; full demineralization → SAC(H) cation then SBA anion.
Single-tank blends save space — carbon + KDF, softening resin + fine mesh, calcite + corosex, catalytic carbon over garnet — but you can't backwash or regenerate the layers independently, so only blend media that share a maintenance cycle.
Why this is step 2 and not step 5
You can't size (step 3), install (step 4) or price (step 5) a system you can't spec. And you can't run the kitchen-table sale (step 6) if the homeowner asks "why does the well smell?" and you don't know it's H₂S and that Katalox or catalytic carbon handles it. Every question at the sink is a row in this table. Learn the table.
What you'll learn in the program
Step 2 inside Water CheatCode is the full field guide — every media and resin with its application and its gotchas, the treatment-train rules, the standard trains for city and well water, and how to go from a step-1 test result to a technology choice without guessing. Free to join.
Previous: Step 1 — test before you spec. Next: Step 3 — system selection and sizing.
FAQ
What's the difference between filtration media and ion-exchange resin?
Media is a working material in a backwashing tank that physically or chemically removes something — sediment, chlorine, iron, sulfur. Resin swaps ions: a softener's strong-acid-cation resin trades calcium and magnesium for sodium and recharges from a brine tank. Most whole-house systems use both, in sequence.
What order should water treatment stages go in?
Sediment first; oxidize and remove iron, manganese and sulfur before the softener; remove chlorine before the softener on city water; fix low pH early; soften once the water is clean; RO at the kitchen sink for drinking water; UV last on clear water. Wrong order destroys the next stage — iron and chlorine both ruin softener resin.
Why does carbon go before a water softener on city water?
Because chlorine above roughly 1 ppm oxidizes and destroys softener resin beads. Carbon (or KDF plus carbon) removes the chlorine first, which protects the resin and fixes the taste and odor at the same time. On chloraminated water, use catalytic carbon.
What media removes iron from well water?
Birm (needs dissolved oxygen and pH around 6.8+; chlorine destroys it), manganese greensand (regenerated with potassium permanganate), Katalox Light (light, wide pH range, also handles manganese, H₂S and some arsenic), and Pyrolox/Filox for heavy loads (very dense; needs high backwash flow). Iron is handled before the softener so it doesn't foul the resin.
Can you mix different media in one tank?
Yes, in blends like carbon + KDF, calcite + corosex, or resin + fine mesh, to save space. The tradeoff is that blended layers can't be backwashed or regenerated separately, so only blend media that share the same maintenance cycle.