Treatment process
Ion exchange softening
Exchanges calcium and magnesium for sodium on a resin bed. The most common household hardness treatment, and the one whose trade-off is most often overlooked: hardness is not removed from the water so much as swapped for something else.
Key facts
- Scale
- Municipal and household
- Treatment stage
- Softening
- Certification
- Certified to NSF/ANSI 44.
How it works
Water passes through a bed of sulphonated polystyrene resin loaded with sodium ions. Calcium and magnesium bind more strongly and displace sodium. When capacity is exhausted the bed is regenerated with concentrated brine, which reverses the exchange.
What it removes — and what it does not
| Row | Removes | Does not remove |
|---|---|---|
| 1 | Calcium and magnesium hardness | Nitrate, chloride, sulphate or sodium |
| 2 | Some iron and manganese at low concentrations | Bacteria, viruses or protozoa |
| 3 | Barium and radium | PFAS, pesticides or most organics |
| 4 | — | Lead — and by lowering hardness it can make plumbosolvency worse |
Trade-offs
- Adds sodium in proportion to the hardness removed, roughly 46 mg/l per 100 mg/l of hardness as CaCO₃
- For this reason softened water is conventionally not plumbed to the kitchen drinking tap
- Regeneration discharges a saline brine to the sewer, which is a growing concern for wastewater treatment and reuse
- Potassium chloride can substitute for salt at higher cost
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Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- NSF International / American National Standards Institute — NSF/ANSI drinking water treatment unit standards. Public but restricted · Publisher terms — public access, reuse not clearly granted
- American Water Works Association — AWWA standards, water audits and utility benchmarking. Public but restricted · Publisher terms — public access, reuse not clearly granted