Hardness is dissolved calcium and magnesium, which leave scale in pipes and pots and stop soap lathering, and iron is a metal that makes water reddish-brown and metallic-tasting; neither is usually a danger to drink, but both cost you something. Each 1 milligram per liter of dissolved iron uses about 0.64 milligrams per liter of chlorine in turning to rust-coloured particles, which cuts into the dose before any germ is touched, and iron that appears only after water has stood in a pipe points to corroding pipe, not to the source.
Part of Community Water Testing
Hardness and iron are two mineral properties of water, tested by simple means, that affect taste, staining, scale and the performance of chlorination.
These Minerals Are Mostly A Nuisance, With One Real Interaction
Hardness comes from calcium and magnesium picked up as water passes through rock. Hard water forms scale on pots and inside pipes, uses up soap, and leaves a scum instead of lather. It is generally a nuisance rather than a health problem, as I recall from general drinking-water guidance, but scale can narrow pipes and clog valves over years.
Iron gives water a reddish-brown colour, a metallic taste, and orange-brown stains on laundry and basins. The community water testing page lists reddish-brown colour as iron or rust, to be checked for corroding pipes, and says iron removal may be needed. Iron can come from the source, which carries dissolved iron, or from the pipes, which release rust.
A simple test separates the two:
| Observation | Meaning |
|---|---|
| Water clear at first, turns reddish when left standing in a clear glass | Dissolved iron that oxidises in air: from the source |
| Water reddish at once, especially the first draw after standing, clearing after flushing | Rust from corroding pipe or a rusty tank |
| Reddish at the source and everywhere | Iron in the source |
| Reddish at one tap only | That branch or fitting is corroding |
What You Need
- Clear glass jars, all the same size
- A plain bar of soap and a clear bottle with a lid
- A reference sample of soft water, such as clean rainwater
- A log of what you see and where
Method: Test For Both And Place The Cause
Step 1: Hardness (soap test). Put equal amounts of water in two bottles, a reference of soft water and your sample, and shake each with the same amount of soap (the same number of shaves or the same swirl of a bar). Compare the lather: lasting foam means soft water, a thin scummy lather means hard water. It is relative, not a number.
Step 2: Iron (standing test). Fill a clear jar and let it stand for an hour or two. A reddish-brown cloud or sediment that appears is iron oxidised by air.
Step 3: Place the iron. Compare the source, the tank outlet and the taps, and the first draw against flushed water.
Step 4: If the iron is in the source, remove it by aeration, settling and filtering. Let the water fall through the air in a cascade or pour it between containers so it picks up oxygen, give it time in the settling basin so the rust-coloured particles drop, then filter (Sedimentation).
Step 5: If the iron is from pipes, flush the system, find the corroding section and replace it (Pipe Materials).
Worked Example: Iron Uses Up Chlorine
Chlorine oxidises dissolved iron: 2 Fe²⁺ + Cl₂ → 2 Fe³⁺ + 2 Cl⁻. The masses give the ratio. Cl₂ is 70.9 g/mol and 2 Fe is 2 × 55.85 = 111.7 g/mol, so each mg of iron consumes 70.9 ÷ 111.7 = 0.635 mg of chlorine.
- Water with 1 mg/L iron uses about 0.64 mg/L of chlorine.
- Water with 2 mg/L iron uses 2 × 0.635 = 1.27 mg/L.
- Water with 5 mg/L iron uses 5 × 0.635 = 3.2 mg/L.
Against the treatment page’s dose of about 5 mg/L, with a residual of 0.5 mg/L, which implies a demand of 4.5 mg/L, an extra iron demand of 3.2 mg/L means the water would not reach the target residual unless the iron was removed first. A residual that mysteriously will not appear, in water that is rusty, points to iron. The remedy is to remove the iron upstream; the dose follows the treatment procedure and is judged by the residual (Dosing Calculation).
Common Mistakes
- Blaming the source for rust that comes from the pipe.
- Chlorinating iron-rich water and expecting a residual.
- Treating hardness as a health hazard and iron as harmless in every case. Each is mostly a nuisance, but iron interferes with disinfection.
- Descaling pipes with strong acid and then flushing with chlorine. Never mix them.
Safety
Acid descalers and bleach must never meet
If you remove scale with an acid, flush the system thoroughly and wait before any chlorine goes in: acid and bleach together release chlorine gas. Wear eye protection and gloves with any acid.
What’s Next
- Learn next: Chlorine Residual — seeing whether iron is eating the dose
- Combine with: Treatment Selection — choosing steps that handle iron