If you're using a water softener to remove small amounts of dissolved iron or manganese, the hardness figure on your water analysis may not be the correct setting to enter into the control valve.
That's because iron and manganese can increase the effective load on the resin. To account for this, some water softener manufacturers recommend entering a compensated hardness figure — higher than the measured calcium and magnesium hardness alone.
Getting this right helps the softener regenerate before its performance deteriorates. However, the compensation is a programming allowance, not a claim that 1 mg/L of iron chemically equals 70–85 mg/L of calcium carbonate hardness. The figures discussed below are rules of thumb, not universal settings.
What is compensated hardness?
A conventional water softener removes hardness minerals, principally calcium and magnesium, using ion-exchange resin. Under suitable conditions, the same resin can also exchange dissolved iron (ferrous iron) and small quantities of dissolved manganese.
If the valve is programmed using only the measured hardness, it may overestimate how much water can be treated before regeneration, increasing the risk of iron or manganese breakthrough and resin fouling.
Compensated hardness increases the programmed hardness value to make allowance for those additional contaminants. It usually makes a metered softener regenerate sooner, not remove more iron or manganese in a single pass.
Rules of thumb: how much hardness should you add?
For a preliminary estimate, you may see the following rules of thumb quoted:
| Contaminant in untreated water | Illustrative rule-of-thumb allowance (not a universal setting) |
|---|---|
| 1 mg/L (1 ppm) dissolved iron | 70–85 mg/L as CaCO₃ |
| 1 mg/L (1 ppm) dissolved manganese | 70–100 mg/L as CaCO₃ in some approaches |
Important: These are rules of thumb only. They are not the true chemical hardness equivalence of the metals, and they are not a standard that applies to every softener. Manufacturer instructions vary, particularly for manganese. For example, some equipment uses 85 mg/L as CaCO₃ per 1 mg/L for both iron and manganese, while another manufacturer's sizing guidance uses approximately 68 mg/L for iron and 137 mg/L for manganese per 1 mg/L present.
Always use the factors specified by the manufacturer of the complete softener. Where the control valve has separate iron and manganese settings and calculates the correction itself, enter the measured values in those fields rather than adding the allowance again to hardness.
How to calculate the setting
For a controller that requires one combined hardness figure, the general method is:
Compensated hardness = measured hardness + iron allowance + manganese allowance
When using mg/L as CaCO₃:
Compensated hardness = measured hardness + (iron in mg/L × iron factor) + (manganese in mg/L × manganese factor)
Worked example
Suppose your water analysis reports:
- Total hardness: 150 mg/L as CaCO₃
- Dissolved iron: 1.0 mg/L
- Dissolved manganese: 0.3 mg/L
Using illustrative rule-of-thumb compensation factors of 85 for iron and 100 for manganese:
| Item | Calculation | Contribution |
|---|---|---|
| Measured hardness | — | 150 mg/L |
| Iron allowance | 1.0 × 85 | 85 mg/L |
| Manganese allowance | 0.3 × 100 | 30 mg/L |
| Compensated hardness | 150 + 85 + 30 | 265 mg/L as CaCO₃ |
With those factors, the illustrative compensated hardness is approximately 265 mg/L as CaCO₃. Don't enter that figure automatically: check the specific equipment's method before programming it, because different manufacturers' factors will give different results.
What if the softener uses grains per gallon or degrees of hardness?
Control valves don't all use the same units. Check the label on the screen or the programming manual.
For hardness expressed as CaCO₃:
- Grains per US gallon (gpg): divide mg/L by approximately 17.1.
- French degrees (°fH): divide mg/L by 10.
- German degrees (°dH): divide mg/L by approximately 17.8.
Our worked example of 265 mg/L as CaCO₃ is therefore approximately 15.5 gpg, 26.5 °fH or 14.9 °dH. Don't enter 265 into a controller expecting gpg — the number would be far too high.
Can a normal water softener remove iron and manganese?
Sometimes — but this is where the type and condition of the water matter.
Conventional softening resin can remove some dissolved (clear-water) iron and low concentrations of dissolved manganese, provided the softener is suitable for the water chemistry and the manufacturer permits that use. Results depend on pH, concentrations, hardness and whether the metals remain dissolved before entering the resin.
It is a different situation if the iron has already oxidised into visible rusty particles, or manganese has formed black deposits. Those particles can foul the resin bed. A conventional softener should not generally be treated as a mechanical iron or manganese filter.
High iron or manganese concentrations, iron bacteria, oxidised metals or unsuitable water chemistry may require a dedicated iron/manganese removal system, such as appropriately designed oxidation and filtration, rather than relying on a softener.
Increasing the hardness setting does not make an unsuitable softener suitable for high metal concentrations.
Does increasing the hardness setting use more salt?
Potentially, yes. When a metered softener is given a higher compensated hardness figure, it calculates that its capacity will be used more quickly and may regenerate more frequently. This can increase total salt and water consumption.
However, setting the hardness too low simply to save salt can lead to iron, manganese or hardness appearing in the treated water and can contribute to resin fouling. The aim is accurate programming, not the highest possible setting or the lowest possible number of regenerations.
Where iron is present, follow the manufacturer's guidance on resin cleaning and maintenance as well as regeneration. Compensation alone cannot prevent all fouling.
Check the treated water after programming
Once the softener has been configured, test the outlet water for hardness, iron and manganese, preferably including a check towards the end of a normal service run. Soft water on its own is not proof that iron and manganese removal is satisfactory.
For a private water supply, a suitable analysis of the untreated water should also establish the pH, hardness, iron and manganese concentrations, and whether the metals are dissolved or particulate. Other contaminants may also affect the treatment choice.
The takeaway
If a softener is being used to remove dissolved iron or manganese, don't automatically enter the measured hardness without checking whether compensation is required.
As a rule of thumb for an initial estimate, 1 mg/L of dissolved iron is sometimes allowed an additional 70–85 mg/L as CaCO₃, and 1 mg/L of manganese may be allowed around 70–100 mg/L. These are not universal conversion factors: some manufacturers specify a considerably higher manganese factor.
Use the instructions for the particular softener, check that the equipment is suitable for the water, and confirm performance by testing the treated water.
At GAPS Water Treatment, we supply water softeners, control valves and specialist iron and manganese treatment equipment. If you're unsure which approach is appropriate, contact us by email with your water analysis and the model of the softener or control valve.
Powered by Discount Bot