Summary
In the reverse osmosis concentrate, salts are concentrated 2–4 times; when calcium carbonate and sulphate reach saturation, they precipitate on the membrane surface. There are two ways to prevent this.
Softening (ion exchange)
- Removes hardness completely; the risk of scaling drops to zero and recovery can rise to 75–80%.
- Higher investment, salt consumption and brine waste.
- If iron/manganese is present, prefiltration is needed to protect the resin.
- Not used for water with high conductivity and salinity; in such water, scale is controlled with an antiscalant.
Antiscalant dosing
- Low investment; a small dosing pump and chemical tank are enough.
- The hardness stays in the water; the dose calculation depends on the water analysis, and scaling begins at the wrong dose.
- On its own, it may be insufficient with silica and very high hardness (> 40 °fH).
- The preferred method for water with high conductivity and salinity; softening is not used for such water.
Decision table
| Situation | Recommendation |
|---|---|
| High conductivity and salinity | Antiscalant (softening is not used) |
| Hardness < 25 °fH, low silica, 60–70% recovery | Antiscalant |
| Hardness > 30 °fH or a high recovery target | Softening (+ antiscalant if necessary) |
| Product water to be used in a boiler | Softening (the boiler is also protected from scale) |
| Small capacity (< 1 m³/h), low hardness | Antiscalant or cartridge polyphosphate |
In medium and large industrial systems, the safest arrangement is softening + a low dose of antiscalant; for water with high conductivity and salinity, softening is not used and scale control is provided by an antiscalant. For the resin volume, see the softener calculator.
