Calcium and magnesium are the two minerals that make water hard. In a filtration plant, a softener removes them to protect the RO membrane, using sodium salt to keep working. After the membrane, the plant adds measured amounts of calcium, magnesium and sodium bicarbonate back for taste and stability.
Key points
- Calcium and magnesium cause hardness and scale, which can damage RO membranes.
- Sodium chloride (softener salt) regenerates softener resin. It goes to drain, not into drinking water.
- Sodium bicarbonate is a different, food-grade compound. Plants dose it after RO to raise alkalinity and pH.
- Remineralisation puts calcium and magnesium back after RO for taste and to make the water less corrosive.
Each mineral, stage by stage
| Mineral | In raw water | In the softener | After the RO membrane |
|---|---|---|---|
| Calcium | Main cause of hardness and scale | Captured by the resin | Added back as food-grade calcium chloride for taste and stability |
| Magnesium | Second cause of hardness | Captured by the resin | Added back as food-grade magnesium sulphate for mouthfeel |
| Sodium | Present in varying amounts | Released by the resin; softener salt (sodium chloride) regenerates it | Most is removed by the membrane; plants dose sodium bicarbonate to correct pH |
Where these minerals come from
Rain soaks into the ground and dissolves minerals from soil and rock as it moves down to the aquifer. That is how calcium, magnesium and sodium end up in groundwater across Punjab. Together with other ions they make up the dissolved solids measured by a TDS meter (see our guide to TDS in water). In moderate amounts they are harmless and give natural water its taste.
Calcium and magnesium: the minerals behind hard water
Water hardness is the amount of dissolved calcium and magnesium, usually reported in milligrams per litre as calcium carbonate (mg/L as CaCO3).
When hard water is heated or concentrated, these minerals come out of solution as scale, the white crust you see inside a kettle. Inside a treatment plant the same scale forms on RO membranes, cutting the flow of water and raising operating pressure. That is why plants put a softener before the RO stage.
| Hardness | mg/L as CaCO3 |
|---|---|
| Soft | 0 – 60 |
| Moderately hard | 61 – 120 |
| Hard | 121 – 180 |
| Very hard | More than 180 |
Source: U.S. Geological Survey, Hardness of Water.
Sodium in the softener
A softener is filled with ion-exchange resin beads loaded with sodium. As hard water flows through, the resin holds on to calcium and magnesium and releases sodium in their place.
After some hours the resin is used up. The plant then regenerates it by washing the bed with strong brine (sodium chloride solution). The high concentration of sodium pushes the calcium and magnesium off the beads, and they are flushed to drain.
Softener salt is not added to drinking water. The softener sits before the RO membrane, and the membrane removes most of the sodium the softener releases, along with other dissolved salts.
Sodium bicarbonate after RO
RO product water is not only low in minerals. It is also often slightly acidic, because carbon dioxide passes through the membrane while the minerals that would buffer it do not. Acidic, low-alkalinity water tastes sour and is corrosive to metal pipes and fittings.
To fix this, many plants dose a small, measured amount of food-grade sodium bicarbonate (NaHCO3) after the membrane. It raises alkalinity and brings pH into the normal drinking range. This is a different compound from softener salt and is made for use in food and drinks.
Why plants add calcium and magnesium back
Reverse osmosis removes minerals without choosing between them. The water is very pure, but it tastes flat and empty. Plants therefore add measured amounts of food-grade minerals back, a step called remineralisation:
- Taste: calcium and magnesium give the water body and a rounded mouthfeel.
- Stability: water with almost no minerals tends to dissolve the metal and concrete it touches. Adding calcium and alkalinity makes it gentler on tanks and pipes.
Plants do this with calcite contact filters, or by dosing solutions of calcium chloride (CaCl2), magnesium sulphate (MgSO4) and sodium bicarbonate (NaHCO3) with metering pumps.
How it all fits together in one plant
- Raw water: groundwater comes in with natural hardness (calcium and magnesium) and some sodium.
- Pre-filtration: sediment and carbon filters remove suspended particles, turbidity and chlorine.
- Softener: resin swaps calcium and magnesium for sodium to protect the membrane from scale. Softener salt regenerates it.
- RO membrane: removes most dissolved salts, including sodium.
- Remineralisation: calcium, magnesium and sodium bicarbonate are dosed to reach the target TDS, pH and taste.
- Disinfection and filling: UV and/or ozone treatment before bottling.
Softener or RO: they fix different problems
A softener only swaps calcium and magnesium for sodium. It stops scale, but it doesn't lower TDS, so salty water stays salty. RO removes most dissolved salts of every kind.
| Your water problem | Softener | RO |
|---|---|---|
| Scale on heaters, kettles and taps (hard water) | Fixes it | Fixes it |
| Salty or brackish taste, high TDS | No | Fixes it |
| Need to protect an RO membrane from scale | Often used in front of RO | Needs antiscalant or softening first |
Many plants in Pakistan run both: a softener first, then RO. Our guide on how an RO plant works goes through each stage, and the membrane and filters guide explains antiscalant.
Questions people ask
Does a water softener add sodium to drinking water?
A softener swaps calcium and magnesium for sodium, so softened water carries more sodium than the raw water. In an RO plant the softener sits before the membrane, and the membrane then removes most of that sodium along with other dissolved salts.
Why do RO plants add minerals back?
Reverse osmosis removes most dissolved minerals, which leaves water tasting flat and slightly corrosive to pipes and tanks. Adding measured amounts of food-grade calcium, magnesium and sodium bicarbonate restores taste and stabilises pH.
Is sodium added to drinking water on purpose?
Sodium chloride (softener salt) is only used to regenerate softener resin and is not added to finished water. Sodium bicarbonate is different: many plants dose a small, measured amount after RO to raise alkalinity and bring pH into the normal drinking range.
Is high TDS the same as hard water?
Not exactly. TDS measures everything dissolved in the water, including sodium, chloride and sulphate. Hardness measures only calcium and magnesium.
How do I test water hardness?
Use a hardness test kit (drops or strips) or get a lab test. A TDS meter can't measure hardness, because it reads every dissolved salt together, not just calcium and magnesium.
Does a water softener lower TDS?
No. It swaps calcium and magnesium for sodium, so the total dissolved solids stay about the same. To lower TDS you need reverse osmosis.
What to ask your mineral supplier
- Purity: what is the assay of each mineral, and can I see the certificate?
- Food grade: is it a food-grade product (for example E509 calcium chloride, E518 magnesium sulphate)?
- Traceability: does the certificate show heavy metal limits such as lead and arsenic?
Minerals by Mr. RO
Food-grade minerals for water plants across Pakistan, Rs 800 per kg, with certificates.
We make no health claims. For questions about minerals in your diet, please speak to a doctor.