2.10 Lime-Soda Softening, Ion Exchange & Water Stabilization
Key Takeaways
- Carbonate hardness is removed by lime alone, while noncarbonate hardness additionally requires soda ash, which is why the split between the two governs chemical cost.
- Excess lime softening raises pH to about 11 to remove magnesium hardness, and the treated water must then be recarbonated with carbon dioxide to stabilize it.
- Practical softening limits are about 30 to 40 mg/L of calcium carbonate and 10 mg/L of magnesium hardness, so finished water is normally blended back to 80 to 120 mg/L.
- A sodium cation exchange softener exchanges sodium for calcium and magnesium, is regenerated with brine, and adds sodium to the finished water without changing total dissolved solids much.
- The Langelier Saturation Index compares actual pH to the pH of calcium carbonate saturation, so a positive value indicates scaling tendency and a negative value indicates corrosive water.
Hardness and why it is treated
Hardness is the sum of polyvalent metallic cations in water, dominated in practice by calcium and magnesium, and expressed as milligrams per liter as calcium carbonate (CaCO3). It is picked up as groundwater passes through limestone, dolomite, and gypsum, which is why many Colorado groundwater systems are hard while mountain surface supplies are soft.
Hardness is a nuisance rather than a health parameter: it consumes soap, forms scale in water heaters and boilers, and shortens appliance life. General classification:
| Hardness as CaCO3 | Description |
|---|---|
| 0 - 75 mg/L | Soft |
| 75 - 150 mg/L | Moderately hard |
| 150 - 300 mg/L | Hard |
| Over 300 mg/L | Very hard |
The critical operational split is by anion association:
- Carbonate hardness (temporary hardness) — calcium and magnesium associated with bicarbonate and carbonate. Removable by lime alone.
- Noncarbonate hardness (permanent hardness) — calcium and magnesium associated with sulfate, chloride, and nitrate. Requires soda ash (sodium carbonate) in addition to lime.
The working rule: if total hardness exceeds total alkalinity, the difference is noncarbonate hardness. If alkalinity equals or exceeds total hardness, all hardness is carbonate hardness. Because soda ash is the more expensive chemical, this single comparison drives the chemical budget.
The lime-soda ash reactions
Softening works by converting soluble calcium and magnesium into insoluble precipitates that settle out:
- Carbon dioxide is neutralized first and consumes lime without removing hardness: CO2 + Ca(OH)2 → CaCO3↓ + H2O
- Calcium carbonate hardness: Ca(HCO3)2 + Ca(OH)2 → 2 CaCO3↓ + 2 H2O
- Magnesium carbonate hardness requires excess lime: Mg(HCO3)2 + 2 Ca(OH)2 → 2 CaCO3↓ + Mg(OH)2↓ + 2 H2O
- Noncarbonate calcium hardness: CaSO4 + Na2CO3 → CaCO3↓ + Na2SO4
- Noncarbonate magnesium hardness needs both: lime converts it to noncarbonate calcium hardness, then soda ash precipitates the calcium.
Two pH thresholds anchor the whole subject:
- Calcium carbonate precipitates efficiently around pH 10.3.
- Magnesium hydroxide requires roughly pH 11.0 or above, which is why removing magnesium demands excess lime beyond the stoichiometric requirement.
Process variants
| Variant | What it does | When used |
|---|---|---|
| Straight (conventional) lime | Lime only; removes calcium carbonate hardness | Alkalinity ≥ hardness, low magnesium |
| Excess lime | Lime beyond stoichiometry to reach pH ~11 | Magnesium removal required |
| Lime-soda ash | Lime plus soda ash | Noncarbonate hardness present |
| Excess lime-soda ash | Both, at pH ~11 | High magnesium and noncarbonate hardness |
| Split treatment | Softens part of the flow at high pH, blends with bypassed raw water | Reduces chemical cost and recarbonation demand |
Practical limits. Softening cannot drive hardness to zero. The practical floor is about 30 to 40 mg/L calcium carbonate hardness and about 10 mg/L magnesium hardness. Finished water is deliberately blended back to roughly 80 to 120 mg/L total hardness, because water that is too soft is aggressive and corrosive.
Recarbonation
Softened water leaves the basin at high pH, supersaturated with calcium carbonate. Delivered as-is it would deposit scale throughout the distribution system and in customer plumbing, and would fail secondary standards for pH. Recarbonation adds carbon dioxide to lower pH and convert remaining carbonate to soluble bicarbonate.
- Single-stage recarbonation follows the softening basin and drops pH to roughly 8.5 to 9.5.
- Two-stage recarbonation is used with excess lime: the first stage drops pH from about 11 to about 10.3 to precipitate the excess lime as calcium carbonate in a second settling basin, and the second stage drops pH to the stable finished-water range.
Carbon dioxide is generated on site by submerged combustion burners or supplied as liquid CO2. Under-recarbonation leaves scale-forming water that cements filter media; over-recarbonation redissolves the calcium carbonate and returns hardness to the finished water.
Ion exchange
Ion exchange replaces unwanted ions in water with acceptable ones held on a synthetic resin. Regulation 100 lists cation or anion exchange technology as its own treatment type, classified C, C, and B across the three flow ranges.
Sodium cation exchange (zeolite softening) is the classic municipal and household softener. Resin beads charged with sodium exchange two sodium ions for each calcium or magnesium ion:
2 Na-R + Ca²⁺ → Ca-R2 + 2 Na⁺
Characteristics an operator must know:
- It removes hardness to near zero, far below what lime softening achieves, so treated water is normally blended with bypassed raw water to hit the target hardness.
- It adds sodium to the finished water — roughly 8 mg/L of sodium for each 1 grain per gallon (17.1 mg/L as CaCO3) of hardness removed. This matters for customers on sodium-restricted diets.
- It does not remove alkalinity or reduce total dissolved solids appreciably; it substitutes one cation for another.
- Regeneration uses a concentrated sodium chloride brine, typically 8 to 12 percent, and the cycle is: backwash → brine draw and slow rinse → fast rinse → return to service.
- Capacity is expressed in grains of hardness per cubic foot of resin, and exchange capacity falls as salt dose falls.
- The brine waste stream is high in chloride and total dissolved solids, which is a genuine disposal constraint in Colorado where discharge to a wastewater plant can jeopardize the plant's own chloride limits.
- Iron, manganese, and suspended solids foul the resin; oxidized iron in particular coats the beads and must be prevented by pretreatment.
Anion exchange on a strong-base resin regenerated with sodium chloride is the standard treatment for nitrate and is also used for arsenic, uranium, perchlorate, and increasingly PFAS. The key operational hazard is chromatographic peaking: when a nitrate-selective column is run past exhaustion, sulfate displaces previously captured nitrate and the effluent can carry more nitrate than the influent. Running past breakthrough on a nitrate unit is therefore an acute public health risk, not merely a treatment inefficiency.
Stabilization and the saturation indices
Whether water scales or corrodes is a balance, and operators use indices to describe it.
The Langelier Saturation Index (LSI) is the actual pH minus pHs, the pH at which the water would be exactly saturated with calcium carbonate at its measured calcium, alkalinity, total dissolved solids, and temperature:
LSI = pH − pHs
| LSI | Interpretation |
|---|---|
| Positive (> 0) | Supersaturated; tends to deposit calcium carbonate scale |
| Zero | At equilibrium; neither scaling nor dissolving |
| Negative (< 0) | Undersaturated; aggressive and corrosive, dissolves protective scale |
Related tools include the Ryznar Stability Index, computed as 2(pHs) − pH, where values above about 7 indicate corrosive tendency and below about 6 indicate scaling; the Aggressive Index, used for asbestos-cement pipe; and calcium carbonate precipitation potential (CCPP), which expresses in milligrams per liter how much calcium carbonate the water can actually deposit and is generally more useful than LSI for setting a corrosion control target.
The practical takeaway for a treatment operator: a softening plant that fails to recarbonate delivers strongly positive-LSI water that cements filter media and scales mains, while a plant that over-recarbonates or delivers very soft water delivers negative-LSI water that strips protective scale and mobilizes lead and copper. Softening and corrosion control are the same problem viewed from two ends.
A groundwater supply has a total hardness of 340 mg/L as calcium carbonate and a total alkalinity of 215 mg/L as calcium carbonate. What does this tell the operator about chemical requirements?
Why is excess lime required when magnesium hardness must be removed?
A nitrate removal anion exchange column is operated past its breakthrough point. What is the specific hazard?