3.3 Lime-Soda Ash Softening, Iron & Manganese Removal

Lime-Soda Ash Softening, Iron & Manganese Removal

Groundwater across Illinois—from sand-and-gravel formations such as the Mahomet Aquifer to deep sandstone strata—frequently contains high levels of dissolved hardness, iron, and manganese. Left untreated, these minerals cause scaling in boilers and distribution piping, stain plumbing fixtures, and produce colored water complaints. Water utilities employ chemical precipitation softening and oxidation-filtration to eliminate these inorganics.

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1. Water Hardness Chemistry: Carbonate vs. Noncarbonate

Water hardness represents polyvalent metallic cations, predominantly calcium (Ca2+) and magnesium (Mg2+), expressed as calcium carbonate equivalent (mg/L as CaCO3): $\text{Total Hardness (TH)} = 2.50 \times [\text{Ca}^{2+}, \text{mg/L}] + 4.12 \times [\text{Mg}^{2+}, \text{mg/L}]$

Hardness is divided into two operational categories:

Carbonate Hardness (CH)

• Hardness chemically equivalent to bicarbonate (HCO3-) and carbonate (CO3 2-) alkalinity.
• Known as "temporary hardness" because heating drives off carbon dioxide, precipitating calcium carbonate scale: $\text{Ca(HCO}_3)_2 + \text{Heat} \rightarrow \text{CaCO}_3\downarrow + \text{CO}_2\uparrow + \text{H}_2\text{O}$
• Rule: If $\text{Total Hardness} > \text{Total Alkalinity}$, then $\text{Carbonate Hardness} = \text{Total Alkalinity}$.

Noncarbonate Hardness (NCH)

• Hardness associated with sulfate (SO4 2-), chloride (Cl-), and nitrate (NO3-) anions.
• Known as "permanent hardness" because boiling does not precipitate it; removal requires soda ash (Na2CO3) or ion exchange.
• Rule: $\text{Noncarbonate Hardness} = \text{Total Hardness} - \text{Total Alkalinity}$. (If $\text{Total Hardness} \le \text{Total Alkalinity}$, all hardness is carbonate hardness, and $\text{NCH} = 0$).

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2. Stoichiometry of Lime-Soda Ash Softening

Chemical softening uses lime to raise pH, precipitating calcium as calcium carbonate (CaCO3↓) and magnesium as magnesium hydroxide (Mg(OH)2↓).

Chemical Feed Forms

• Quicklime (Calcium Oxide, CaO): Slaked with hot water (160°F to 180°F) in an exothermic reaction to produce hydrated lime slurry: $\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{Heat} \quad (\Delta H = -65.3 \text{ kJ/mol})$
• Hydrated Lime (Calcium Hydroxide, Ca(OH)2): Dry powder slurried directly into solution feeders.
• Soda Ash (Sodium Carbonate, Na2CO3): Dry chemical used to precipitate noncarbonate hardness.

The Five Softening Reactions

1. Carbon Dioxide Neutralization: $\text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3\downarrow + \text{H}_2\text{O}$ Dissolved CO2 consumes lime without removing hardness: 1 mole of CO2 consumes 1 mole of Ca(OH)2.
2. Calcium Carbonate Hardness Removal: $\text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \rightarrow 2 \text{ CaCO}_3\downarrow + 2 \text{ H}_2\text{O}$ Occurs efficiently at pH 9.0 to 9.4 (optimal ~9.3). For each mole of calcium bicarbonate removed, 2 moles of CaCO3 precipitate.
3. Magnesium Carbonate Hardness Removal: Magnesium carbonate is soluble (~70 mg/L) and will not precipitate at pH 9.3. Excess lime must be added to raise the pH to ≥ 10.6, precipitating magnesium hydroxide: $\text{Mg(HCO}_3)_2 + 2 \text{ Ca(OH)}_2 \rightarrow \text{Mg(OH)}_2\downarrow + 2 \text{ CaCO}_3\downarrow + 2 \text{ H}_2\text{O}$ Magnesium carbonate hardness requires twice the molar lime dose of calcium carbonate hardness.
4. Calcium Noncarbonate Hardness Removal (Soda Ash): $\text{CaSO}_4 + \text{Na}_2\text{CO}_3 \rightarrow \text{CaCO}_3\downarrow + \text{Na}_2\text{SO}_4$ Soda ash supplies carbonate ions (CO3 2-) to precipitate calcium as CaCO3↓, leaving soluble sodium sulfate.
5. Magnesium Noncarbonate Hardness Removal (Lime + Soda Ash): $\text{MgSO}_4 + \text{Ca(OH)}_2 + \text{Na}_2\text{CO}_3 \rightarrow \text{Mg(OH)}_2\downarrow + \text{CaCO}_3\downarrow + \text{Na}_2\text{SO}_4$ Lime precipitates magnesium at pH ≥ 10.6, and soda ash precipitates the resulting calcium sulfate.

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3. Recarbonation & Finished Water Stabilization

Water leaving lime softening clarifiers has a high pH (10.6 to 11.2) and is supersaturated with CaCO3 and Mg(OH)2. If sent to filters, mineral scale cements media grains into rock ("filter growth") and clogs distribution pipes.

Recarbonation Chemistry

Carbon dioxide gas (CO2) is bubbled through recarbonation basins to neutralize hydroxide and convert carbonates into soluble bicarbonates: $\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3\downarrow + \text{H}_2\text{O}$ $\text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(HCO}_3)_2$

• Single-Stage Recarbonation: Water is dosed with CO2 in a single post-softening basin to drop pH to 8.6–8.8 prior to filtration. This precipitates fine CaCO3 floc that can load filters.
• Two-Stage Recarbonation:
  • Stage 1: Located between primary softening and secondary settling. CO2 lowers pH to ~9.5, precipitating unreacted lime as CaCO3 floc that settles out in secondary basins.
  • Stage 2: Located immediately prior to filtration. CO2 drops pH to 8.3–8.8, converting all remaining carbonate ions to soluble bicarbonates to prevent precipitation on filter media.

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4. Iron & Manganese: Chemistry & Secondary Standards

In anaerobic groundwater aquifers, iron and manganese exist in dissolved, colorless, divalent reduced states: ferrous iron (Fe2+) and manganous manganese (Mn2+).

Exposure to air or chlorine oxidizes them into insoluble colored precipitates: ferric hydroxide (Fe(OH)3↓, reddish-brown) and manganese dioxide (MnO2↓, brownish-black).

• Iron: Secondary MCL = 0.3 mg/L. Causes red-brown water, laundry/porcelain staining, and metallic tastes.
• Manganese: Secondary MCL = 0.05 mg/L. Causes black water, dark staining, and bitter taste (complaints occur at >0.02 mg/L).
• Iron Bacteria: Organisms (Gallionella, Sphaerotilus, Crenothrix) feed on ferrous iron to produce voluminous slime sheaths that clog well screens, distribution mains, and customer meters.

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5. Treatment Mechanisms: Aeration, Chemical Oxidation & Greensand

Iron and manganese are removed through oxidation into insoluble precipitates followed by filtration:

1. Aeration

Aeration towers introduce atmospheric oxygen to oxidize ferrous iron: $4 \text{ Fe}^{2+} + \text{O}_2 + 10 \text{ H}_2\text{O} \rightarrow 4 \text{ Fe(OH)}_3\downarrow + 8 \text{ H}^+$

• Stoichiometry requires 0.14 mg/L of O2 per 1.0 mg/L of Fe2+. Stripping dissolved CO2 raises pH, accelerating iron oxidation (effective at pH ≥ 7.5).
• Limitation: Aeration oxidizes manganese too slowly to be practical; aeration alone cannot remove manganese unless pH exceeds 9.5.

2. Chemical Oxidation

• Chlorine (Cl2): Rapidly oxidizes iron (1.0 mg Fe consumes 0.64 mg Cl2). Manganese oxidation by chlorine requires free residuals, pH ≥ 8.0, and 1 to 2 hours of contact time.
• Potassium Permanganate (KMnO4): Powerful oxidant effective across pH 6.2 to 8.5: $3 \text{ Fe}^{2+} + \text{KMnO}_4 + 7 \text{ H}_2\text{O} \rightarrow 3 \text{ Fe(OH)}_3\downarrow + \text{MnO}_2\downarrow + \text{K}^+ + 5 \text{ H}^+ \quad (0.94 \text{ mg } \text{KMnO}_4 / \text{mg Fe})$ $3 \text{ Mn}^{2+} + 2 \text{ KMnO}_4 + 2 \text{ H}_2\text{O} \rightarrow 5 \text{ MnO}_2\downarrow + 2 \text{ K}^+ + 4 \text{ H}^+ \quad (1.92 \text{ mg } \text{KMnO}_4 / \text{mg Mn})$ Underdosing leaves soluble metals; overdosing permits pink/purple unreacted permanganate into the distribution system.

3. Manganese Greensand Filtration

Glauconite sand coated with manganese oxide (MnO2) acts as a catalytic contact medium:

• Continuous Regeneration (CR): KMnO4 is continuously fed ahead of the greensand filter. Precipitates are captured in the bed while the greensand catalytic coating buffers chemical feed fluctuations. CR is the standard municipal design in Illinois.
• Intermittent Regeneration (IR): Water passes through greensand until the coating's oxidative capacity is exhausted. The bed is backwashed, regenerated with concentrated KMnO4 (1.5 oz/cu ft), soaked for 1 hour, and rinsed to waste before returning to service.