Water Softening: Lime-Soda Treatment
An experimental study optimizing lime–soda ash dosing and pH control for efficient hardness removal and balanced water quality.
Overview
This experiment explored the effectiveness of lime-soda ash softening for reducing water hardness caused by calcium and magnesium ions. Through controlled jar tests with varied reagent dosages, the study investigated how chemical dosing and pH influence hardness removal efficiency, alkalinity balance, and final water quality.
Objective
To determine the optimal lime (CaO) and soda ash (Na2CO3) dosages that achieve maximum hardness removal while maintaining desirable alkalinity and pH in treated water.
Methodology
Six 1-L jar test samples were prepared using a Phipps & Bird apparatus. Lime and soda ash were dosed in varying ratios and mixed at sequential speeds (165, 100, and 30 RPM). After settling, each sample was filtered through a mixed-cellulose membrane and analyzed for:
Total, calcium, and magnesium hardness
P- and T-alkalinity
pH
All titrations followed standard methods using EDTA and H2SO4, with concentrations expressed in mg/L as CaCO3.
Ca²⁺ + CO₃²⁻ → CaCO₃↓
Mg²⁺ + 2OH⁻ → Mg(OH)₂↓
Results
Parameter | Raw Water | Jar 1 | Jar 2 | Jar 3 | Jar 4 | Jar 5 |
pH | 7.46 | 9.12 | 9.52 | 9.55 | 10.41 | 10.48 |
Total Hardness (mg/L as CaCO₃) | 178 | 64 | 78 | 64 | 62 | 76 |
Calcium Hardness (mg/L) | 144 | 54 | 28 | 30 | 35 | 66 |
Magnesium Hardness (mg/L) | 34 | 10 | 50 | 34 | 27 | 10 |
Total Alkalinity (mg/L) | 130 | 66 | 57 | 61 | 62 | 41 |
Observations:
Hardness decreased significantly with increasing lime/soda dosage, reaching optimal removal between pH 9.3 – 9.6.
Excessive lime caused over-softening and pH drift above 10, reducing efficiency and altering alkalinity ratios.
The balanced combination of lime and soda ash produced the most stable and effective hardness removal.
Discussion
The experiment effectively modeled the lime-soda softening process used in municipal treatment facilities. Results highlight how precise dosing control ensures:
Sufficient removal of calcium and magnesium
Stabilization of alkalinity for corrosion control
Avoidance of excessive reagent use and sludge formation
Minor analytical inconsistencies (e.g., TH = 0 mg/L in Jar 3) were attributed to endpoint overshoot or incomplete settling, not true zero hardness.
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