Water Softening: Lime-Soda Treatment
An experimental study optimizing lime–soda ash dosing and pH control for efficient hardness removal and balanced water quality.
Course: ENV 4005L – Environmental Engineering Laboratory II
Lab Type: Group Laboratory
Role: Jar testing, chemical dosing, titration analysis, data interpretation
Focus: Diagnostic evaluation of hardness removal and alkalinity control
Overview
This laboratory experiment assessed the efficiency of the lime-soda ash softening as a chemical treatment process of removing calcium- and magnesium-based hardness in water. Instead of considering the experiment as a dosing exercise per se, the study was considered as a chemical treatment diagnostic, with jar tests being used to explain the effects of reagent dosage and pH on removal efficiency, alkalinity balance, and treatment stability.
This kind of diagnostics is necessary in the water treatment processes where chemical corrections should be informed by the observed system behavior and not just by the stoichiometry.
Objective
To determine lime (CaO) and soda ash (Na₂CO₃) dosing conditions that:
Maximize calcium and magnesium hardness removal
Maintain stable alkalinity for corrosion control
Avoid excessive pH elevation and chemical overuse
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 & Interpretation
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:
Jar testing revealed clear relationships between pH control, hardness removal, and alkalinity stability:
Optimal hardness removal occurred between pH 9.3–9.6, where both calcium carbonate and magnesium hydroxide precipitation were effective.
Excess lime dosing increased pH above 10, resulting in diminishing hardness removal efficiency and undesirable alkalinity reduction.
Balanced lime–soda ash dosing produced the most consistent removal while preserving treated-water stability.
Observed analytical anomalies (e.g., unrealistically low hardness values) were attributed to endpoint overshoot or incomplete settling, underscoring the importance of diagnostic interpretation rather than blind reliance on numerical outputs.
Engineering Significance
This experiment demonstrates how lime–soda softening functions as a diagnostic control process, not merely a chemical reaction. Effective treatment requires:
Monitoring pH as a controlling variable
Interpreting hardness trends rather than single-point measurements
Recognizing when results reflect analytical limitations instead of true system behavior
These diagnostic skills are directly transferable to municipal water treatment operations, where chemical dosing must be continuously adjusted to changing raw water conditions
Reflection
This lab reinforced the role of engineering judgment in chemical treatment systems. Rather than maximizing removal at all costs, the results highlighted the importance of balancing effectiveness, stability, and chemical efficiency. Interpreting imperfect data and identifying realistic operating windows mirrored real-world treatment decision-making more closely than idealized calculations.
Project Gallery
