Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
D3739-19 – Standard Test Method Technical Guide
ASTM D3739-19 defines the standard practice for calculating and adjusting the Langelier Saturation Index (LSI) specifically for the concentrate stream of reverse osmosis (RO) devices. Accurately determining this index is critical for predicting the need for calcium carbonate (CaCO₃) scale control in RO system design and long-term operation.
📐 Scope and Applicability
This practice is strictly applicable to RO concentrate streams with a total dissolved solids (TDS) concentration between 10 and 10,000 mg/L. For streams exceeding 10,000 mg/L TDS, engineers must refer to Practice D4582 (Stiff and Davis Stability Index). The LSI calculation helps operators determine if CaCO₃ scaling presents a risk to membrane performance and flux sustainability.
⚠️ Regulatory Compliance: The standard emphasizes that it does not address all safety concerns (Section 1.3). Users must establish appropriate health, safety, and environmental practices and determine the applicability of regulatory limitations before use. All values stated in SI units are to be regarded as standard.
⚙️ Calculation and Adjustment of LSI
Section 4 outlines the calculation methodology for the LSI using the feedwater’s TDS, calcium ion concentration, total alkalinity, pH, and temperature, combined with the system’s recovery rate. The resulting index value indicates the saturation state of the water with respect to CaCO₃.
When the LSI predicts a scaling condition, Section 4.2 provides three distinct adjustment techniques to bring the concentrate water chemistry into a less scaling range:
- Decrease Recovery: Lowering the conversion of feed to permeate reduces the concentration factor of scaling minerals in the concentrate stream.
- Reduce Calcium Content: Pretreatment of the feedwater to remove calcium ions (e.g., via softening or anti-scalant dosing) directly tackles the scaling species.
- Adjust Alkalinity/CO₂ Ratio: Modifying the water’s acid-base balance alters the solubility equilibrium of calcium carbonate.
📊 Key Parameters and Referenced Test Methods
Accurate measurements are the foundation of a reliable LSI calculation. ASTM D3739-19 mandates specific standard test methods for the input parameters to ensure consistency and repeatability.
| 🟦 Parameter | 📏 Applicable Range / Unit | 📐 Designated Standard |
|---|---|---|
| Total Dissolved Solids | 10 to 10,000 mg/L | D1888 |
| Calcium Content | Measured as mg/L Ca | D511 |
| Total Alkalinity | Measured as mg/L CaCO₃ | D1067 |
| pH | Standard pH Units | D1293 |
The following table summarizes the LSI adjustment strategies derived directly from the standard’s guidance.
| 🎯 Strategy | ⚡ Mechanism of Action | 📌 Practical Consideration |
|---|---|---|
| Decrease Recovery | Lowers concentration factor of scaling ions | Reduces net permeate production per element |
| Decrease Calcium | Removes the primary scaling precursor ion | Typically requires ion exchange or lime softening pretreatment |
| Adjust Alkalinity/CO₂ | Alters the CaCO₃ solubility equilibrium | Commonly achieved by acid (e.g., H₂SO₄) or caustic injection |
✅ Integrated System Approach: For comprehensive performance validation, D3739-19 should be used alongside Guide D4195 (Water Analysis for RO) and Test Methods D4194 (Operating Characteristics of RO/NF Devices) to correlate calculated scaling indices with actual system behavior.
❓ Frequently Asked Questions
🔍 What does the Langelier Saturation Index indicate?
Per Section 3.2.1, the LSI is a calculated index showing the tendency of a water solution to either precipitate or dissolve calcium carbonate. A positive LSI suggests scaling potential, while a negative index suggests a corrosive or dissolving tendency.