A thermodynamic index that predicts whether water will deposit or dissolve calcium carbonate, calculated from the difference between the actual pH and the saturation pH for calcium carbonate equilibrium.
The Langelier Saturation Index, or LSI, predicts whether water will deposit or dissolve calcium carbonate. Wilfred F. Langelier introduced the index in a 1936 paper in the Journal of the American Water Works Association, and it remains the most widely used scaling indicator in reverse osmosis system design ninety years later. The calculation is straightforward: subtract the saturation pH from the actual pH of the water. The saturation pH is the value at which the water would sit in equilibrium with solid calcium carbonate at a given temperature, calcium concentration, alkalinity, and total dissolved solids.
A positive LSI value indicates supersaturation with respect to calcium carbonate and a tendency to form scale on membrane surfaces. A negative value indicates the reverse, with undersaturated water that has corrosive potential toward metallic components. Zero is equilibrium. In RO applications, calculate the LSI for the concentrate stream, not the feed water. By the time water reaches the tail end of the array, ion concentrations are several times higher than at the inlet, and that is where scaling actually occurs.
Modern antiscalants enable stable operation at LSI values well above zero, often +1.8 to +2.5 with advanced phosphonate and polymer chemistries, provided dose rates are calculated correctly for the concentrate conditions, typically through specialized water chemistry modeling tools. The index predicts thermodynamic tendency, not the rate at which scale will form. For high-TDS feeds above approximately 10,000 mg/L, the Stiff and Davis Stability Index is generally preferred because it accounts for ionic strength effects that the original Langelier formulation does not address.
