The Langelier Saturation Index (LSI) is a calculated value that predicts calcium-carbonate scaling tendency in water. LSI > 0 means CaCO₃ will precipitate (scale-forming); LSI < 0 means water is undersaturated (potentially corrosive). LSI = pH − pH_s, where pH_s depends on TDS, Ca²⁺, alkalinity, and temperature. Software (Genesys, ROSA) computes LSI in the RO concentrate stream — the highest-risk point in the system. LSI determines whether the RO concentrate will precipitate CaCO₃ on the tail elements. LSI > 0 in the concentrate is the trigger for antiscalant dosing or recovery reduction. Without antiscalant, keep LSI < 0. With antiscalant, LSI up to +2.5 is typically safe per vendor projections. LSI is calibrated for low-TDS water. The Stiff & Davis Saturation Index (S&DSI) extends the LSI calculation to high-ionic-strength brackish and seawater brines. Lower pH (acid dosing), lower recovery, or add antiscalant. Antiscalant is by far the cheapest of the three. ForeverPure has supplied desalination, high-pressure pumps, and energy-recovery devices to commercial and industrial customers since 2003. Contact our engineers for sizing, quotes, or technical support.LSI (Langelier Saturation Index) (LSI)
How It Works
Why It Matters
Related Products & Resources
Frequently Asked Questions
What LSI is safe for RO concentrate?
How is LSI different from S&DSI?
How do I lower LSI?
Need Engineering Help?
