laminar flow- The flow of fluid in which the flow paths are in smooth, parallel lines, with essentially no mixing and no turbulence.

Langelier's Index - A calculated number used to predict whether or not a water will precipitate, be in equilibrium with, or dissolve calcium carbonate. It is sometimes erroneously assumed that any water which tends to dissolve calcium carbonate is automatically corrosive.

LAS - Abbreviation for "Linear Alkyl Sulfonate".

leakage- The presence of a consistent concentration of ions in the effluent of an ion exchange system due to incomplete removal of the ions; caused by incomplete regeneration, excessive flow rates, low temperatures, the concentration or characteristics of the influent ions, or other factors. (See hardness leakage.)

lime- The common name for calcium oxide (CAO); hydrated lime is calcium hydroxide, Ca(OH)₂.

lime scale- Hard water scale containing a high percentage of calcium carbonate.

limestone - A sedimentary rock, largely calcium carbonate, and usually also containing significant amounts of magnesium carbonate.

linear alkyl sulfonate - A term applied to a family of straight chain chemical compounds, widely used as detergents; sometimes called "soft" detergents because they are more readily degraded to simpler substances by biological action than the previously used alkyl benzene sulfonate. (See detergent, alkyl benzene sulfonate.)