limesoftening

Lime softening, also known as lime-soda softening, is a water treatment process used to reduce hardness in drinking water by removing calcium and magnesium ions as insoluble compounds. The primary chemical is calcium hydroxide (lime), and some plants also use sodium carbonate (soda ash) to help balance alkalinity. It is employed in municipal and industrial facilities to prevent scale, improve downstream treatment, and enhance coagulation.

How it works: Lime is dosed into the raw water and mixed to promote reactions. As pH rises to typically around 9–11, carbonate species form from the water’s alkalinity and react with calcium and magnesium to precipitate as calcium carbonate and magnesium hydroxide. The precipitates are removed in a downstream clarifier, with the clarified water often undergoing filtration and a final pH adjustment. The process also consumes alkalinity and can reduce dissolved inorganic alkalinity, requiring careful dose control.

Design and operation: Lime softening requires precise dosing, mixing, and detention time to balance effective hardness removal with sludge production. A substantial amount of sludge—largely calcium carbonate and magnesium hydroxide—is produced and must be settled and disposed of or dewatered. Some plants supplement lime with soda ash to stabilize pH and maintain adequate carbonate levels. After softening, water may be further treated or conditioned for distribution.

Applications and limitations: The process effectively reduces calcium and magnesium concentrations, lowers hardness, and can improve downstream coagulation. Limitations include high chemical consumption, significant sludge handling needs, sensitivity to water composition and temperature, and the need for careful pH and alkalinity management to avoid residual lime in finished water.

See also: Water treatment, Hardness removal, Lime.