Lime‐soda ash softening is used in most larger treatment plants to remove dissolved minerals, especially calcium and magnesium, which cause hardness in water. When lime and soda‐ash are added, the hardness‐causing minerals form nearly insoluble precipitates. These precipitates are removed by the conventional processes of coagulation/flocculation, sedimentation, and filtration. The following types of lime‐soda ash processes are discussed: conventional lime‐soda ash treatment; excess‐lime treatment; split treatment; and caustic soda treatment. This article is adapted from the AWWA publication, Introduction to Water Treatment, Module 5, Softening.