Raw water can usually be softened with lime alone when it contains what?

Raw water can typically be softened effectively using lime alone when it contains little or no noncarbonate hardness. This is due to the way lime softening works.

Lime (calcium hydroxide) is primarily effective in removing calcium and magnesium ions from water, which are the main contributors to hardness. When the water has low noncarbonate hardness, this means there aren't significant amounts of dissolved hardness that would remain after lime is added. In such cases, the reaction with lime will precipitate out the majority of calcium and magnesium ions as insoluble compounds, thereby softening the water effectively.

In contrast, if raw water contains high noncarbonate hardness, it would require additional treatment methods or chemicals, such as sodium carbonate, to handle the remaining hardness after the initial lime reaction. This makes lime alone insufficient in those circumstances. High organic matter can interfere with the softening process, leading to other complications. Additionally, high suspended solids can impede the effectiveness of the softening treatment because they might clog up treatment processes or affect the precipitation of insoluble compounds.

In summary, low levels of noncarbonate hardness indicate that lime can efficiently reduce the hardness without the complications associated with other types of hardness or water quality issues.