Notes on Chemical Precipitation Softening Processes

Although the basic processes using lime (calcium hydroxide) and soda ash (sodium carbonate) to soften water by precipitating insoluble hardness salts have been known for 200 years, they are still relevant today. Many large industrial cooling systems around the world use lime-soda-softened makeup water, especially in some newly industrializing countries, where local water supplies may be particularly hard and the economics favor this process rather than, say, ion exchange. 

Both hot and cold processes are employed, although the hot process, which takes place at or above 212°F (100°C) is usually reserved for boiler feed-water applications, as it produces water of lower hardness levels and usually a lower silica content as well. Also, less lime is needed, as the carbon dioxide it would normally react with is driven off at the higher temperatures. Sometimes caustic soda (sodium hydroxide) is used in place of soda, depending on the alkalinity of the water and the chemical costs however, irrespective of the process or chemicals used, the major precipi-tants are always calcium carbonate and magnesium hydroxide. 

The steps involved in converting soluble hardness salts in raw water to insoluble precipitants that can be removed by filtration or clarification are  

Chemical analysis of raw water and calculation of amounts of lime and soda needed. (Analysis is usually carried out at least daily on both raw and treated water to confirm expected results and to make adjustments due to changing raw water quality.) 

Addition of measured amounts of chemicals, either in batches or continuously followed by a period of rapid mixing followed by flocculation. 

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