Precipitation softening

Makeup. Makeup treatment depends extensively on the source water. Some steam systems use municipal water as a source. These systems may require dechlorination followed by reverse osmosis (qv) and ion exchange. Other systems use weUwater. In hard water areas, these systems include softening before further purification. Surface waters may require removal of suspended soHds by sedimentation (qv), coagulation, flocculation, and filtration. Calcium may be reduced by precipitation softening or lime softening. Organic contaminants can be removed by absorption on activated carbon. Details of makeup water treatment may be found in many handbooks (22—24) as well as in technical Hterature from water treatment chemical suppHers. 

Precipitation softening processes are used to reduce raw water hardness, alkalinity, siHca, and other constituents. This helps prepare water for direct use as cooling tower makeup or as a first-stage treatment followed by ion exchange for boiler makeup or process use. The water is treated with lime or a combination of lime and soda ash (carbonate ion). These chemicals react with the hardness and natural alkalinity in the water to form insoluble compounds. The compounds precipitate and are removed from the water by sedimentation and, usually, filtration. Waters with moderate to high hardness and alkalinity concentrations (150—500 ppm as CaCO ) are often treated in this fashion. 

Gold Lime Softening. Precipitation softening accompHshed at ambient temperatures is referred to as cold lime softening. When hydrated lime, Ca(OH)2, is added to the water being treated, the following reactions occur ... 

Filtration is used in addition to regular coagulation and sedimentation or precipitation softening for removal of soHds from surface water or waste water. This prepares the water for use as potable, boiler, or cooling makeup. Waste water filtration helps users meet more stringent effluent discharge permit requirements. 

Filtration does not remove dissolved soHds, but may be used together with a softening process, which does reduce the concentration of dissolved sohds. For example, anthracite filtration is used to remove residual precipitated hardness salts remaining after precipitation softening. 

Precipitation Softening. This process depends on sufficient holdup time within a vessel to allow sedimentation and clarification to occur. A coagulation chemical such as alum or iron salts added to the solution will improve the process efficiency. 

Supply of MU water for a medium-pressure (450 psig) WT boiler, from a surface water source with very variable suspended solids and hardness (sugar refinery, South Africa). The process used is a. carbonate removal using hot-lime precipitation softening coupled with silica adsorption by magnesia addition b. clarification in anthracite filters and c. cation ion-exchange for the balance of hardness removal. 

Traditional types of gravity and pressure media bed filters (such as sand and anthracite filters) are widely used to remove suspended solids and improve the clarity of raw water. They are also used following aeration or oxidation and chemical precipitation softening. 

Kedem, O. and G. Zalmon (1997). Compact accelerated precipitation softening (CAPS) as a pretreatment for membrane desalination I. softening by NaOH, Desalination, 113, 1, 65-71. 

U.S. Environmental Protection Agency (1999). Enhanced Coagulation and Enhanced Precipitative Softening Guidance Manual, EPA 815-R-99-012, Washington, DC. 

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