Chemical precipitation softening processes

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. 

In a related study, Khedr (2013) assessed the removal of radium, U, as uranyl cation, or carbonate complexes, and radon by reverse osmosis (RO) and nanofiltration (NF) in comparison with the conventional methods of ion exchange resins (lERs), chemical precipitation/softening, coagulation, and adsorption on surface active media. lERs and chemical softening achieved radionuclide rejection from 32 to 95%, but with loss of process efficiency due to undesired... 

Sodium alumiaate is used ia the treatment of iadustrial and municipal water suppHes and the use of sodium alumiaate is approved ia the clarification of drinking water. The FDA approves the use of sodium alumiaate ia steam generation systems where the steam contacts food. One early use of sodium alumiaate was ia lime softening processes, where it iacreases the precipitation of ions contributing to hardness and improves suspended soHds removal from the treated water (17). Sodium alumiaate reacts with siHca to leave very low residual concentrations of siHca ia hot process water softeners. Sodium alumiaate is often used with other chemicals such as alum, ferric salts, clays, and polyelectrolytes, as a coagulant aid (18,19). 

Chemical precipitation Precipitation induced by addition of chemicals the process of softening water by the addition of lime and soda ash as the precipitants. Chloramines Compounds formed by the reaction of hypochlorous acid (or aqueous chlorine) with ammonia. 

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. 

Part III covers the various unit processes employed in water and wastewater treatment including water softening water stabilization coagulation removal of iron and manganese by chemical precipitation removal of nitrogen by nitrihcation-denitrihcation removal of phosphorus by chemical precipitation ion exchange and disinfection. 

CHEMICAL PRECIPITATION - When the chemicals react with the dissolved minerals in the water to produce a relative insoluble reaction product. A typical example of this takes place with the lime-soda softening process. 

Chemistry. Chemical processes are used to treat water and wastewater, to control air pollution, and for site remediation. These chemical treatments include chlorination for disinfection of both water and wastewater, chemical oxidation for iron and manganese removal in water-treatment plants, chemical oxidation for odor control, chemical precipitation for removal of metals or phosphorus from wastewater, water softening by the lime-soda process, and chemical neutralization for pH (acidity) control and for scaling control. 

While the ambient-temperature operation of membrane processes reduces scaling, membranes are much more susceptible not only to minute amounts of scaling or even dirt, but also to the presence of certain salts and other compounds that reduce their ability to separate salt from water. To reduce corrosion, scaling, and other problems, the water to be desalted is pretreated. The pretreatment consists of filtration, and may include removal of air (deaeration), removal of CO2 (decarbonation), and selective removal of scale-forming salts (softening). It also includes the addition of chemicals that allow operation without scale deposition, or which retard scale deposition or cause the precipitation of scale which does not adhere to soHd surfaces, and that prevent foam formation during the desalination process. 

Precipitation and dissolution phenomena are extremely important in both natural waters and water treatment processes. Dissolution of minerals is a prime factor in determining the chemical composition of natural waters. Natural water chemical composition can be altered by precipitation of minerals and the subsequent sedimentation of these solids from supersaturated solutions. Water and wastewater treatment processes such as lime-soda softening, iron removal, coagulation with hydrolyzing metal salts, and phosphate precipitation are based on precipitation phenomena. 

For surface water, treatment is aimed primarily at removal of turbidity (fine suspended matter) and perhaps softening the water. The typical treatment processes for removal of turbidity involve the addition of chemicals such as alum or ferric chloride. The chemicals are rapidly mixed into the water so that they react with alkalinity in the water, then slowly mixed (flocculation) to form a settleable precipitate. After sedimentation, the water passes through a sand filter and finally is disinfected wdth chlorine. If the water is to be softened as part of the treatment, lime, Ca(OH)2, and soda ash, Na COj, are used in place of alum or ferric chloride, and the water hardness (calcium and magnesium ions) is removed along with its turbidity. 

It has been suggested that this property may obviate the need to use softened water in chemical EOR processes based on ASP flooding in high-water-hardness carbonate reservoirs [19]. The use of conventional strong alkalis can result in precipitation with hardness ions, leading to loss of alkali and scaling problems, unless soft injection water is employed [20],... 

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