Sedimentation lime treatment 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. 

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. 

A very limited amount of individual plant specific data for the aluminum forming industry is available. Data available on the influent and effluent streams are discussed briefly in the following subsections for specific plants. This aluminum processing plant uses lime precipitation (pH adjustment) followed by coagulant addition and sedimentation as its treatment system. Data... 

Water goes through the processes of sedimentation, filtration, aeration, lime-soda treatment and chlorination to make it safe for drinking. 

TREATABILITY/REMOVABILITY Process, Removable Range (%), Avg. Achievable Cone. (i g/L)) Gravity oil separation, not available, 11 Sedimentation, 0, negative removal Sedimentation with chemical addition (lime, polymer), 99, 10 Sedimentation with chemical addition (lime), >92, <10 Activated sludge (based on synthetic wastewater), 65, 1750 Continuous activated sludge biological treatment simulator 9% removal... 

Generally, sedimentation processes are used primarily for the removal of suspended material from water, but the removal of dissolved mineral impurities such as in the Ume or Ume-soda ash softening process is an equally important aspect of sedimentation in the treatment of water. In the softening process, hydrated lime or hydrated lime and soda ash are added to react with the dissolved CO2 and the calcium and magnesium salts that commonly cause the hardness of water. The following equations describe some of the reactions that take place in the formation of the calcium carbonate and magnesium hydroxide precipitates. 

While alum (Al2(SO )31 SH O) is commonly used as a flocculant in general water treatment, the amounts used in defluoridation are much higher, atleast 150 mg/mgF . As is typical with flocculation processes, the water must be thoroughly stirred to ensure dispersal of the flocculating agent. Since the reaction results in an excess of ions. Lime (Ca(OH)2) is added to the water during the process to help maintain a neutral pH and hasten the settling of the sediment. The amount of lime added is typically 5% of mass of the aluminum sulfate added [10]. 

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