Removal of hardness

REMOVAL OF HARDNESS

• The process of removing hardness from water is called softening
• Hardness is defined as the property of water to consume soap (prevention of formation of lather by forming precipitates)
• Hardness of water is also responsible for forming scales (in industrial and domestic heaters, hot water lines, etc)
• Hardness is expressed in terms of ppm as CaCO3
• Primary constituents of hard water are divalent cations of Calcium and Magnesium (Ca2+, Mg2+) and Iron and Manganese (Fe3+, Mn2+)
• Hardness due to calcium bicarbonate (CaHCO3) and magnesium bicarbonate (MgHCO3) is called carbonate hardness or temporary hardness as it can be removed by boiling.
• However, hardness due to sulphate, chloride and nitrate salts of calcium is called permanent hardness as it cannot be removed by boiling. 
• Hardness of water varies the source from where the water originated.
• The following are the methods to remove hardness
• Lime/soda ash method
• The is the most common method of water softening used as lime and soda ash are readily available and cost efficient
• In this process, hydrated lime is fed at a dosage sufficient to cause the carbonate hardness to precipitate out, settle, and be removed from the water.
• It's not uncommon to feed large doses of hydrated lime
• The advantages of using caustic soda are:
• Ease of handling and feeding,
• Lack of deterioration in storage
• Less calcium carbonate sludge is formed
• Can precipitate both carbonate and non-carbonate hardness, so the caustic soda can actually replace some of the need hydrated lime.
• Use of soda ash or caustic soda is used is based on chemical costs and raw water characteristics.
• Ion-exchange method
• Ion exchange is not nearly as cost-efficient as lime softening when producing large amounts of water. Therefore, water plants typically do not use ion exchange to soften the finished product water. 
• Most in-home water softeners are ion-exchange units.
• Ions can exist with either a positive charge or a negative charge. 
• A positive charge ion is called a cation. A negative charge ion is called an anion.
• The basic premise of which ion-exchange softening uses is trading one ion for another.
• An ion-exchange water softener has three main parts:
• a resin tank containing small beads of synthetic resin
• a brine tank and
• the control valve
• The resin is treated with sodium (Na) from the brine tank. Sodium is a positive charged cation with a charge of plus one
• As water passes through the resin tank, the sodium ions are exchanged with calcium and magnesium ions, because the calcium and magnesium ions are of a higher positive charge than the sodium ions.
• Therefore, the resin behaves as a negative charge. 
• Since the calcium and magnesium ions have a stronger positive charge than the sodium ions, the resin “exchanges” the sodium for calcium and magnesium because of higher attraction between the resin and the calcium and magnesium than between the resin and  sodium. 
• This process of ion exchange will continue until all available exchange sites are filled, at which point the resin is exhausted and must be regenerated.
• Zeolite softening
• Zeolite resin exchanges sodium ions for ions causing hardness in the water
• To replenish the sodium ions used, the units need to be regenerated with material containing high amounts of sodium, normally a salt brine. This allows the resin to be reused many times
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