Water Treatment and Purification of Water | Chemical Process Industries

Water Treatment and Purification of Water:

Water conditioning and purification of water have been the most important function of municipalities for a long time. The importance of the use of suitable and treated water has been recognized and efforts are made to discover the best ways to treat water to be used in industries by Chemical Engineers. Besides moral and community considerations, laws prohibiting and limiting the pollution of streams and air require these problems to be considered as a necessary operating expense. Although solution is specific to each industry, a few general principles may be observed: increasing reuse of wastewaters, control of pollution and, if feasible, recovery of by-products at their source to lessen the expense of treatment, and lagoons the wastes to keep pollution at a minimum level or to effect a saving in neutralization costs.
While starting a chemical plant at a place, it is kept in mind that if a good quality and quantity of water is available or not. For this purpose, both surface water and groundwater are considered. Groundwater is mostly used for cooling purposes because of its almost constant temperature but this water is usually hard and causes scaling which may interfere with heat transfer. Hard water is those which contains dissolved salts of calcium and magnesium. Usually, these salts are present in water in the form of Carbonates, Bicarbonates, Chlorides, Sulfates, and Nitrates. In boilers, this hard water forms insoluble carbonates along with soap and cause clogging. Also, heat transfer processes may be affected.

Types of Hardness in water:

There are two types of hardness in water. Let's briefly discuss these types.

1. Temporary Hardness:

Temporary hardness is usually caused by dissolved bicarbonates of Calcium and Magnesium in water. Temporary hardness can be removed by the simple heating process. Upon heating these bicarbonates are converted into insoluble carbonates which can be removed easily.

2. Permanent Hardness:

Permanent hardness is usually caused by dissolved chlorides, sulfates, nitrates, etc of Calcium and Magnesium along with a small amount of alumina, silica, iron or manganese. This type of hardness cannot be removed by simple boiling. We can use the ion-exchange method, lime-soda process or some other process discussed below to remove permanent hardness. 
Generally, purification of water is done for three basic purposes:
1. For Drinking
2. Industrial wastewater
3. Municipal wastewater

Methods used Purification of Water:

Ion Exchange Method:

In industries, hard water causes scaling in boilers or other types of equipment is used for utilization, therefore, it is mandatory to convert it into demineralized water having low electrical conductivity. Ion Exchange is a chemical reaction in which mobile hydrated ions of a solid are exchanged with ions of like charges in solution. Ions present in water react with ions present on solid to neutralize them, groups attached to the solid matrix are called Ion Exchanger. Cation Ion Exchange takes place when positive charge ions in solution react with negative charge ions on the solid matrix. Similarly anionic Ion Exchange takes place when negative ions in the solution or water react with positive ions present on Ion Exchanger.

Lime Soda Processes:

The use of slaked lime and of soda ash to remove hardness in water has long been important. The modern application has been divided into the cold lime process and hot lime soda process. The Calcium ions in hard water are removed as CaCO3 and Magnesium ions as Mg(OH)2. Typical equations for these reactions are:

For Carbonate Hardness:

Ca(HCO3)2 + Ca(OH)2 --> 2CaCO3 + 2H2O

Mg(HCO3)2 + Ca(OH)2 --> MgCO3 + CaCO3 + 2H2O

Then since MgCO3 is fairly soluble, further addition of lime is necessary, causing

MgCO3 + Ca(OH)2 --> Mg(OH)2 + CaCO3

For Noncarbonate soluble Calcium and Magnesium salts:

MgCl2 + Ca(OH)2 --> Mg(OH)2 + CaCl2

CaCl2 + Na2CO3 --> CaCO3 + 2NaCl

CaSO4 + Na2CO3 --> Na2CO3 + Na2SO4

MgSO4 + Na2CO3 + Ca(OH)2 --> Mg(OH)2 + CaCO3 + Na2SO4

The cold lime process is employed chiefly for partial softening and ordinarily uses only cheaper lime for its reagent reactions. It can reduce calcium hardness to 35ppm if the proper opportunity is given for precipitation. This cold lime process is applicable to the partial softening of municipal water, to the conditioning of cooling water where calcium bicarbonate hardness may be the cause of scale formation, and to the processing of certain paper mill waters where calcium bicarbonate is troublesome.