Water is widely used in various process applications in industry. Other major industrial uses are boiler feed water and cooling water. The king and degree of treatment of water in these applications spends upon the end use. As examples, cooling water may require only minimal treatment, removal of corrosive substances and scale-forming solutes is essential for boiler  feed water, and water used in food processing must be free of pathogens and toxic substances.

Numerous factors must be taken into consideration in designing and operating an industrial water treatment facility. These include the following:

1. Water requirement
2. Quantity and quality of available water sources
3. Sequential use of water.
4. Water recycle.
5. Discharge standards.

Here external treatment, include processes such as aeration, filtration, and clarification to remove material that may cause problems from water. Such substances include suspended or dissolved solids. Hardness, and dissolved gases.

Whereas internal treatment is designed to modify the properties of water for specific applications. Examples of internal treatment include the following:

1. Reaction of dissolved oxygen with hydrazine or sulfite.
2. Addition of chelating agents to react with dissolved Ca⁺² and prevent     
3. Formation of calcium deposits.
4. Addition  of precipitants, such as phosphate used for calcium removal.
5. Addition of inhibitors to prevent corrosion.
6. Adjustment of pH.
7. Disinfection for food processing uses or to prevent bacterial growth in cooling water.