SOLUTION

External Treatment of Water

External water treatment includes the following steps: filtration, chlorination, clarification, softening, precipitation, ion exchange, de-alkalization, demineralization, deaeration, and reverse osmosis (RO).

Filtration:- Filtration removes or minimizes such impurities as coarse suspended matter, silt, clay, and some organic materials. It also removes floc or sludge produced by coagulation or softening. Filtration is a mechanical process and therefore does not remove dissolved solids. Undissolved solid particles in water are removed by using either gravity filters or pressure filters. Filtration also removes the rust particles formed due to corrosion in the boiler and steam piping or tubing. Coarse suspended matter in water is removed by using gravity or pressure filters. However, when the suspended solids are very fine, clarification of the solution using a coagulation and flocculation step is mandatory to ensure effective filtration.

Clarification:- The objective of clarification is to remove suspended matter and color from water. The term clarification is used in connection with the turbidity in water. High-purity water is colorless and completely transparent. However, raw water used in boilers is not normally high purity and has several suspended impurities in it, which makes it turbid. Lack of clarity in water is referred to as turbidity. The suspended particles range from a few microns in diameter to the submicron and nano levels with a high surface area. They usually have a net negative charge on their surface, which keeps them in suspension in a medium such as water (as the like charges on the neighboring particles’ surfaces repel them from each other). Clarification is a process of removal of such insoluble suspended impurities from water and consists of steps including coagulation and flocculation.

Coagulation:- Because ultra-fine-sized suspended solid particles are present in raw water, cartridge filters are at times unable to filter them. In such a situation, the water is treated before filtration with chemicals known as coagulants, and the treatment is known as coagulation. It is a process of neutralization of the negative charges from the surface of the fine particles, which makes them unite to one another by providing a nucleus to form masses or agglomerates that can be filtered. Chemicals such as alum (aluminum sulfate), sodium aluminate, ferric sulfate, ferric chloride, activated silica, and high-molecular-weight polyelectrolytes are often used as coagulants. They cause impurities that are otherwise present as fine suspensions in the solution to settle. Hard and high-TDS water is treated more easily than soft and low-TDS water.

Flocculation:- Stable agglomerates of small particles are converted into large floes by a process called flocculation. This is done by stirring or gentle agitation of a coagulated solution. Flocs, by virtue of their higher specific gravity, tend to settle at the bottom. The clear water is then separated from the floes by Recantation.