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Processes of domestic water treatment and purification. Water for domestic use is chiefly obtained from rivers, springs, and wells; and sometimes from lakes and seas. However, lake and sea waters may be too salty for drinking or washing and hence are not normally used for such purposes. The gravel traps any large floating substances. The coarse sand prevents small particles from passing.
There are four steps of purifying water for domestic use.
- Decantation. This is a process to separate mixtures by removing a liquid layer that is free of a precipitate, or the solids deposited from a solution. The purpose may be to obtain a decant (liquid free from particulates) or to recover the precipitate.
- Filtration. This is the process of separating suspended solid matter from a liquid, by causing the latter to pass through the pores of some substance, called a filter. The liquid which has passed through the filter is called the filtrate.
- Coagulation and sedimentation. In water treatment, coagulation and flocculation involves the addition of compounds that promote the clumping of fines into larger flocs so that they can be more easily separated from the water. Coagulation is a chemical process that involves neutralization of charge, whereas flocculation is a physical process and does not involve neutralization of charge. The coagulation-flocculation process can be used as a preliminary or intermediary step between other water, or wastewater, treatment processes like filtration and sedimentation.
- Disinfection. This is the process of cleaning something, especially with a chemical, in order to destroy bacteria.
We obtain our water supply from surface water (for example, rivers, lakes and reservoirs) and ground water (for example, underground aquifers and lakes). Water from these sources is never completely pure, particularly if it is drawn from a river. The water may contain:
- Bacteria; most are harmless, but some can cause diseases.
- Dissolved substances; for example, calcium and magnesium compounds dissolved from rocks, and gases from the air.
- Solid substances and debris; particles of mud, sand, grit, twigs, dead plants and perhaps tins and rags that people have dumped.
- To kill harmful disease-causing microorganisms such as bacteria, fungi, amoeba, salmonella, etc.
- To remove toxic substances dissolved in water.
- To remove solid substances and debris from the water such as tins, rags, plant remains, sand, algae, Spirogyra, etc.
- To remove suspended earthy material (clay matter).
- To remove odor and unpleasant smells caused by different contaminants dissolved in water.
- To remove water hardness. Sodium carbonate is added to water to remove both temporary and permanent hardness in water to make the water soft.
- To add sodium fluoride to water in some areas to help fight tooth decay.
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