Water Purification Techniques

Desalination of seawater, or distillation, and reverse osmosis are particularly important.

Popular methods for purifying water, especially for private suppliers, are listed below. Some countries use some of these methods to treat water for municipal supplies. Desalination of seawater, or distillation, and reverse osmosis are particularly important.

Boiling
Water is heated to its boiling point. Near sea level, the water is boiled for at least 1 minute. At altitudes greater than a kilometer or a mile, water is boiled for 3 minutes. Boiling water inactivates or kills microorganisms that live in water at room temperature.

When “hard” water (water containing calcium salts) is boiled, the dissolved calcium in the water is precipitated as calcium carbonate and forms the “fur” that builds up on kettles in areas with hard water. Boiling does not remove solutes of a higher boiling point than water. In fact, it increases their concentration, as some water is lost as steam.

Carbon Filtration
Because of the way charcoal is made, it has a lot of surface area and is able to absorb many different compounds. Filtering water through activated charcoal removes many contaminants, including some toxic compounds, from the water. Charcoal filters are commonly used in households and fish tanks. Filters for drinking water sometimes contain silver, as silver has a bactericidal effect.

Distillation
In distillation, water is boiled to produce water vapor. Solutes remain in the solution while the water vapor rises to a cooled surface where it condenses back into water and is funneled into a container. Although contaminants with the boiling point of water and droplets of unvaporized water rise with the steam, the resulting water is 99.9% pure.

Reverse Osmosis
In normal osmosis, pure water would move through a semi permeable membrane to dilute impure water. In reverse osmosis, mechanical pressure is applied to impure water to force it through a semi permeable membrane to filter it and render it pure. Theoretically, reverse osmosis is the most thorough method available for purifying water on a large scale. However, it is difficult to manufacture perfect semi permeable membranes.

Ion Exchange
Common ion exchange systems using a zeolite (hydrous silicate) resin bed replace unwanted calcium and magnesium ions with soap-friendly sodium or potassium ions. Common water softeners use this method. Another type of ion exchange system swaps hydrogen ions for unwanted cations and hydroxide ions for unwanted anions. The system is recharged with hydrochloric acid and sodium hydroxide, resulting in deionized water.

Electrode ionization
As water is passed between a positive electrode and a negative electrode, ion-selective membranes direct the positive ions toward the negative electrode and the negative ions toward the positive electrode. Deionized water of high purity results. Often, the water is first passed through a reverse osmosis unit to remove organic particles and other contaminants.

Conditioning Hot Water
To reduce the effects of hard water, hardness salts are put into water systems subject to heating. When bicarbonate ions break down, carbonate ions crystallize out of the saturated solution of calcium or magnesium carbonate. Water high in hardness salts can be treated with soda ash (sodium carbonate) to cause precipitation of the excess salts. The result is calcium carbonate of high purity. Traditionally, calcium carbonate is sold to toothpaste manufacturers.

Plumbo-Solvency Reduction
Acidic waters of low conductivity, such as rainwater in mountainous areas with igneous rocks, can dissolve lead in lead pipes. To reduce this solvent effect, small quantities of phosphate ion are added to the water and the pH is increased. This creates insoluble lead salts that adhere to the inside of the pipes, thus reducing plumbo-solvency.

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