Our drinking water is treated to clean up solids, unsafe levels of chemical contaminants, and harmful microorganisms.
Drinking water doesn’t need to be pure water. In addition to water molecules, it contains minerals, dissolved gases, chlorine, and tiny amounts of various contaminants.
Water treatment plants remove a few main categories of contaminants from water:
Disinfecting Water / Killing Microbes
Removing or killing microorganisms such as bacteria, fungi, and protozoa prevents humans from getting waterborne diseases. Microbes can be removed in multiple ways:
- Chlorination: adding chlorine to water is usually the most cost effective way to disinfect drinking water on a large scale.
- Bromine / Iodine: Similar to adding chlorine, adding these other halogens kills microbes. Sometimes bromine or iodine tablets are used to disinfect small amounts of water for personal use where clean drinking water is not available.
- Ultraviolet Light: ultraviolet light can be used to kill microbes.
- Boiling: boiling water kills most microorganisms. This method is only practical for small quantities of water (i.e. when camping) because it takes so much energy ($) to boil water.
Removing Solids: large and small solids need to be removed from water prior to putting it in the water supply.
- Filtration: sand and gravel can be used to filter out solids; screens and meshes can also be used.
- Flocculation: removes “cloudiness” in water. This process removes tiny suspended particles by adding chemicals, such as aluminum sulfate, Al2(SO4)3, and slaked lime, Ca(OH)2. Those chemicals cause the suspended particles to settle out into the bottom of a tank.
Chemical Contaminants: many chemical contaminants can be present in water, and they require different types of treatment.
- Aeration, or spraying the water into air, can remove chemicals that cause unpleasant tastes and odors.
- Ion Exchange can remove metal ions, very similarly to the way water softeners remove hard minerals from household water.
- Heavy metals can also be removed through flocculation (adding chemicals to react with metal ions, then let them settle out)
- Adsorption is the use of a solid to attract some types of contaminant; after the solids pick up contaminants, they are filtered out or allowed to settle out. Charcoal (carbon) adsorption is used in drinking water filters found in many homes.
Some types of water treatment are practical for smaller amounts of water but cost prohibitive on large scales:
- Reverse Osmosis: using a pump to force water through a semi-permeable membrane. This membrane allows water through, but not contaminants.
- Distillation: boiling water and then recollecting the vapors. This removes nearly all contaminants, with the exception of substances that have a lower boiling point than water. It is a very effective way to purify water, but it is also very expensive because of the energy cost of boiling the water.