Metropolitan Water District of

          Salt Lake & Sandy

Disinfection

In order to ensure that the water is safe for public consumption, chlorine is added to inactivate bacteria and other pathogenic (disease causing) organisms.

Chlorine was first used in drinking water in the late 19th century to control the spread of water-borne diseases such as typhoid, cholera, dysentery and gastro-enteritis.  These pathogens have killed more people than all the wars in history. Chlorine acts as a powerful disinfection agent inactivating bacteria and other microbes.

Chlorine is a greenish-yellow gas with a penetrating and distinctive odor.  The gas is two-and-a-half times heavier than air. Chlorine has a high coefficient of expansion, and for this reason no chlorine container should be filled to more than 85 percent of its volume.  One liter of liquid chlorine can evaporate and produce 450 liters of chlorine gas.  When mixed with water, Hypochlorous acid and Hydrochloric acid are formed.  The Hypochlorous acid is the most effective disinfectant of the two.

Disinfectants such as ozone, ultraviolet light, and heat are only a temporary solution.  Chlorine makes sure that clean water comes out of your tap since it leaves a residual in the water throughout the system.

Besides purifying water, chlorine helps remove tastes and odors, and controls the growth of slime and algae in main pipes and storage tanks.

Safe water is becoming harder to find.  In the past, safe water could be found in remote areas; but with population growth and related pollution of waters, there are very few natural waters left that are safe to drink without some type of treatment.

Water carries all types of dissolved materials and biological life forms that can cause diseases.  Chlorine is vital in controlling these diseases.  As a result of chlorine use, organisms and the diseases they transmit are no longer a problem in the United States.  However, many developing regions of the world still experience serious outbreaks of various waterborne diseases. The World Health Organization estimates that more than three million people still die each year as a direct result of drinking unsafe water.

Chlorine Operation As Disinfection

The exact mechanism of chlorine disinfection action is not fully known.  One theory maintains that chlorine exerts a direct action against the bacterial cell, thus destroying it.  Another states that the toxic characteristic of chlorine inactivates the ENZYMES which enable living microorganisms to use their food supply.  As a result, the organisms die of starvation.

When chlorine is added to water containing organic and inorganic materials, it will combine with these materials and form chlorine compounds.  If you continue to add chlorine, you will eventually reach a point where the reaction with organic and inorganic materials stops.  At this point, you have satisfied the "CHLORINE DEMAND."

When chlorine is added to the water, several chemical reactions takes place.  Some involve the molecules of the water itself, and some involve organic and inorganic substances suspended in the water.

The chemical reaction between chlorine and these organic and inorganic substances produce chlorine compounds.  Some compounds have disinfecting properties, others do not.  Chlorine reacts with water and produces substances with disinfecting properties.

The total of all the compounds with disinfecting properties, plus any remaining free (uncombined) chlorine is the "CHLORINE RESIDUAL".  The presence of this measurable chlorine residual indicates to the operator that all possible chemical reactions have taken place and there is still sufficient "AVAILABLE RESIDUAL CHLORINE" to inactivate the microorganisms present in the water supply.

If you add the amount of chlorine needed to satisfy the chlorine demand and the amount of chlorine residual needed for disinfection, you will have the "CHLORINE DOSE".  This is the amount of chlorine you will have to add to the water to disinfect it.

Example:

Chlorine Dose, mg/L = Chlorine Demand, mg/L + Chlorine residual, mg/L

Reaction with water forms hypochlorous and hydrochloric acids: Chlorine + Water = Hypochlorous Acid + Hydrochloric Acid