BACKGROUND:

What has saved more lives than all the doctors and hospitals in history?  The treatment of drinking water. If you’re surprised, don’t be.  Waterborne disease is one of the world’s most serious health threats, but is virtually eliminated by treating water.

The history of water treatment dates back to antiquity.  Ancient Egyptian inscriptions describe water purification by boiling, exposure to sunlight, charcoal filtration, and settling in an earthen jar.  In 400 B.C., Hippocrates, the Father of Medicine, suggested that water should be boiled and strained through a piece of cloth to remove particles.  It wasn’t until the early 20th century,  that a method of purification was developed that virtually eliminated water borne diseases such as cholera and typhoid fever.  We call it chlorination, and it is the most widely used method for disinfecting water in the United States today.  It has been so successful that many people in developed countries take the safety and purity of our water for granted.

In the United States the following is a common way that water districts clean drinking water.  

Ozonation - Ozone is the primary disinfectant of the water coming into the plant. Ozone-rich air is bubbled up  through the water in a series of ozone contact chambers. In addition to being a highly effective disinfectant, the  ozonation process also destroys compounds which cause unpleasant tastes and odors in the water. 

Coagulation/Flocculation - After ozonation, the water moves on through a flash mixer where chemicals called coagulants are added. These coagulants react with particles in the water, causing them to clump together. The water then goes to the flocculation basins, which use the hydraulic energy of the water for mixing to create "floc"  particles which are large and heavy enough to settle to the bottom. 

Sedimentation - The water then moves onto a double decked sedimentation basin where the heavier floc particles settle to the bottom, while the clearer water moves on to be filtered. A vacuum system removes the settled solids and deposits them in a solids holding basin. 

Filter Press - The material removed in the sedimentation and filtration process contains a significant amount of water. In order to separate the water from the solids so that it can be recycled, the material is sent to a gravity thickener and then to a filter press where the remaining water is squeezed out. The compacted solids are then  transported to a landfill for disposal. 

Filtration - Following sedimentation, the clarified water is filtered through layers of anthracite coal and sand. The process removes any remaining particles that did not previously settle out.  This  "polishing" provides a high level of clarity. As the water leaves the plant, it receives a small dose of chlorine to keep it fresh and clean as it travels through the distribution system to customers. The pH of the water is adjusted for corrosion control and fluoride is added to benefit the community's dental health.

 

PROCEDURE:

1. Students have learned the properties of water, but many still do not appreciate the value of water.  Instruct students to read, "Nu, You, and Water" and use it as a discussion of how water is obtained and cleaned throughout the world.  The "homeland" country in the story refers to Sub Sahara Africa.
   
   
2. Contact local water district, to see what information they may have for your class.  You may want students to use the Internet to find out information on how water is managed throughout your state.  Many cities have water departments that may give presentations on how they treat your water systems.  
     
3. Once you get information you may want to chart the information.  For example, in a state like California water is very important.  Agriculture accounts for 87% of water use, domestic uses account for 8.5%, manufacturing accounts for 2%, and 2.5% is used for other purposes. You may want the students to make a pie chart using this information.
     
4. Ask students how much water does a leaky faucet waste.  A leaky faucet causing a slow drip, wastes the following  amount of water:
      
   1/32 inch leak wastes 25 gallons in 24 hours 
   1/16 inch leak wastes 100 gallons in 24 hours 
   1/8 leak wastes 400 gallons in 24 hours
   
   A 1/32 inch leak will waste in one year - 9,000 gallons of water!!
     
5. Ask students how much water is used in the following common activities.  You may want to have them do a math exercise to add the amount of water used in a normal day by a household.
     
    Shower = 25 gallons
    Brushing teeth = 10 gallons
    Bath tub = 36 gallons
    Shaving = 20 gallons
    Dishwashing = 30 gallons
    Automatic dishwasher = 16 gallons
    Washing hands = 2 gallons
    Toilet flushing = 5 to 7 gallons
    Washing machine = 60 gallons
    Outdoor watering = 10 gallons per minute

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