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What happens at a water treatment plant?


For thousands of years people have treated their drinking water to make it look cleaner and taste better. Medical lore from ancient India to Egypt advised that water should be filtered through sand and coarse gravel. In Greece, Hippocrates, the Father of Medicine, recommended boiling water and straining it through a cloth to remove particles.


Early in the 1800s, scientists began to suspect that diseases could be transmitted by drinking water. Since the discovery of bacteria in the 1870s, treatment to eliminate disease-causing microorganisms has dramatically reduced the incidence of diseases transmitted through water.
Water treatment facilities process water through many steps before it reaches your tap as drinking water for your family. The overall process generally begins with intake at the source, followed by pretreatment, mixing, coagulation and flocculation, sedimentation, filtration, disinfection, and distribution to the tap. See Figure 1. Other steps may be necessary depending on the quality of the water supply.


Preliminary Treatment Processes Preliminary treatment processes to purify water depend on the nature of the water supply and the type and the amount of contaminants. Quality may vary seasonally, thus requiring more or less treatment during certain water-flow conditions. Most groundwater in Alabama receives only the disinfection treatment process. All surface water and water from wells susceptible to contamination from surface water receive full treatment through a filter plant. The various preliminary treatment processes may include the following.

Screening. Large objects such as logs, sticks, fish, and plants are usually screened out at the intake or as the water is drawn into the treatment plant from a river, lake, or other surface water source. If the source is groundwater, the screening is done by nature as the water travels under the surface of the earth.


Gravel, sand, some silt, and other gritty materials may be removed by fine screening.


In some cases, algae, aquatic plants, and other very small debris may be removed by still finer screening.
Chemical Pretreatment. The water is conditioned for removal of natural organics, primarily algae and other aquatic microorganisms, as well as their by- products. This may precede micro-straining.


Main Treatment Processes Chemical Feed and Rapid Mix. Chlorine and other chemicals, such as alum or lime, are added to the water to help remove impurities, destroy any taste or odor, raise pH, disinfect, and sometimes remove excess minerals such as iron that may cause rust or staining problems. The water is then mixed rapidly to distribute the chemicals evenly.
Since the early 1900s, chlorine (as a solid, liquid, or gas) has been the primary disinfectant used in the United States because it is effective and inexpensive and can provide a disinfectant residual in the distribution system. Ozone and ultraviolet radiation can also be used as primary disinfectants, but chlorine or an appropriate substitute must also be used as a secondary disinfectant after the main treatment processes to prevent re-growth of microorganisms in the distribution system.

  Chemicals may be added to oxidize ferrous iron (Fe++), which is relatively high in some groundwater, to the ferric state (Fe+++). If pH of the water is above 7 (either naturally or by adding lime), the insoluble compound of ferric hydroxide is precipitated.

Sometimes chemicals are included to reduce the “hardness” or mineral content of drinking water. This usually involves the exchange of sodium for calcium and magnesium and, sometimes, the removal of iron and manganese. However, softening is not as popular as it once was for several reasons. The increased sodium in softened water is unhealthy for people with high blood pressure. In soft water lead is more easily leached from plumbing. Finally, detergents that clean favorably in hard water are readily available. There are no softening plants in Alabama since total dissolved solids are not excessive.

Coagulation and Flocculation
The water is sent into large basins where the alum clings to other chemicals and impurities in the water (coagulation), causing them to form larger, and heavier particles called floc. Gravity causes these larger particles to settle to the bottom.

The water is allowed to sit undisturbed long enough so that solid particles completely settle to the bottom. This process removes chemical precipitates as well as extremely fine clay and organic particles, including dead microorganisms.


After flocs (large, heavy particles) settle to the bottom, the water continues on its trip through filters. Layers of sand, gravel, and sometimes hard coal are used to remove any other impurities that are left in the water. Filtration helps to control biological contamination and turbidity. (Turbidity is a measure of the cloudiness of water caused by the presence of suspended matter.) Turbidity can shelter harmful microorganisms and reduce the effectiveness of disinfection. Removing organics prior to final chlorination of drinking water supplies is important.


After most impurities have been removed from the water, a small amount of chlorine is added to keep the water from developing bacteria as it travels throughout the distribution pipes. The amount of chlorine (usually no more than 3 parts per million) is carefully measured to be the lowest possible amount needed to keep the water free of germs. Residual chlorine at the tap should be near 0.5 parts per million.


A primary health concern with chlorination is the formation of disinfection by-products. When chlorine combines with organic matter in water, such as decaying plants or animals, it forms substances called trihalomethanes (THMs). These have been shown to cause cancer in laboratory animals. Chloroform is a common THM which has been linked to bladder cancer in those who drink from treated public water supplies. There is a drinking water standard for
total trihalomethanes (TTHMs) of 0.10 milligrams per liter, but it applies only to those systems that serve more than 10,000 people.


In some places fluoride at concentrations up to 1 part per million is also added to help prevent tooth decay. This is not as common as it once was because of the health concern for excess fluoride. EPA has established a maximum contaminant level (MCL) of 4 milligrams per liter (4 parts per million) for fluoride in drinking water.

If conventional steps in the main treatment process are not adequate, certain inorganic and organic contaminants must be removed by other methods.

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