Water Treatment Process

The Water Treatment Process

Click the treatment schematic to the right for an interactive description of the treatment process.

Pure water is not found in nature. Chemicals, bacteria,
and suspended sediment particles enter the water through exposure to air and runoff. Before arriving at your tap, raw water is treated at the Hanahan Water Treatment Plant to eliminate the presence of harmful bacteria and unpleasant coloration, taste, and odor. Raw water drawn from Charleston Water System's two sources of supply goes through a multiple barrier treatment process.

Charleston Water System Raw Water Sources
Charleston Water System has multiple water sources,
but primarily draws raw water from the Edisto River and
the Bushy Park Reservoir, both of which are surface water sources. Surface water requires different treatment processes than groundwater, as bacteria, chemicals, organics, and other contaminants enter lakes, rivers, and reservoirs through runoff. Surface water, as compared to
well water, typically contains lower concentrations of undesirable minerals. Before water enters the plant for treatment, it flows through intake screens that remove
large objects such as plants and logs.

Chemical Treatment
During what is called the pretreatment process, chlorine, ammonia, and lime are injected into the untreated water. Chlorine and ammonia together form a chemical called chloramines, which disinfects the water by killing harmful bacteria and viruses. Lime is added to adjust the pH of the water, which affects the reaction efficiency of process treatment chemicals to water.

Click the image to enlarge

Physical Treatment
After the injection of pretreatment chemicals, the water is rapidly mixed with aluminum sulfate (alum) to evenly distribute the chemicals. The alum acts as a coagulant, which means it neutralizes particles and other impurities in the water to form large, heavy particles called floc. This is called flocculation, and it takes place in flocculation basins where the water movement is slowed, and the floc is given time to
form. From the flocculation basins, the water flows into sedimentation basins, where the floc settles
to the bottom and the resulting residuals are removed. These residuals contain harmless sediment particles and are disposed of by even application to a forested area on the treatment plant grounds.

The water then travels through filtration, in which it moves through large filters made of sand, gravel,
and anthracite. This physical process removes the very fine suspended particles not removed during sedimentation. As a result of the pH of the water and other chemicals present, particles become chemically attached to the filters and are removed from the water.

Once the water is filtered, other chemicals are added to disinfect the water and adjust pH. Chlorine
is added as the primary disinfectant. Lime is injected to readjust the pH to normal levels and fluoride
is added to help the prevention of tooth decay. Orthophosphate is also added to the water to prevent corrosion of water pipes. It works by forming a molecular coating on the inside of metal pipes that prevents lead and copper in home plumbing fixtures from leaching into the water.

Disinfection
Finally, chlorine and ammonia (chloramines) are added to disinfect against harmful bacteria and
viruses to protect the storage tanks and distribution pipes.