Introduction to Water Treatment

Water treatment in a typical water treatment plant is shown in the picture below. Based on the characteristics of the raw water and on other factors, this treatment process may vary considerably from place to place.

 

As water is pumped from the source (a well, spring, river, or lake) it is screened to remove debris. Then, at the water plant, various characteristics of the raw water are tested.

The water may be prechlorinated to kill microorganisms, control odors and taste, and aid in coagulation and setttling. The water may also be aerated, which removes carbon dioxide (CO₂) and raises pH, oxidizes iron (Fe) and manganese (Mn), removes hydrogen sulfide (H₂S), and removes organic contaminants. Potassium permanganate (KmnO₄) may be added to the water in the collection tray of the aerator in order to remove iron and manganese from the water. Ozone may be added to the water to treat iron and manganese, remove algae, neutralize hydrogen sulfide (H₂S), and aid in flocculation. Many of these are dependent upon the plant and the amount of water being distributed daily. 

In most cases, the water then enters a flash mix chamber. Here, various chemicals are added and are mixed into the water. Coagulants cause fine particles to clump together into larger particles. Alkali are added to adjust the pH as well as to oxidize iron and manganese. Hexametaphosphate may be added to prevent corrosion of pipes.

After flowing out of the flash mix chamber, the water goes through a chamber which causes coagulation and flocculation to occur. Here, the fine particles of contaminants gather together into large clumps called floc. When the water flows into the sedimentation basin, some of the floc settles out of the water and is removed. Next, the water is passed through filters which remove particles too small to settle out in the sedimentation basin.

Finally, chlorine is added to the water. The water may also be fluoridated to reduce tooth decay in the consumers. The water is left in the clear well for a period of time to allow the chlorine to kill bacteria in the water and to oxidize hydrogen sulfide. The water is now treated and ready to be distributed.

That was a quick overview of the water treatment process and what happens to water as it goes through the plant step by step. We will get into greater detail in later lessons so that you will understand each step of the process, why it is done and how to make sure each step is being done efficiently.