Drinking Water Treatment Cycle

Step 2

Step 2

Screening and Straining - If you think about surface water sources, lakes, rivers, and reservoirs, you

realize they contain varying amounts of suspended and dissolved materials. This material may include

turbidity, color, taste, odor, microorganisms, fish, plants, trees, trash. So the first process in conventional water treatment is to screen

or strain out the larger items. This is often accomplished using a large metal screen, often called a bar screen, which is placed in front of the water source intake. Large items are trapped on the screen as the water passes through it. These screens must routinely be raked or cleaned off.

Step 3

Step 3

Chemical Addition- – Once the pre-screened source water is received into the treatment plant, chemicals

are added to help make the suspended particles that are floating in the water clump together to form a

heavier and larger gelatinous particle, often called floc. In this process, a chemical is added that reacts

with the natural alkalinity in solution to form an insoluble precipitate. There are many different

chemicals on the market that are used in this process.

Step 4

Step 4

Coagulation and Flocculation- A rapid mix unit is usually used where the coagulant is added to the

water to provide a very quick and thorough mixing. The water mixing is then slowed to allow the water

to come in contact with the forming floc and allow it to increase in size. The continued mixing must be

gentle to allow the floc to grow and gain weight, but fast enough to keep it in suspension until you are

ready for it to settle in the clarifiers. The process of adding a chemical to cause the suspended material

to “clump” into larger particles is called flocculation or coagulation. The treatment unit where

coagulation and flocculation is performed is called the “flocculator”.

Step 5

Sedimentation and Clarification- – Once the flocculation process is complete, the water then passes over the weir in the flocculator and travels to the center of the clarifier, or sedimentation basin. Here, the water makes its way from the center of the clarifier to the saw tooth weir at the perimeter of the unit. As the water makes its way towards the weir, the large floc particles are allowed to settle out to the bottom of the clarifier. A rake continuously travels across the bottom of the clarifier and scrapes the settled floc to the center of the unit. Pumps are used to pull the settled “sludge” out of the clarifier and send it to a sedimentation / disposal pond. The water that passes over the weir is collected and transferred to the filters. The reason clarification occurs before filtration is so the majority of suspended material can be removed prior to filtration, which avoids overloading the filters and thus allowing much more water to be filtered before the filters must be backwashed.

Step 6

Filtration- – Clarified water enters the filters from the top. Gravity pulls the water down through the

filters where it is collected in a drain system at the bottom of the unit. There are many different types

of materials used in filters. The most common being sand and gravel. Many conventional

plants are now using granular activated carbon as the media of choice because it not only provides

excellent mechanical filtration of particulate matter, but also removes organic compounds which can

cause taste and odor problems.

Step 7

Disinfection - Once the water has gone through the filtration process, it is about as clear and clean as it

can get. However, there may still be bacteria and viruses remaining. To ensure these are destroyed,

there must be a disinfection process employed. The most common disinfection process used in the

United States is chlorination.