Water Treatment Process Steps

Water is one of the most important resources in our daily lives, and all of the water we use goes through the water treatment process before we use it. Water treatment is the process of removing impurities from the water, to make it safe for us to drink and use. Therefore water treatment is a very important process, that keeps people safe and healthy. However, not many people know exactly what happens during the water treatment process.

Water Treatment Process Steps

Broadly speaking the water treatment process can be divided into five steps. These are screening, coagulation, sedimentation, filtration, and disinfection. Each of these steps is vital to ensuring the purity of our water. We will explain each of the water treatment process steps in more detail below.

Screening

Many water treatment plants get their water from surface water sources like lakes, surface water, and rivers. However, these water sources can have many suspended materials in them. Materials like leaves, branches, plastic, and trash. These materials can be harmful to the equipment of the water treatment plant. Therefore the first step in the water treatment process is screening.

Screening refers to the process where the water passes through a metal screen, called a bar screen. Often there are multiple screens with various sized openings. These range from coarse screens, with openings of 5-15cm, fine screens, with openings of 5-20mm, and micro strainers, with openings of 15-64µm (micrometers). These screens catch all the large solid materials in the water. The screens are raked regularly to remove the captured material and dispose of it.

Coagulation

After the water has passed through the screening process, the next step of the water treatment process is coagulation. Coagulation removes the fine particles (less than 1µm in size) from the water.

In this step of the water treatment process, a chemical called a coagulant is added to the water, while it is in a rapid mixing tank. Common coagulants used are aluminium sulphate, poly alunimium chloride and ferric sulphate. The rapid mixing tank ensures the reaction of the coagulant throughout the water. The coagulant causes the particles to come together and form larger particles called floc. This process is called flocculation and it is helped as the water passes through the flocculation basin, which has a number of compartments with progressively slower mixing speeds. The mixing must be gentle so the floc can grow, without the mixer impeller breaking it up, but fast enough to keep the floc suspended in the water.

Sedimentation

Once the water has undergone flocculation it moves to the next step of the water treatment process which is sedimentation. Sedimentation is the process that removes the floc from the water.

For this step of the water treatment process, the water is pumped into a sedimentation tank. The sedimentation tank is designed to slow the flow of the water. This allows the floc and suspended solid in the water to settle to the bottom. This layer of sediment on the bottom is called sludge, and it is pumped out of the sedimentation tank to a disposal pond or sludge thickener. This step of the water treatment process helps to remove a lot of suspended material before the water proceeds to the filters. This allows the filters to work longer without the need to be backwash frequently.

Filtration

After the sediment has been removed from the water, it moves to the next step of the water treatment process which is filtration. Filtration is defined as the process of removing solids from a liquid.

In the water treatment process filtration is the final step that removes solids from the water. This is done through the use of various filter media that trap the particles as the water passes through. The most commonly used media is gravel and sand, anthracite, though some treatment plants also use granular activated carbon as it can remove organic compounds.

Once the filter media is full of trapped particles, it is cleaned by a process called backwashing. This involves clean water and air being pumped through the filter in reverse, which removed the trap particles. This backwash water is then disposed of in the sewage system, and the filter can be used again.

Disinfection

Once the water has been filtered it is stored in a closed tank for the final step of the water treatment process. The water in this tank is clean however there may be bacteria and viruses in the water. This is why the final step in the water treatment process is disinfection.

Disinfection is the process of treating water with chemicals that will kill bacteria, viruses, and other microorganisms. In the water treatment process, the most commonly used disinfectant is chlorine. Once the chlorine is added to the water it is given time to react with any pollutants in the water, this is called the contact time. There will still be residual chlorine in the water after this period, which serves to protect the water from future contaminants as it is distributed to the consumer and end users.