What are the Stages of Water Treatment?

Feed Water

In a municipal water treatment plant, the source water is typically a river, lake, or reservoir nearby.  Pumps and pipelines are often used to transport water to the treatment plant. Large pumps are used to transport the water ‘up’ to the treatment facility when needed. To reduce pump and piping costs, it advised to have the treatment facilities in positions that can allow a gravity flow, majority of the time.

The water treatment industry utilizes water from various sources. The process of blending groundwater with surface water is often used to improve the final product

Screening - Application Specific

Water sources, such as lakes, rivers, and reservoirs, contain varying levels of dissolved and suspended solids. These solids may include odour, colour, micro-organisms, turbidity,  taste, unwanted contaminants etc. It may be organic or inorganic, inert or biologically active, and vary in size from a colloidal particle to large rocks. In some cases, these larger items can obstruct the treatment equipment, such as sand and plastic, creating a blockage in the water pumps. A standard water treatment process starts by screening or straining out the bigger particles. A large metal screen, often called a bar screen. These screens come as manual or mechanical. They are often placed in front of the feedwater pump to accomplish this process. As the water passes through the screen, large items are separated from the water. It is essential to routinely scrape, rake or clean these screens.

Chemical Pre-treatment

Chemicals are now added to the treatment plant once the  source water has been screened and the sludge. Chemicals are added during this procedure in order to create precipitates that will not dissolve in water. These processes use a variety of chemicals on the market. Chemicals such as these are called coagulants. Aluminium sulphate, or alum, has been used for many years as one of the most common chemicals. Another very popular coagulant is ferrous sulphate combined with ferric chloride, sodium aluminate, activated silica, and compounds called polymers, which are low molecular weight compounds composed of dozens of small molecules combined into very large molecules. Catalytic polymers (positively charged), anionic polymers (negatively charged), and non-ionic polymers (neutrally charged) make up the three types of polymers. Coagulants must be mixed very well with water before heavy clumps can form, regardless of the coagulant or combination of coagulants used.

With a rapid mix unit, the coagulant is added to the water for a quick and thorough mixing. When water mixing is slowed, water comes into contact with flocs as they form and enlarge. During the next mixing procedure, you must continue mixing gently so that the floc can grow and gain weight, but fast enough so that it remains suspended until you are ready for it to settle in the clarifiers. Flocculation or coagulation is the process of adding chemicals to cause suspended material to “clump” into larger particles. It is referred to as a flocculator when coagulation and flocculation are performed in a treatment unit.

Once flocculation is complete, water is forced to pass over the weir and travel to the centre of the sedimentation basin. The water flows through a saw tooth weir at the perimeter of the unit, from the centre of the clarifier to the centre of the unit. When the water flows towards the weir, the floc particles are allowed to settle at the bottom of the clarifier. Clarifiers have rakes that continuously scrape settled floc to the centre. The sedimented “sludge” is moved to a sedimentation pond or disposal pond by pumps. During the weir-to-filter cycle, the collected water is transferred to the filters. Clarifying before filtration prevents the filters from overloading, allowing for a much greater amount of water to be filtered before they need to be backwashed, as it removes the majority of the suspended material prior to filtration


Clear, clarified water enters from the top of the filter. Water passes through the filters and is collected in a drain at the bottom of the unit by gravity. In filters, a wide range of materials (media) are used. Typically, these are made up of sand or gravel. Today, granular activated carbon is commonly used in conventional water treatment plants to filter particulates and remove organic compounds that can cause taste and odour problems.


The filtration process leaves the water as clean and clear as possible. However, bacteria and viruses may still remain. Using a disinfection process will ensure they are destroyed. Chlorination is a commonly used method to disinfect water. Many forms of chlorine are available, including chlorine gas (most common), hypochlorite (bleach), chlorine dioxide, and others. The water is treated with chlorine in a manner that ensures all microorganisms are killed, regardless of technique. In the water treatment plants, chlorine levels are continuously monitored and very carefully controlled. Adding sufficient chlorine to ensure complete disinfection is crucial, but avoiding excess that affects the taste and smell of the water when delivered to consumers is also important. 

Distribution and Storage

Following the disinfection process, the water is stored. Storage usually takes place in an underground tank called a “clear well,” as well as in elevated tanks visible throughout the city. In the event of an emergency, there must always be sufficient water available. Examples include floods, fires, or power outages.

How does your kitchen tap get its water? Through underground pipelines all over town, the stored water is distributed through a “distribution system”. Water distribution involves huge water pumps at the treatment plant, large overhead tanks, small pipelines, fire hydrants, valves, and meter boxes on your street.