Water Treatment Processes

Water treatment is the process that makes water safe to drink. Natural water contains bacteria, viruses and parasites that must be removed before it can be used.


The most common method of treating water is to boil it. Other methods include using chemical disinfectants and filters. Water is also treated with ultraviolet light to kill germs.


In water treatment, processes are used to make water safe for a specific end-use. These can include drinking, industrial water supply, irrigation, maintaining river flow maintenance, or water recreation (such as swimming). Water treatment makes the incoming raw water suitable for the intended use and reduces the amount of contaminants it contains. The contaminants are usually removed through physical methods such as settling and filtration, or chemical processes such as disinfection and coagulation.

In some cases, it may be necessary to pretreat the water before starting the main treatment process. This can be done to improve the permeability of the water, to reduce the amount of particles in the raw water, or to remove pathogens. Physical pretreatment processes may include screening, aeration, and mechanical pretreatment (size reduction and separation by density). Chemical pretreatment includes oxidation of the dissolved substances, such as iron or manganese, to convert them to an insoluble form, which is then easily removed through filtration.

In the United States, public water systems are required to pretreat wastewater before discharging it to a natural body of water. This is known as the National Pretreatment Program, which is outlined in 40 CFR Part 403. Communities approved to implement this program have the legal authority to issue industrial user permits, conduct inspections of commercial and industrial sources, sample industrial discharges, and enforce regulations.

Primary Treatment

The primary treatment of wastewater removes large solids and grit that could damage the water-treatment equipment in the next stages. The sludge is scraped off the bottom of the tanks and can be used for a variety of purposes, including creating biogas or incinerated for energy. The lighter wastewater goes on to the secondary treatment stage.

In this stage, the wastewater is aerated to help bacteria break down organic material more quickly. Air is pumped into tanks of wastewater, and the microorganisms consume the waste to form a sludge that can be removed from the water. This aeration process also helps the water absorb oxygen, which makes it cleaner and safer for discharge.

Aeration is followed by a chemical process called chlorine contact that kills any remaining bacteria and prevents them from growing back. The treated wastewater is then disinfected before being released into a river, creek, bay or lagoon.

Some treatment plants also perform tertiary treatment to remove excess plant nutrients and toxic compounds from the water before it is discharged. This is especially important in areas with limited space or water scarcity. Usually this involves reducing nitrogen and phosphorus, which can promote algae growth and cause eutrophication of lakes. Nitrogen can be reduced through nitrification-denitrification, while phosphorus is typically precipitated using chemicals or by chemically removing it from the sludge.

Secondary Treatment

Wastewater is a mix of human and animal waste, food scraps, oils, soaps and chemicals. It flows into treatment plants from sinks, showers, bathtubs, dishwashers and washing machines in homes, offices and factories. It also flows into natural bodies of water such as rivers, lakes and oceans where it takes in nutrients from leaves, grass and other plants. All this wastewater contains organic material that needs to be removed before it is released into the environment.

The physical process of settling removes heavy solids from the raw wastewater. This is done in primary tanks that are large enough to slow down the velocity of the wastewater and allow heavy constituents such as grease to coagulate and form floating masses. A long-chain polymer is often added to improve the coagulation process. Once the particles float, they are skimmed off and piped to sludge-management systems.

Following the coagulation step is aeration. In this process, the wastewater is aerated for a period of time to promote the growth of microorganisms that consume the organic matter in the water and help to reduce its BOD content. This is commonly accomplished using wastewater stabilization ponds, also known as oxidation ponds.

Next is a process called flocculation or sedimentation. In this step, the water is gently mixed to form larger, heavier particles called flocs, which then settle to the bottom of the tank because they are heavier than water.


After sedimentation and filtration, the water is disinfected to kill any remaining bacteria or viruses. Disinfection can be done with chlorine, which is added as a liquid (sodium hypochlorite) or gas (chlorine dioxide). The water is exposed to the disinfectant for a period of time known as contact time to ensure that any microorganisms are killed. The amount of chlorine left after this process is called the chlorine residual. This chlorine protects the water throughout the distribution system from any new contamination and provides a bacteriostatic effect against regrowth of unwanted organisms. It also blocks the development of protozoan cysts, which could have passed through the plant in resistant (endospore) or reproductive forms (cysts).

Chlorine works by attacking the cell walls of the microorganisms. This disrupts the cell’s ability to take in nutrients and oxygen, which results in the cell dying. Chlorine disinfection has been used for over a century and has nearly eradicated water-borne diseases such as cholera and typhoid.

Other chemical methods for disinfection include oxidation with permanganate or nitrates, ozone, and ultraviolet radiation. Some of these methods have been shown to produce disinfection byproducts that are potentially harmful to health. However, it has been determined that the short-term benefits of disinfection outweigh the risk of these byproducts at the low levels typically found in drinking water.