The Basics of Water Treatment

Water treatment removes pollutants, germs, chemicals and toxins from public and private water supplies. These contaminants can come from glacial sediment, silt, leaves and other debris stirred up by rainstorms or from specific chemicals or toxins used in industrial processes.


The screened water is aerated to make it less corrosive and to get rid of gases that cause unpleasant tastes or staining. It also goes through coagulation and sedimentation.


Almost 2 billion people around the world do not have access to safe drinking water (Chandra Bhomick, 2017). It is important that we treat the water that we have and that we use for other purposes to ensure that it is clean. The main purpose of pretreatment is to remove all substances that are not part of the natural water cycle from the water supply and to make it safe for use, i.e. it needs to be odourless, tasteless and colourless, as well as non-corrosive to avoid damage to the pipes carrying it.

The pretreatment process starts with aeration to release any volatile gases that may be in the water. Coagulating agents then clump small particles together to form soft, fluffy particles called flocs. These are then filtered out by sedimentation, usually in long tanks with gravel, sand or granulated activated carbon to remove any remaining bacteria and other particles from the water. Chemicals are added to the water during these stages to facilitate the treatment processes. These include chemicals for disinfection, corrosion inhibition and pH balance as well as fluoride for dental health.

In many areas, the energy demand of pretreatment is very high due to pumping and transporting the water over long distances. This can be reduced by using technologies that do not require any pumping, such as trickling filters and gravity aqueducts.

Secondary Treatment

After primary treatment, water must go through the secondary treatment phase to remove any remaining dissolved substances. This involves aeration, flocculation and sedimentation. Aeration involves pumping air into tanks of water to encourage microorganisms to break down organic material more quickly. This helps to eliminate organic grit, grease and oils.

Another method used to help coagulate the finer particles in wastewater is sand filtration or contact filters. Then a chemical called a coagulant is added to the water. This binds together the finer particles, which are now heavier than water and sink to the bottom of the tank. The resulting particles are known as flocs and are removed by a process of sedimentation.

Other types of secondary treatment methods include waste stabilization ponds (WSPs), oxidation lagoons, biotowers and trickling filters. During these processes, oxygen is introduced to the wastewater and a symbiotic relationship between bacteria and algae takes place. Bacteria convert the organic waste into carbon dioxide, hydrogen and methane through a biological process. The resulting decomposed organic matter is known as sludge and can be treated further in an anaerobic digestion process or processed for reuse as a soil amendment.

Reclaimed water can be used to supply drinking water, replenish environmental sources and meet industrial water needs. Water can also be treated to achieve a higher quality standard, such as for water recreation or to reduce lime scale around fixtures in homes and businesses.

Tertiary Treatment

The tertiary treatment step is any process that follows primary and secondary water treatment to achieve higher levels of cleanliness, usually with filtration or additional disinfection. This can include reducing organics, turbidity, nitrogen and phosphorous, metals and pathogens. This is often the most rigorous part of the water treatment sequence, and is required for a variety of uses, especially where wastewater is reused as water for irrigation of food crops or other plants, or when it is discharged into sensitive aquatic ecosystems like estuaries or sluggish rivers.

A popular tertiary water treatment method is to have the water flow under banks of ultraviolet light, which sterilizes bacteria and viruses by damaging their genetic structures. Another option is to use a biological method, which involves growing aerobic bacteria that consume organic matter in a controlled environment.

Finally, a common approach is to use physico-chemical methods to reduce the nitrogen and phosphorous content of water. This can be done by precipitating the ions from solution using aluminum (Al) or ferric (Fe3+) salts, or by adsorption on activated carbon.

Nitrogen and phosphorous are important for plant growth, but they can also degrade water quality by encouraging algae growth that competes with the oxygen needed by other organisms. To protect the surrounding ecosystems, a number of methods are used to restrict their release from sewage treatment plants, including natural processes such as wetlands, and mechanized methods such as microbial denitrification.


Water treatment plants use several methods to disinfect water, which is the last step before it leaves the plant for distribution. This process kills any remaining bacteria, parasites and viruses, and prevents them from spreading throughout the distribution system. It is important to remember that while disinfection of drinking water significantly reduces the concentration of microorganisms, it cannot produce sterilization. Sterilization would have a negative impact on the health of consumers and is impractical for public drinking water systems, which must maintain disinfectant levels in a variety of environmental conditions.

During this process, screens are used to remove large floating and suspended solid materials. These materials can clog pumps and pipes and are a source of pollution. In addition, they may contain dangerous substances such as chemicals, oils, poisonous substances or sewage.

The treated water then flows through filters with varying pore sizes to separate the clear water from the dirt and other particles on top. In addition, chemical disinfectants such as chlorine and chlorine dioxide are added to kill any remaining unwanted organisms.

The water that is left after the disinfection process is called treated drinking water. It must meet strict governmental standards and must be safe for human consumption. Drinking untreated or contaminated water can expose people to diseases such as typhoid and paratyphoid fevers, cholera, campylobacter, salmonella, and hepatitis A and E.