Water Treatment – What Happens in Water Treatment Plants?

Water treatment helps make sure that our drinking and washing water is safe to use. This process removes harmful bacteria, metals and toxins.


Chemicals are added to cause fine, small particles in water to clump together and be removed from the water through settling, skimming or draining. This is called coagulation and flocculation.


A water treatment plant uses a variety of physical and chemical methods to clean surface and groundwater. This process ensures that the water you use is safe and meets or exceeds health standards.

Incoming water is screened to remove large particles that could clog equipment or interfere with the other processes. This step also exposes dissolved gases to air through aeration, which can help remove taste and odor-producing organic matter.

Coagulation uses chemicals to clump together smaller particles and sediments for easier removal by filtration. The water is then sent through long tanks with filters made of gravel, sand or granulated activated carbon to remove any bacteria or contaminants that remain. Water is disinfected using chlorine or other disinfectants.


Screening removes large debris and particles from water before it goes through other treatment processes. This protects the equipment from clogging and makes subsequent steps more efficient.

A screening process uses different sized screens to trap and remove comparatively large size floating matters like sticks, leaves, plastic bags, etc. This helps the sewage water to flow freely without getting clogged inside the pipes of the water treatment plant.

Coarse screenings are based on steel bars that allow water to pass through but stop larger debris. They can be manually or automatically cleaned, depending on the facility and type of waste that pollutes the sewage water. Fine screens are based on wire cloth, wedgewire elements or perforated plates. They can also be static or rotary.


Aeration brings water and air into close contact, allowing dissolved gases to escape from solution. Undesirable gases, such as hydrogen sulfide and carbon dioxide, interfere with the treatment process and negatively impact water taste and odor.

Air is also a powerful oxidizer that rapidly turns unfilterable iron and hydrogen sulfide into filterable ferric iron and elemental sulfide, respectively.

In a mechanical system, paddlewheel aerators use power to agitate the water, transferring oxygen from the air to the water body. A more energy efficient submersible system uses fine bubble diffusers to disperse a high volume of oxygen in the water. This type of aeration is commonly used in the activated sludge treatment process. It is important to ensure that oxygen is evenly distributed throughout the tank.


Coagulation is a water treatment process that prevents particles from repelling each other by neutralising their electrical charge. This destabilises the forces that hold the colloids apart, and encourages them to form clumps or flocs that can be easily separated from the water.

Inorganic chemical coagulants such as aluminum sulfate (alum) and ferric sulfate or chloride are the most commonly used types of coagulants. These are effective at reducing turbidity but produce a large volume of sludge for treatment and disposal.

Organic coagulants such as melamine formaldehydes or tannins also work effectively, producing a smaller volume of less hazardous sludge for treatment and disposal. They can be used in conjunction with metal salts to improve coagulation efficiency and performance. They are also less sensitive to raw water temperatures, so can be more easily adjusted for each water source.


Sedimentation is the process of allowing particles heavier than water to sink and settle. Water treatment plants use sedimentation to remove fine sand, silt and other solids from the water. This can help prevent these particles from reaching and blocking the sewer system, which is a common cause of waterborne diseases like diarrhea, typhoid, and cholera.

Coagulants are added to raw water to neutralize their positive charge and encourage them to bind together into larger flocks, called flocs. The water then flows into a settling tank or clarifier, where the flocs are allowed to settle.

This tank is typically designed with inclined plates or tube settlers that speed up the settling process. Outlets are arranged to skim the water evenly off the top of the tank.


Water treatment plants disinfect water to kill bacteria, viruses, and other microorganisms that may be in it. This helps ensure that the water is safe for end-uses such as drinking and bathing.

The type of disinfectant used depends on the source of the water and its condition. For example, chlorination is a common method for treating drinking water. However, it reacts with organic materials in the water to form disinfection byproducts (DBPs). This is a greater concern when the source of the water comes from lakes and rivers.

Other options for disinfection include ozone and UV light. Both are effective at killing pathogens but don’t produce DBPs. These methods are also less expensive than chlorine. Some communities use these alternatives to treat their water.


Filtration is the final stage in water treatment and consists of passing clarified water through filters which contain different materials with varying pore sizes. The oversized particles are caught and removed from the filtered water leaving the clear water on top.

This process can remove a wide range of contaminants such as dust, chemicals, germs, and parasites. Activated carbon filters are also used to reduce bad odors.

In general granular media are employed for filtration but other solutions such as membranes and fibres are being developed. Filtration can be operated at a constant rate or on a declining basis which minimises the requirement to backwash the filter. The latter mode is favoured as it avoids contamination of the water supply and allows the water to be filtered longer.