The 5 Main Steps of Water Purification

Whatever your water source, drinking purified water is essential if you want to avoid unnecessary illness and disease from pathogens and contaminants in water. For it to meet federal quality guidelines, water we consume needs to go through purification.

Throughout the purification process, water is tested and analysed to ensure that it meets the required federal standards.

Here’s how that happens.


The first step of the purification process is to screen the raw water that enters the purification plant and remove large contaminants. Municipal water is typically drawn from reservoirs, lakes, rivers, streams, wells or underground aquifers. Water that comes from open sources like rivers and lakes, need to be screened because it may contain large contaminants that have to be removed at the very start. This includes things like leaves and wood or other solid particles floating in water. However, if the water comes from an underground source, it would typically not need screening, as the natural percolation that it goes through removes large contaminants.


Coagulants are chemicals that act like magnets, causing the particles in the water to collect and stick together. This is done by treating water in storage tanks with precipitating agents like alum and other chemicals. This results in the creation of floc, or groups of particles, which floats on the surface of the water. When dirt particles become attracted to and start sticking together as floc, the floc eventually becomes heavy and sinks to the bottom of the tanks.


The water, including the floc, next undergoes the sedimentation process. Here the water is directed to flow into a sedimentation basin where it’s kept until the floc sinks to the bottom of the tanks, separating the water from these large solid particles. When floc and water is separated, it’s called clarification.


The floc-free water is then filtered, as it percolates through gravel, sand and even charcoal layers. This stage removes contaminants and particles, including herbicides and pesticides if present. If the water has an unpleasant odor activated carbon is sometimes used for filtration, as this neutralizes the odor. However, while the water may look and smell clean at this stage, it will still have pathogens and other microorganisms still alive inside.  


The water is then directed towards a storage tank or reservoir, where it is treated with pathogen-killing chemicals like chlorine or iodine. This is sometimes used in conjunction with ultraviolet lighting, which is an excellent way to kill pathogens in the water. UV light attacks the genetic code of the microorganism, effectively killing them and making the water safe to consume. The disinfected water is then stored, until it’s needed for use for homes and businesses.

If groundwater is used, as opposed to surface water sources, the only step typically required is disinfection, as this type of water has already gone through the other steps naturally.

Some Facts to Note

While these are the most common steps of water purification, different communities may follow different processes. In general, surface water needs more treatment than ground water so depending on the community’s water sources, it’s common to have variations in the above steps.

One other step for tap water is that it’s typically fortified with fluoride, which helps to prevent tooth decay and keep your teeth healthy. While some appreciate this being in their water, others prefer to filter it out along with other chemicals via a filtration system in their home.

If you have a water filter installed in your home, the water then goes through a whole new process of purification, which rids it of the chemicals used to disinfect it and any other minerals, heavy metals and pathogens that may have made its way into your home.

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