7 Steps of the Water Purification Process at a Public Water Treatment Plant

Covering the process by which water is sourced, treated, and distributed at a municipal water treatment plant

Updated: June 25, 2023
Jeremiah Zac


There are over 148,000 public water systems in the US, supplying water to over 286 million Americans, or 87% of the population.

If you’re one of the many getting their tap water from a public source, it’s important to understand how these utilities operate and deliver water to your home.

While many of the contaminants that can cause major health problems are removed before reaching the tap, there may be some trace particles that make it past the filtration process—knowing what these are, how to test for them, and how to properly filter them may be beneficial for the household.

This article will cover what you need to know about public water sources, the 7 steps involved with the water purification process, and how you can ensure you’re receiving clean water.

What is Public, City, or Municipal Water?

Whether referred to as public, city, or municipal water, the entity that manages the collection, treatment, and distribution of water to a community is called a utility.

A public water supply refers to water being supplied to at least 25 people or having a minimum of 15 connections—this equates to about 87% of the population relying on public water.

That means, if your home isn’t receiving water from a private well, it’s most likely getting water from some public utility.

How do public water utilities work?

Regulations on water treatment facilities are upheld by the EPA under the Safe Drinking Water Act, which states that the quality of drinking water coming from the facility must meet specific quality standards.

It’s important to note that it’s only the water flowing out of the treatment facility and toward residents’ homes that must meet these standards, but the process by which the facility obtains this level of quality isn’t monitored.

This means that water civil engineers have quite a bit of freedom in how a water treatment facility operates.

There are several common treatment processes used across many facilities across the United States, but many vary greatly in design. The actual design of the treatment process is largely dependent on the quality of the water being sourced.

For example, water sources containing high levels of microorganisms may require a different treatment process that water sources higher in organic chemicals.

The following is the most common process for public water treatment facilities.

7 Steps of the Water Purification Process

I. Source Water Collection

In public water treatment, the initial process starts with locating and accessing suitable bodies of water—whether surface waters like rivers, lakes, and reservoirs, or underground aquifers reached through wells—that are adequate enough to supply the hefty water demands of a municipality.

Determining the choice of water source necessitates considering a myriad of factors such as the source's closeness to the served population, the available water quantity and quality, and the regional environmental and geological conditions—an urban setting near a substantial river may lean heavily on this river for its water supply, whereas a township nestled in a more parched landscape could find itself relying on well water, extracted from an aquifer.

After settling on a water source, water gets pumped and channeled to the water treatment facility via an intricate pipe system, and at this juncture, the water's quality is scrutinized in an initial assessment to identify the degree and variety of present impurities, an evaluation that empowers water civil engineers to customize the treatment process for optimal results based on the specific water characteristics in play.

II. Screening

Screening is the first step of the actual water treatment process after the water has been transported to the facility—it is designed to remove large debris materials from the water, such as leaves, branches, and even trash.

This screening process is essential as it prevents large debris from damaging or clogging the machinery used in later stages of treatment, allowing only water and smaller particles to pass through, ultimately improving the effectiveness of subsequent treatment processes.

III. Coagulation and Flocculation

The coagulation and flocculation stages are tasked with large-scale removal of suspended particles, priming the water for subsequent, more refined purification techniques.

As suspended particles have a natural tendency to repel each other, the introduction of a coagulant like aluminum sulfate swings into play, offsetting these particles' charges and enabling them to come together.

Subsequent gentle stirring transforms these particles into more substantial clusters, known as 'floc,' facilitating their filtration and extraction - this phase is termed flocculation.

As the floc enlarges in size and weight, it readies itself for the forthcoming sedimentation process.

IV. Sedimentation

The sedimentation tank is the next stop for the water, now laden with larger and denser floc clusters; in this calm setting, the floc particles gradually descend to the bottom, creating a sludge layer at the tank's base.

Concurrently, the cleaner water, now mostly devoid of solid particles, stays afloat and is carefully transitioned to the subsequent treatment phase, often filtration. Periodic sludge disposal from the tank's bottom ensures the sedimentation process's continued efficacy.

V. Filtration

Post sedimentation, the water journey advances to the filtration stage, where it navigates a sequence of media purposefully designed to ensnare the smaller particles that might have slipped past the sedimentation process, thereby purifying the water that finally emerges from the filter.

This filtering medium usually features distinct layers of sand and gravel, but an omnipresent layer of activated carbon—renowned for its exceptional contaminant-trapping prowess—solidifies the arrangement.

VI. Disinfection

Upon completing the steps of sourcing, screening, coagulation, flocculation, sedimentation, and filtration, the water reaches the stage of disinfection, a process vital to its safety through the eradication of harmful bacteria, viruses, and other health-threatening microorganisms.

The most commonly employed disinfection method is chlorination, a practice that involves the introduction of chlorine compounds into the water, used for their potent microorganism-killing capabilities and their residual effect safeguarding against potential contamination post-treatment.

Another prevalent disinfection technique utilizes ultraviolet (UV) light, where infectious microorganisms undergo DNA damage, inhibiting their reproduction and neutralizing their harm.

There are occasions where disinfection methods may differ, employing techniques like ozonation or advanced oxidation processes, dictated by the treatment plant's specific requirements and abilities.

VII. Storage and Distribution

Once water has been effectively treated – having gone through the stages of screening, coagulation, flocculation, sedimentation, filtration, and disinfection – it is ready to be stored and distributed to the public.

Storage is a crucial stage in the water treatment process as large reservoirs provide a buffer capacity, allowing the treatment plant to meet variations in demand throughout the day – demand often peaks in the morning and early evening, and is lower late at night.

Secondly, the storage reservoirs provide an emergency reserve of water in case of an interruption to the supply, such as a power failure at the treatment plant or a break in a distribution pipe.

Is it Safe to Rely on Public Drinking Water?

The EPA is the governing body that ensures the water leaving the treatment facility and entering residents’ homes meets quality standards that are safe for drinking.

Under the Safe Water Drinking Act, all public water services are required to keep certain contaminants at concentration levels that the EPA determines to be safe.

They employ experts to run extensive studies to determine safe concentration levels for each contaminant and have strict requirements for updating and maintaining these standards.

The entire list of contaminants and their required levels can be seen on the EPA website.

Then why are contaminants still showing up in the water coming from my faucet?

Even with all of these treatment standards, the total number of contaminants making it through the treatment process will never be zero. It is nearly impossible to completely remove all contaminants from water apart from distillation.

The long-term effects of consuming even trace amounts of some of these chemicals, particularly pharmaceuticals and microplastics, are yet to be studied thoroughly.

While the United States is still far better at public water treatment than many developing countries, a home water filtration system is the best bet in taking full control of your water supply.

What if the public treatment system fails?

An accident or natural disaster may cause unexpected complications or failure at the treatment facility, resulting in a compromised public water supply. In the event that the quality of the water supply comes into question, the city may issue a boil water advisory to its residents, which instructs them to boil the water before drinking until the advisory has been lifted.

What about those who rely on private wells?

It is also important to note that SDWA standards only apply to public water systems, not private ones. So, a home relying on a private well for its drinking water source won’t be monitored by the EPA and will be responsible for testing and filtering independently.

Thank you for taking the time to read our article on the 7 steps of the water treatment process. We'd love to hear your feedback in the comments section below. If you've found this article to be useful and are interested in learning more, be sure to sign up for our newsletter.

Share this post!

I'm Jeremiah, the owner of World Water Reserve. I'm a writer and researcher with a particular interest in sustainability and rural living, water scarcity, and innovative water purification methods. I utilize my multimedia and communication experience in the NGO and humanitarian fields to bring light to important topics. My passion is to educate others on the reality of the global water crisis and on ways to sustain themselves and their families in the midst of it.
Jeremiah Zac