April 26, 2024

What is Wastewater Treatment and What are its Stages?

Jane Cooper

You would never drink the water straight out of your toilet, would you?

Of course you wouldn't, but there's a good chance you've consumed water that was waste at some point.

But can wastewater ever be made clean enough to drink? And how does it get processed to become something that can be used for irrigation and industrial purposes?

Well, the process of treating water starts in the toilet.

American toilets drain to wastewater treatment plants. In fact, 76% of the United States rely on an estimated 14,748 wastewater treatment plants, according to ASCE.

But what happens after the water is sent to a wastewater treatment facility? And what health implications are caused when a step in this process doesn't go according to plan?

Continue reading to find out what is wastewater treatment, how it works, and how wastewater can be processed into a drinkable liquid.

What is Wastewater Treatment

So what is wastewater treatment?

It's the process of removing pollutants from wastewater and converting it into something free of solids and organic matter. This allows the treated water to be returned into an ecosystem with little-to-no impact on the environment.

There are five steps in the wastewater treatment process: pre-treatment, primary treatment, secondary treatment, the disinfection process, and the release.

Improper execution and/or skipping any of these steps can have disastrous effects on local water supplies and public health.


Once wastewater arrives at a water treatment plant, it goes through the process of pre-treatment.

This begins with the wastewater flowing through a bar screen. A bar screen is a mechanical filter used to separate larger things from the wastewater.

Unfiltered sediments and organic material can accumulate and cause major clogs. This can slow down the water treatment process overall and clogs can result in downtime for a wastewater treatment facility.

They also tend to make a mess.

The items removed from the bar screen then get sent to landfills. After processing through the bar screen, the wastewater gets sent through a grit chamber.

Grit chambers are narrow and long tanks that lower the velocity of flowing water.

This allows small particles like coffee, eggshells, and rocks to settle out. These particles have to be sifted out because they can't be removed with chemicals.

The sediments settled in grit chambers can cause excessive wear on other pieces of equipment in the wastewater treatment process. The grit can also clog and destroy pumps.

Now that the water has gone through pretreatment, it's ready for primary treatment.

Primary Treatment

After sediments have settled in the grit chambers wastewater can move on to a primary clarifier.

A primary clarifier is a tank that holds wastewater and allows heavier solids to settle at the bottom, becoming sludge. The lighter materials float to the surface of the water, where it becomes scum.

Primary clarifiers rely on settling velocity. Settling velocity is the speed at which a particle settles. As wastewater is pumped into a primary clarifier, its flow rate cannot exceed the settling velocity of the water.

After completing primary treatment, wastewater becomes effluent - a slightly treated wastewater free of solids and organic material.

Now the effluent can move on to secondary treatment.

Secondary Treatment

This is the last step of the water treatment process that removes solids and larger biological matter.

Secondary treatment begins with the effluent flowing into an aeration basin.

An aeration basin holds the effluent and artificially adds air to it. This process, known as aerobic digestion, breaks down the organic matter left in the effluent.

Some older water treatment plants use an additional step ahead of the aeration basins involving biofilters. Here, the effluent is poured over stone or plastic and bacteria eat away at the organic material in the water.

Generally, this process isn't used anymore because it isn't as efficient or effective as modern wastewater treatment practices.

Next, the effluent is pumped into a secondary clarifier. In the secondary clarifier, the sludge is removed and pumped back into aeration basin.

Now, the effluent is ready for disinfection.

The Disinfection Process

By the time effluent reaches the disinfection process, 85% of organic matter has been removed from it.

The disinfection process removes the harmful pathogens still in the effluent. It can be accomplished through one of three steps.

  • Chlorine Disinfection - Imagine chlorine disinfection as a process of bleaching the effluent. This is a chemical form of disinfection, and the chlorine must be removed from the effluent before it can be discharged to stream or lake.
  • Ozone Disinfection - This process involves pumping an electrical current through the effluent to disassociate diatomic oxygen molecules (O2) so they can combine with a free oxygen molecule and form ozone (O3). The process of ozone disinfection can kill as much as 99.99% of bacteria.
  • UV Disinfection - UV disinfection uses ultraviolet light to sterilize the harmful microbiomes left in the effluent. Even though the microorganisms aren't removed, they are dead and harmless.

Now the wastewater has been stripped of all large and microscopic organic and inorganic materials and it has been disinfected. The effluent can finally be released.

Effluent Release

Effluent release generally exists in two forms.

The treated wastewater can be released into local streams or waterways since it is no longer harmful. This allows the effluent to rejoin the water cycle of the local ecosystem.

But in areas where water is scarce, effluent can also be discharged into another treatment plant. Here, the effluent is treated for consumption.

This is typically not practiced in the United States. However, some cities and municipalities are considering pumping effluent into their aquifers to increase their water supply.

Reclaiming Water

Though it may sound like a long journey, the water treatment process only takes about 24 to 36 hours from start to finish.

Water sanitation is a dirty job, but you can thank the 14,748 water treatment plants for the peace of mind you have after you flush the toilet.

If you're still wondering what is wastewater treatment, if you have questions about the wastewater treatment process, or if you are hoping to use water treatment at your business, please contact us.

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