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What Does a Waste Water Treatment Plant Do?

Panoramic View of Modern Wastewater Treatment Plant

Panoramic View of Modern Wastewater Treatment Plant

A mong the advantages that make the comfort of the 21st century, clean water and complex sewer systems are no longer novelties in the big cities. The common citizen doesn’t even need to know how the water gets to their home or what happens with the wastewater unless they come to deal with an incident that may involve improper water treatment or water pollution in their area.

Nonetheless, understanding the process that stands behind wastewater treatment and disposal can offer you a broader view of how the municipality is handling this problem. It can be quite fascinating to break the process into pieces and see how the water is treated before being discharged into a lake or another water source without affecting its ecosystem.

Where Does All the Dirty Water Go?

You do the dishes, wash your hands, and flush several times a day. All the dirty water is carried away in a minute. But where does it go? Sometimes, when a sewer system isn’t available, it will end up in a septic tank in the back of the yard. But if we are talking about a big city, it will be carried through the sewer system to a treatment base, also known as a wastewater treatment plant. This is a complex cleaning and filtering system, usually located next to a river or a lake where almost all the residues in the water are cleaned before the water is discharged back into the nearest body of water.

Wastewater Purification and Treatment Plant

Wastewater Purification and Treatment Plant

How Does a Wastewater Treatment Plant Work?

Filtering wastewater is far from being a simple process, as it isn’t contaminated only with one type of pollutants. Thus, most municipal treatment facilities employ either 2-stage or 3-stage processes. While the treatment can vary from one plant to another, in the following paragraphs we will describe the process that is most often used.

Primary Treatment

For the water to enter the first treatment stage, it needs first to be collected and brought to the treatment facility. The system has been well-thought to require minimal energy costs. Thus, most sewer openings are located in places below the sea level, so the rainwater can flow into them using only the gravitational force. Most pipes are inclined to transport the wastewater without any help. However, in areas where this method doesn’t work, pumps are used to drive the water to the facility.

The primary treatment consists of the following stages:

  • Screening – as the water prepares to enter the treatment process, it first goes through a screen, which is in charge of removing large debris that can include sticks, rags, or even dead rats. As the system consists of a series of pipes and reservoirs, these floating objects can end up clogging them.
  • Settling – the next stop is the grit chamber, where sand and small rocks are left to settle. These elements usually end up in the sewer when it rains, as they are carried by rainwater.
  • Sedimentation – there are still plenty of solids floating at the surface of the water that won’t settle in the grit chamber. In the sedimentation tank, the water flow is diminished to allow the floating solids to sink. They are further pumped out and usually used as fertilizers.

Secondary treatment

As it was proven, primary treatment isn’t sufficient to release clean water back into nature, as this stage only cleans the inorganic elements. The water is still filled with organic residues and lots of bacteria. This is why modern wastewater treatment facilities use a second treatment stage, which includes the following steps:

  • Activated sludge process – after going through the first cleaning stage, the water goes through an aeration tank, where it is pumped with oxygen and bacteria-filled sludge. The bacteria will break down organic matter into compounds that are considered harmless. The oxygen is added to enhance this process, which takes a few hours. At the end of the step, the sludge is preserved to be used with another flow of wastewater.
  • Sedimentation – this stage has the purpose of removing the excess of bacteria.
  • Disinfection – sedimentation cannot remove all the bacteria in the water, so chlorine is added to kill up to 99% of them.
  • Dechlorination – as the chlorine acts upon the bacteria, it gets consumed, so its traces are rather weak in the resulted water. Nonetheless, some governments require for the chlorine to be removed before the water is discharged.

More advanced treatment plants can add a third or even a fourth treatment stage to remove pathogens, metals, nutrients, and odors and make the water safe to drink.

Panoramic View of Modern Wastewater Treatment Plant

Panoramic View of Modern Wastewater Treatment Plant

Is This Method Reliable?

In theory, it is. The system is performant and, as explained, all the water that goes through it will be released clean in a proportion of 99%. Then, how do you always hear about wastewater polluting our lakes and oceans? Are people or companies responsible for this? They can be. Among the main reasons natural waters get polluted is that wastewater doesn’t follow the path to treatment. Some people discharge it directly into nature, and sometimes even municipalities do. Because it is cheaper.

Regarding the big companies that use water as a part of the manufacturing process, they cannot send the water as it is to a treatment plant, as it won’t be able to cope with heavy metals and other chemical substances. They need to do some treatments of their own first. Nonetheless, they cost, and this is the reason highly-polluted wastewater ends up in nature.


Wastewater treatment plants are the ones keeping our cities clean. They work continuously to remove debris and pollutants from the wastewater so the fish and other underwater creatures can multiply and continue to populate our lakes, seas, and oceans. Nonetheless, it seems that it isn’t sufficient to send our wastewater to the plant and sleep sound, as the water pollution problem is still present and seems to gain even bigger proportions.

Tobey Hunter
Tobey Hunter
Tobey is the editor-in-chief at P2Rx™, his experience both in the field of journalism and a keen interest in the topic of pollution, a subject he previously covered in his career in multiple pieces, making him the expert of our team. What Tobey learned from his background is that researching each topic thoroughly is the only guarantee that an article will depict a truthful picture, a policy that he strictly follows. In his off time, he indulges in reading modern literature and binging on the latest TV shows.

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