Wastewater treatment – an unknown privilege

Wastewater treatment (=Abwasserreinigung) is a crucial process that we often take for granted. We rely on it to keep our water (lakes, rivers, etc.) clean and free from harmful pollutants. But how does it actually work?

Understanding the basics of wastewater treatment can help us appreciate the privilege of having access to clean water and inspire us to take action to protect it…

What I want you to learn

To put it simply, nothing works without wastewater treatment. It’s important for me to raise awareness about this issue, as we depend on water every day. If you’re interested in what happens to water after it leaves the drain, this article is for you. I also tried to get an impression of another country or continent. Actually, an interview about the wastewater situation in Ethiopia was planned, but it never took place. So, unfortunately, I can only give you a very superficial impression.

Conclusion about the wastewater process

800 wastewater treatment plants throughout Switzerland provide a lot of treated wastewater. But to do this, the wastewater must first reach the treatment plants (=Reinigungsanlagen). This is ensured by Switzerland-wide sewers (=Kanäle) with a total length of 130’000 kilometers. [1]  Every wastewater treatment plant is different. Sometimes a different system is used, other times it needs a different composition of chemicals.

For my experience, I was able to visit the wastewater treatment plant in Rüti ZH (rueti.ch). The small sewage treatment plant is responsible for three municipalities and about 14,500 inhabitants. The master of the sewage plant was able to give me a tour and show me how their wastewater treatment plant works.

The first step

The wastewater first flows through a massive rake (=Rechen), where items such as wood or textiles get stuck while the remaining water can pass through. The wastewater then enters a grease and sand trap (=Fett- und Sandfangbecken), where, thanks to simple physical processes, the grease collects on the surface and the sand sinks to the bottom. The water then continues on to the “pre-clarification basin,” (=Vorklärbecken) while the grease and sand are periodically removed by a machine.

Massive rake through which the waste water flows
Massive rake (=Rechen) through which the waste water flows


Grease and sand catch basin in a water purification plant
Grease and sand catch basin

The second step

In the primary clarifier, mainly fats and sludge are separated from the wastewater. For this purpose, a so-called “scraper” (=Schaber) is used, which removes the fat from the water surface on one hand and removes the sludge that has sunk to the bottom on the other hand. Now a scraper slowly moves in a circle and takes the fats and sludge with it. At one point in the circle, a hole opens up, into which the substances to be removed can flow.

Primary clarifier - a large, round cleaning basin - in a wastewaster treatment plant
Primary clarifier

The third step

The previous stage of wastewater treatment is also known as “mechanical treatment”, (=mechanische Stufe) while the next stage is called “biological treatment” (=biologische Stufe). The previously treated wastewater still contains many nutrients and carbon compounds that can cause hygiene problems in bodies of water. In the “aeration tank” (=Belebtschlammbecken), these substances are converted into carbon dioxide and water through a biochemical process involving bacteria and other tiny organisms. The bacteria are essential for the purification process, and fortunately they reproduce naturally.

Aeration tank in a wastewater treatment plant
Aeration tank

The last stage of wastewater treatment is the “chemical treatment.” (=chemische Reinigung) The water flows into a “final clarifier (=Nachklärbecken).” In essence, the water is treated in the same way as in the primary clarifier, but the process is refined many times over. In the preceding activated sludge tank, activated sludge flocs are generated, which are now filtered out in this final step. The flocs settle in the water and are separated from the rest. In addition, the wastewater can pass through a sand filter to ensure that all activated sludge flocs are removed. After this final step, the wastewater has become water again and is clean enough to be discharged into a river.


final clarifier in a wastewater treatment plant
final clarifier

A look at another place

Taking a closer look at the world around us, one quickly realizes that clean water is far from being a given. With this in mind, I set out to explore the state of water supply in Africa. To get a good impression, I was looking for a suitable expert.I was even lucky enough to find someone to get more information about Ethiopia. Despite a successful email promise, it never came to an interview. This is because the expert herself often travels abroad and is therefore difficult to reach. But since I still wanted to find out something about it, I gathered some general info:

One thing that surprised me right from the start of my research was the apparent regional disparities. For instance, while 99% of people in Egypt have access to clean drinking water, in the Central African Republic, this number drops to a mere 36%. Another significant issue is the inefficient or non-existent utilization of groundwater, resulting in an alarming loss of water resources. Additionally, the lack of infrastructure poses a major challenge. Despite various projects worldwide, African countries often lack wastewater treatment facilities, leaving them with no means to purify their water. Only time will tell what the future holds and whether we can eventually overcome water pollution and scarcity on a global scale. [2]


Polluted water can cause big damage and destroy entire ecosystems. Only thanks to wastewater treatment plants do we benefit from clean water in Switzerland: a privilege that we often only become aware of when we no longer encounter it as a matter of course in other parts of the world. Of course, wastewater treatment plants also pose new challenges in the form of new substances that enter our wastewater. Microplastics or hormones are two that come to my mind currently. But thanks to ongoing technological progress, we don’t have to worry in the near future about whether or not we can drink water from a well in Switzerland.

Janis G.

In addition to the information I have taken from the interview, I have supplemented my text with information from the following pages:
[1]  Zahlen und Fakten, (bafu.admin.ch),  2017

[2]  Situation in Afrika (science.orf.at), Raphael Krapscha, 2022

The photos are self made and I own all rights for the pictures.

Reviewers: Yuri D. and Damian K. 

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