When we take a medicine, a part of it is eliminated from our bodies through pee or poop, which then makes a long journey to reach the sewage system. Here the story begins. In the sewage system, human excretions pass through a treatment plant, where many pollutants are removed. This less polluted water is then returned to the rivers, which will then reach the sea. Unfortunately, however, in many places in the world, there are no sewage systems and human excretions end up directly in the environment, which creates a lot more pollution.
Generally, the water of rivers is also used for irrigation of crops and is used by humans for drinking and other purposes. Have you ever seen water blowers in crop fields? This is irrigation at work. What happens if there is something in the water that is not supposed to be there, like a pollutant? You are right. It will be spread all over the plants! Almost every city has some kind of water treatment that happens in specific places, called water treatment plants, or WTPs. In this case, the water goes through another stronger process of purification, so that we can safely drink it.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2019.00081