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I’ve seen that there is an unofficial series of sorts on Ricochet, where members write posts explaining what they do in their day jobs. I work for a company that does wastewater treatment (for industries, so my perspective is a little different from your local municipal wastewater treatment plant). This post is my contribution.
The motivation for treating wastewater is to avoid or mitigate negative impacts on the river, lake, ocean, or other area into which it flows. Common negative impacts include filling waterways with debris or sediment, causing fish kills or dead zones by depleting oxygen levels, promoting algae blooms, and spreading pathogens that can harm other people who use that water. In some cases, there may also be concerns related to specific heavy metals or other chemicals. I’ll focus on oxygen depletion, but feel free to ask about other impacts in the comments.
When organic matter (and there is plenty of that in wastewater) gets discharged into the environment, it gets degraded by microorganisms. These decay processes consume oxygen, which dissolves fairly poorly in water. If the amount of organic matter is high enough, the rate of decay can exceed the rate at which more oxygen can dissolve into the water, and the level of oxygen can drop to the point that fish and other things living in the water start to die.