is it done mechanically or by people?
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Wastewater is treated by physical, chemical, and biological means.
A standard wastewater treatment plant begins with screening. This removes large items – everything from diapers to wet wipes to milk cartons – from the water.
Grit removal (settling out though gravity) removes dense things that don’t belong in further treatment, like gravel and rocks.
Historically, wastewater treatment plants have done primary sedimentation – settling out of the more solid parts of human waste. A coagulant, or chemical that causes larger particles to clump together, is usually added before this step. Nowadays this step is followed by biological treatment and secondary sedimentation (having microbes eat as much waste as possible, then settling them out of the water). This typically involved aeration, or adding small bubbles of air or even pure O2 to the water to keep aerobic bacteria growing.
This is followed by filtration, generally, to remove smaller particles.
Finally, a chemical disinfectant like chlorine is added to the water – and then removed, because disinfectants can harm wildlife. Other disenfection methods include UV light and ozone – since these only affect water when in contact with it, it’s better than having to add and then remove disinfectant. Just know that disenfection is a combination of method or chemical strength and contact time needed to kill or prevent cells from reproducing.
There are a few other processes wastewater can go through – for example, some plants have a third main step (“tertiary treatment”) designed to add more air to the water.
What’s allowed in the water when it leaves the wastewater treatment plant is based on local and federal regulations, ensuring that the plant effluent is clean enough to go into the environment.
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