Youngin found this article today:
Durham Water Supply
The article mentions aluminum sulfate. See below for more detail on that.
Whether treating drinking water or wastewater, there are two main goals. One is to remove particles that are suspended in the water. The other is to remove pathogens and other contaminants that pose health risk or taste and odor problems. These goals can overlap, as the particles being removed are also contaminants. Certain pathogens or micro-organisms are removed using disinfection.
The basic process of drinking water treatment when using surface water is:
1. Screening - basically this separates the really big debris out of the raw water
2. Coagulation - chemicals are added to change the surface characteristics of particles. The particles are then more inclined to clump together. Aluminum sulfate is commonly used for this purpose, though Durham has switched to ferric sulfate.
3. Flocculation - this is more of a mixing, or mechanical, step that further encourages particles to clump together
4. Sedimentation - this step slows down the flow of water and allows time for the particles, or clumps of particles to settle out of the water. They form a sludge in the bottom of a basin, and are disposed of from there.
5. Filtration - water is filtered
6. Disinfection - the addition of chlorine or other disinfectant to kill pathogens
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