Which of the following is NOT one of the four basic processes for removing iron or manganese?

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Removing iron and manganese from water relies on a few primary ways to get these contaminants out: oxidizing dissolved iron so it can be precipitated, using ion exchange to capture the iron and manganese ions, applying lime-softening to precipitate iron when hardness is high, and then filtering to remove the resulting solids. Coagulation plays a supportive role by helping particles clump together so they settle or get filtered more easily, but it isn’t counted as one of the four fundamental removal methods by themselves. So the process that isn’t considered one of the core four is coagulation, since the four basics focus on oxidation, ion removal, softening, and physical removal by filtration.