CHLORINE is common, cheap, but extremely toxic. It does not decrease physical or chemical contamination, it does increase colesterol formations, is a carcinogen, and causes heart disease.
BROMINE, used in pools and spas, doesn't smell or taste as bad and doesn't kill bacteria very well.
IODINE is not practical, and is mostly used by campers.
HYDROGEN PEROXIDE kills bacteria with oxygen, is chemically made and is very toxic. It is used in emergencies.
SILVER is an effective bactericide but a cumulative poison which concentrates and doesn't evaporate.
NONTOXIC ORGANIC ACIDS should be used with caution in large water plants only.
LIME AND MILD ALKALINE AGENTS should also be used with caution only by large water plants, or only for laundry.
NEUTRALIZING CHEMICALS react with the unwanted chemicals and produce outgases and a sediment, but levels of need vary.
COAGULATION-FLOCCULATION adds chemicals which lump together suspended particles for filtration or separation.
ION EXCHANGE exchanges sodium from salt for calcium or magnesium, using either glauconite (greensand), precipitated synthetic organic resins, or gel zeolite, thus softening the water. Minerals, metals, chemicals or odors are not affected, and the water is salty to drink.