Water can be disinfected with heat, chemicals, or light. Boiling water, at a rolling boil, for 5 to 8 minutes kills bacteria and parasites like giardia and cryptosporidium. Boiling water concentrates nitrate levels. Therefore, don't boil if there's nitrate in the water. Water can also be disinfected with chlorine, bromine, iodine, or ozone.
Chlorine also oxidizes iron and manganese so they can be filtered out.
High concentrations of chlorine give water a bad taste. They may combine with other components to form cancer-causing compounds. Ozone may be used to disinfect public water supplies. It is rarely used for private water supplies. Chlorine is used most often to disinfect private wells contaminated by disease-causing or nuisance bacteria.