The chemical method is to use various chemicals to convert impurities in water into substances that are less harmful to the human body or to concentrate impurities. The oldest chemical treatment method should be regarded as adding alum into the water, after the collection of impurities in water, the volume of impurities in the water becomes larger, and the impurities can be removed by filtration. Commonly used water treatment agents are: poly aluminum chloride, poly aluminum ferric chloride, basic aluminum chloride, polyacrylamide, activated carbon and various filter materials.