Effects of Lime & Alum on Water Purification

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Water treatment requires many chemicals to carry out the goal of making water pure. Chemicals take unwanted substances from the water, destroy dangerous bacteria, help prevent cavities in teeth and help keep water pipes corrosion free.

Two of the many chemicals that are used in the water purification process are aluminum and lime.

Water Purification Process

The water purification process in most municipalities has six steps. They are coagulation/flocculation, sedimentation, filtration, stabilization, fluoridation, and chlorination. Aluminum sulfate, or filter alum, is added to the water coming into the purification facility during coagulation/flocculation. Adding hydrated lime is the next step that occurs during sedimentation.

Aluminum Sulfate

The chemical formula for aluminum sulfate is Al2(SO4)3. Frequently, it is known as filter alum. In water purification, a mixture of 48 percent filter alum in a water solution is combined with the raw incoming water at a rate of 18-24 milligrams per liter.

Alums are found in many household products such as deodorant and baking powder.

However, in water purification processes it is as a coagulant. A coagulant binds extremely fine particles suspended in raw water into larger particles that can be removed by filtration and settling.

This allows for the removal of unwanted color and cloudiness (turbidity). Additionally, the process removes the aluminum itself.


Hydrated lime's chemical name is calcium hydroxide, and its chemical formula is Ca(OH)2. When purifying water, adding hydrated lime to the water for pH adjustment is a part of the process.

Filter alum is an acidic salt that lowers the pH of water undergoing purification. Adding hydrated lime to this process between the sedimentation and filtration steps at the rate of 10 to 20 milligrams per liter neutralizes the effect of filter alum on the processing water.

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