Aluminum sulfate serves as a coagulant and flocculating agent in water treatment, making the water easier to clean. It is mainly used in municipal water treatment plants to treat wastewater and potable water. Aluminum sulfate is also important for lake treatment and restoration.
Untreated wastewater and potable water have negatively charged suspended colloids, and water treatment systems use positively charged metallic salts, such as aluminum sulfate, to destabilize the colloidal particles. As a coagulant, aluminum sulfate enhances the removal of particulates, such as dissolved organic carbon, and other suspended tiny particles at the microorganism level. After coagulation, aluminum sulfate acts a flocculating agent and enables these particles to stick together and form bigger particles, allowing easier purification of the water through sedimentation and filtration. The use of aluminum as coagulants often results in higher levels of aluminum in the water, but usually at safe amounts.
While aluminum sulfate helps remove large amounts of organic compounds in potable water and wastewater, it isn't enough to remove pathogens, such as bacteria and viruses. However, aluminum sulfate is a primary step in water purification, as it prepares the water for further filtration. Because aluminum sulfate reduces dissolved substances in the water, less chlorine is needed to disinfect the water. Most aluminum sulfate is also removed with the tiny sticky particles, or floc, formed during the coagulation and flocculation processes.