One of the most main uses of aluminum sulfate is in water treatment and purification. When added to water, it causes microscopic impurities to clump together into larger and larger particles. These clumps of impurities will then settle to the bottom of the container or at least get large enough to filter them out of the water. This makes the water safer to drink. On the same principle, aluminum sulfate is also sometimes used in swimming pools to decline the cloudiness of the water.