Aluminum Sulfate acts as a flocculent
One of the problems with pool filters is that they are unable to trap dirt or foreign matter below .02 mm (20 microns), resulting in cloudy water (assuming the total alkalinity, pH, calcium hardness and TDS are all within the recommended limits).
Aluminum Sulfate as a Flocculation is the process of adding Aluminum Sulfate to pool water which bind or stick tiny particles together, resulting in larger clumps which settle to the bottom of the pool. This process leaves the water clear, and the "clumps" can be easily removed by vacuuming the floor of the pool.