Hasa 70086 5 lb Pail Alum. Alum has a dual purpose and may be used as an additive to sand filters to improve filtration or can be applied directly to the pool as a floc. When added to a sand filter, Alum coats the filter bed and traps smaller particles that the sand alone cannot. When severe water clarity issues occur, it may be easier to "floc" the pool. This process drops the suspended particles to the bottom of the pool which are then vacuumed to waste, rather than filtering.