Alum or Aluminium Sulphate is a flocculant product used in water treatment to coagulate (bind together) colloidal very fine dirt particles and other organic materials so they fall to the bottom of a tank where they can be removed. This process then leaves the water clear. Alum is also used to remove phosphorus from water. Alum used to be used extensively to treat town water supplies. However it is being phased out due to the carryover of aluminium into the water supply. Other more synthetic chemicals are now generally used.