Some methods for the preparation and use of “Activated Silica” as an aid to aluminium sulphate in the coagulation of polluted waters arc described. It is shown that transfer from laboratory tests to full scale plant operation involves certain modifications for the successful use of activated silica. Suitable ratios of silica to aluminium sulphate are mentioned, and general methods of operating the process arc discussed. Reference is briefly made to relative running costs, and to some advantages that may be derived from the application of this system.