Inorganic pre-polymerized coagulants: current status and future trends

The aim of this paper is to provide an overview of the evolution of inorganic pre-polymerized coagulants by summarizing the latest developments in their composition and properties. Considerable attention has been paid in recent years to preparing pre-polymerized metal-ion coagulants, based on either aluminium (e.g. polyaluminium chloride, PAC), or ferric ion (e.g. polyferric sulphate, PFS), as they exhibit a better performance than the conventional coagulants (aluminium or iron (III) based salts). However, their efficiency is still much lower than the organic polymeric coagulants. Therefore, attempts have been made to improve further the efficiency of pre-polymerized coagulants either by producing a mixed coagulant (containing both aluminium and iron), or by adding other components to produce new composite coagulants. Concerning the second approach, the additive most frequently used is the polysilicic acid, while it is suggested but yet not well established that organic compounds (e.g. polydimethyldiallylammonium chloride, PDMDAAC) may also be employed for such a purpose. Finally, the modified inorganic pre-polymerised coagulants were found to exhibit a better coagulation performance in treating water and wastewater, when compared with the non-modified ones.