In this study the efficiency of various commercially-available coagulants, ie alum and polyaluminium chlorides (PAC1s), for the treatment of surface (river) water, or for the post-treatment of biologically pretreated urban wastewater were investigated. The different coagulants were comparatively evaluated, considering the removal of suspended solids (SS), natural organic matter (NOM), as well as the residual aluminium concentration, and the removal of phosphates (inorganic phosphorus), in the case of urban wastewater treatment. The dynamics of flocculation were also studied using a Photometric Dispersion Analyser (PDA) in order to compare the flocculation rates and the strength of different coagulant flocs. Finally, the optimum operational conditions, ie coagulant dosage, pH, temperature, duration and intensity of (initial) rapid-mixing rate, were determined for the examined cases. It can be generally supported that PAC1 coagulants were superior to alum, as they presented higher SS and NOM removal, stronger and larger flocs, higher flocculation rates and lower residual aluminium concentration, which is a very important factor, especially for the treatment of surface water destinated for potable use. (c) 2005 Society of Chemical Industry.