The effect of chemical settling of tannery wastewaters on the kinetic behaviour and performance of activated sludge process is investigated on the basis of experimentally supported model evaluations. Experiments involved besidese conventional characterization, detailed COD fractionation and assessment of major kinetic and stoichiometric coefficients by means of respirometric measurements. A multi-component model based on the endogenous decay concept was used for the kinetic interpretation and design of activated sludge. Results indicated that chemical settling as a pretreatment step induced a totally different kinetic pattern, significantly improving the biological treatability of tannery wastewaters. Design implications were evaluated in terms of major parameters of practical importance, such as effluent quality, activated sludge composition, aeration volume, oxygen demand and sludge generation. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.