The study involved characterization of denim-processing wastewaters, mainly to generate the necessary experimental data for the modelling and evaluation of the activated sludge process. The striking feature of the wastewater quality was the excessive suspended solids content requiring effective removal before biological treatment. COD fractionation was, however, typical for a textile effluent in general, with a biodegradable fit-action of 90%, a readily biodegradable COD ratio of 20%, a predominantly soluble slowly biodegradable fraction of 55-60% and negligible particulate inert COD. Hydrolysis was identified as the significant step in the biodegradation kinetics with rate coefficients quite specific to plant operation. Evaluation of the hydrolysis kinetics showed that the magnitude of the slowly biodegradable COD could be reduced with a higher hydraulic detention time, effectively improving the quality of the soluble effluent. (C) 2001 Society of Chemical Industry.