This paper evaluates the effects of the pipeline retention time in sewer systems and the dilution by stormwater runoff on COD fractions and kinetic parameters of samples from two large-scale wastewater treatment plants (WWTPs) fed with combined sewers, one collected from adjacent residential areas (Sewer Q) and the other undergoing long pipeline transportation (Sewer B). With four batch tests, COD fractions and kinetic parameters of two sewers were obtained by numerical computation based on activated sludge models (ASMs) modified by a dual hydrolysis mechanism. Long pipeline transportation significantly influenced the content and existence of heterotrophic biomass fractions in sewers, and the percentage of the fraction in Sewer B (17%) was higher than that in Sewer Q (9%). Compared with Sewer Q, long pipeline retention also resulted in lower concentrations of hydrolysable fractions and particulate inert COD in Sewer B, as well as an increase in the hydrolysis rate coefficient of particulate hydrolysable COD from 0.90 day(-1) in Sewer Q to 1.83 day(-1) in Sewer B. Results of samples from Sewer B during a typhoon event showed high variations of COD fractions and parameters, indicating the necessity for COD fractionation and parameter calibration before applying ASMs to wet weather cases. (C) 2008 Society of Chemical Industry