BACKGROUND: Household wastewater was segregated to grey and black water components as part of a recent approach aiming to separate wastewater fractions for more efficient treatment and reuse. Chemical oxygen demand (COD) fractionation and biodegradation characteristics of black and grey water were determined by means of model calibration of the oxygen uptake rate profiles. An ASM1 type of model modified for endogenous respiration was selected for this purpose. RESULTS: Calibration identified, aside from the readily biodegradable COD, two hydrolysable COD fractions in the black water, and only a single, slowly biodegradable COD fraction, in the grey water. The relative magnitude of the readily biodegradable COD fraction was high compared with typical domestic sewage, especially for grey water, and the level of particulate inert COD fraction, was relatively low. Biodegradation of soluble COD fractions in both wastewaters approximated typical rates observed for domestic sewage. However, the hydrolysis rate of the slowly biodegradable COD was quite low, reflecting the characteristics and impact of settleable organic matter. CONCLUSION: Based on the process rates obtained, aerobic biodegradation was equally effective for black and grey water fractions, both for the removal of soluble COD and the stabilization of particulate COD. (C) 2010 Society of Chemical Industry