BACKGROUND: The study rationale was to explore the potential of particle size distribution (PSD) to support COD fractionation of sewage. Accordingly, themain aim focused on establishing the experimental relation between COD fractionation and PSD on two samples of a selected source, one representing the average character of the sewage and the other a weaker sample. PSD analyses were also conducted on effluent samples. RESULTS: PSD profiles of COD yielded the typical bimodal distribution, one major peak representing 65% of COD, higher than 1600nm and the second peak of 10% lower than 2 nm. Results were evaluated to underline the potential of PSD in complementing COD fractionation as related to biodegradation. PSD exhibited a close relationship with COD fractions with different biodegradation characteristics and defined all major COD fractions except the initial soluble and particulate inert COD fractions. CONCLUSION: Selection of appropriate values for inert COD fractions together with default kinetic information available for sewagewould enablemodelling anddesigning of variousbiological treatment alternativeswithout theneedfor timeconsuming experiments on COD fractionation. Furthermore, PSD of biological treatment effluent provided concrete evidence for the generation of soluble residual products, which should be part of all modelling evaluations. (C) 2017 Society of Chemical Industry