The objective of this study was to evaluate the optimal location of ozonation within biological treatment for a typical tannery wastewater by giving special attention to biodegradability-based chemical oxygen demand (COD) characterization. As treating the raw tannery effluent solely by biological treatment is not adequate to meet the discharge standards owing to the high level of biorecalcitrant COD at the outlet, the application of chemical oxidation, i.e. ozone together with biotreatment (pre-ozonation or in mid-ozonation or post-ozonation) was investigated. The tannery effluent under investigation had initially inert soluble COD (S-I1) and particulate COD (X-I1) fractions corresponding to 9% and 13% of the total COD (C-T1), respectively, whereas each component of the biodegradable part - readily biodegradable COD (S-Sl)5 rapidly hydrolysable COD (S-H1), and slowly hydrolysablc COD (X-S1) - accounted for around 26% of the total COD (C-T1). Pre-ozonation, undesirably competing with biotreatment for the removal of degradable organics, was shown to be insufficient both in terms of total COD (C-T1) and inert COD (C-I1) removal efficiencies. The scheme of biological treatment + ozonation + biological treatment could be applied successfully when 42. 8 mg O-3 min(-1) was introduced for S min with a utilized ozone percentage of 76% at a point in biological treatment where the readily biodegradable COD (S-Sl) was depleted through biochemical reactions. Such an alternative yielded satisfactory outcomes when both total COD (C-T) and inert COD (C-I) removal efficiencies per utilized ozone ratios were considered. With post-ozonation, on the other hand, the highest inert COD (C-I) removal efficiencies together with an effluent quality meeting the discharge standards could be obtained. (c) 2006 Society of Chemical Industry