In intensive farming areas, the design of biological nitrogen removal plants for piggery wastewater requires the determination of the chemical oxygen demand (COD) fractions of the effluent. For this purpose, an experimental procedure was developed to quantify the inert soluble (S-I) and particulate (X-I) COD fractions, as well as the readily (S-S) and the slowly (X-S) biodegradable COD fractions. For the four wastewaters tested, the S-1 and the X-1 fractions were equal to 3-4 gO(2)l(-1) and 17-28 g O-2 l(-1), respectively, which resulted in a total inert fraction of 42-84% of total COD. The S-S and the X-S fractions were very variable, ranging 0-5 g O(2)l(-1) and 4-25 g O(2)l(-1) respectively, depending on the farm management practices and the storage conditions prior to biological treatment. From these results, the denitrification potential of the piggery wastewaters for biological nitrogen removal treatment could be assessed. (C) 2004 Elsevier Ltd. All rights reserved.