The production of dye intermediates made from naphthalene compounds such as 6-nitro-1-diazo-2-naphthol-4-sulfonic acid (6-nitro-1,2,4-Acid) caused a major environmental concern, mainly due to their nature of being highly toxic concentrated organics. In this work, an enhanced Fenton process integrating the multi-staged Fenton reaction with inner circulation was practiced in a pilot scale. This creative design exhibited a particular advantage in treating the wastewater with high amount of organic substances. Experiments showed a higher COD removal efficiency (93%) and oxidation efficiency (62%) when two-staged Fenton reaction with inner circulation (flow-to-influent ratio of 5) was applied, whereas only 40.9% of COD reduction was obtained when using the single-staged Fenton without any circulation. It was demonstrated experimentally that the wastewater color went through a fast change via different stages of the reaction. The main intermediates generated along the two-staged reaction have been identified as aromatic and aliphatic acids, but their concentrations demonstrated regular changes in each stage, indicating an enhanced oxidation effect of staged Fenton process. (C) 2012 Elsevier B.V. All rights reserved.