A two-phase experimental plan was designed in order to investigate a new technique to remove color from pulp mill effluents via chemical and biological reactions in series. In the first phase, the chemical characterization of chromophores using the spectra of infrared (IR), ultraviolet (UV) and nuclear magnetic resonance (NMR) analyses was carried out. The results of chemical analyses indicated that the color-causing materials are mainly unsaturated compounds, possibly conjugated double bonds on aromatic rings. Therefore, a reducing agent, sodium borohydride (NaBH4) was chosen in order to change the structure of chromophores via hydrogenation reaction at ambient temperature and pressure as a means of color removal from the wastewater streams. The bench scale results of the first phase of this research demonstrated that the NaBH4 reduction is a viable method for color elimination with no sludge produced. The batch kinetic study of NaBH4 pretreatment indicated that 97% color decrease in 24 h followed first-order kinetics with respect to sodium borohydride consumption and the reaction rate constant was evaluated at 0.6 h(-1). Subsequently in the second phase of the experiments, the pilot plant of an innovative chemical and biological reactor system was investigated. The plant consisted of two 20-1 reactors operating in the batch mode for the overall residence time of 6 days. In the first reactor, chemical hydrogenation with NaBH4 was performed for I day and resulted in a color and COD reduction by 97% and 35%, respectively, and increased BOD up to 85%. No significant change in TSS was observed. The chemically treated effluent was then subjected to a biological oxidation reaction in the second reactor for further treatment with a residence time of 5 days. The results indicated significant decrease in BOD (99%), COD (92%), and TSS (97%). Consequently, a combined chemical and biological reaction system appears to effectively decrease the color as well as BOD, COD and TSS, in contrast to the conventional techniques such as aerated lagoon and activated sludge systems in which color reduction is not observed. The economics of the pilot scale implementation of a NaBH4 color reduction treatment process for 97% color removal was estimated to be in the range of 0.001 US dollar per liter of the most highly colored wastewater sample. (c) 2006 Elsevier B.V. All rights reserved.