BACKGROUND: The use of ozone combined with biological treatment was investigated for molasses fermentation wastewater containing highly concentrated, brown and biorefractory compounds. These persistent compounds, known as melanoidins, generate disposal issues: in the environment, the color is problematic for aquatic life; and in municipal wastewater treatment plants, the molecules are biorefractory. RESULTS: This paper aims to evaluate the impact of ozone pretreatment, applied in the range 0.1 to 1 g g(-1) consumed ozone doses, on both macroscopic physico-chemical parameters such as chemical oxygen demand (COD), total organic carbon (TOC), color and UV absorbance, and batch aerobic biodegradability. Then, performances of ozone pretreatment are assessed in terms of biodegradability improvement in batch and semi-continuous anaerobic processes and, also, in semi-continuous denitrification as a potential carbon source. Ozonation applied at the ozone dose of 0.5 g O-3 g(-1) COD led to an increase in biodegradability in all bioreactors. On average, the pretreatment resulted in an increased biodegradable fraction from zero to 33% without noticeable toxicity on biomass. This ozone dose also achieved 45% nitrogen removal by biological denitrification. CONCLUSION: Ozone pretreatment is a suitable technique for the biodegradabiiity improvement of molasses fermentation wastewater, in aerobic, anaerobic and anoxic conditions. The pretreatment should be optimized in order to maximize the subsequent biodegradability. (C) 2010 Society of Chemical Industry