A study of the aerobic degradation of beet molasses alcoholic fermentation wastewater diluted to 50% (chemical oxygen demand, COD: 82 g/l) was carried out using the following fungi: Penicillium sp., Penicillium decumbens, Penicillium lignorum and Aspergillus niger. These four microorganisms produce a decolorization of the wastewater from the first day of incubation, achieving the maximum decolorization level at the fourth day of treatment in all cases. P. decumbes showed the maximum decolorization with a percentage of 40%. Simultaneously, a significant reduction in the phenolic content of the wastewater was also observed in all cases, reaching average removals of 70% for the four microorganisms studied. Average COD removals were similar in the four cases, achieving maximum values of 52.1 and 50.7%, respectively, on the fifth day of fermentation with Penicillium sp. and P. decumbens. Finally, a comparative study of the mesophilic anaerobic digestion of untreated and previously fermented (with P. decumbens) beet molasses was carried out in laboratory-scale suspended cell bioreactors. Average COD removals of 93% and methane yield coefficient of 305 ml methane at STP conditions per g of COD removed were found in the anaerobic digestion of pre-treated molasses. The combined aerobic-anaerobic process showed the following advantages in relation to the single anaerobic digestion process: higher average percentages of COD removal (96.5 compared with 90.0%) and a decrease of the hydraulic retention time (HRT) necessary to achieve these COD reductions, increasing the decolorization of the wastewater. (C) 2002 Elsevier Science Ltd. All rights reserved.