BACKGROUNDThe aim of this work was the examination of molasses wastewater treatment by the combination of activated sludge with ozonation, for the effective removal of refractories. In addition to biological treatment, samples were subjected to: (a) ozonation prior to activated sludge; (b) ozonation coupled with addition of hydrogen peroxide prior to activated sludge; (c) post-ozonation of activated sludge effluents; and (d) ozonation applied to influents and effluents, as a pre- and post-treatment of the biological process. RESULTSThe activated sludge treatment of the mixture resulted in a 48% reduction of COD, while the combination of ozone pretreatment followed by biological treatment resulted in 66.5% COD removal and the production of an effluent with 240 mg L-1 COD. Therefore, it was concluded that ozone pretreatment might contribute to the enhancement of the biodegradability of the influent. This assumption was further supported by the examination of molecular weight distribution of wastewater samples by size exclusion chromatography. CONCLUSIONThe combined biological and ozone processes resulted in effluents with similar COD values, independent of the stage of the application of the ozone treatment process, either as a pretreatment or as post-treatment step, indicating that a fraction of the compounds of the raw influent was resistant to the particular treatment process. This recalcitrant residual exhibited high organic nitrogen content, and was responsible for the relatively moderate total nitrogen content reduction, although almost complete nitrification and denitrification took place during the biological process, as deduced by the ammonia and nitrates content of the effluents. (c) 2015 Society of Chemical Industry