The production of ethanol by fermentation and subsequent distillation generates wastewater known as 'vinasse'. This waste is highly polluting on account of its high organic load and antibacterial activity-which arises from the presence of phenolic compounds that reduce the efficiency of the detoxification treatments best suited to this type of waste (eg anaerobic digestion). This can be avoided by using a pre-treatment based on a chemical oxidant (ozone) at an acid or alkaline pH. Ozonation in acid media provides more selective elimination of phenolic compounds and a more readily biodegradable waste (ie an increased BOD 5 /COD ratio).