No purification procedure exists for treating the waste waters from the Spanish-style green olive industry. This study shows that an activated sludge process can be used successfully, yielding a 75-85% COD reduction, due mainly to the removal of organic acids and ethanol present in the waste. In contrast, only a small proportion of polyphenols was consumed. These residual polyphenols can account for most of the remaining COD and the residual brown colour. Grau's model for substrate removal rate was applied to take account of the effect of influent-substrate concentration on the effluent COD concentration. The constant value with this model (k) was 9.8 day(-1). Likewise, increasing the hydraulic retention time and temperature improved the sludge removal. Thus, a COD of 200-300 mg dm(-3) in the effluent was routinely achieved. Concentrations of NaCl up to 3% did not affect the aerobic system although the sludge volume index was higher than 200 cm(3) g(-1).