Electrochemical oxidation of olive oil mill wastewaters.

The electrochemical oxidation of olive oil mill wastewaters over a titanium-tantalum-platinum-iridium anode was investigated. Batch experiments were conducted in a flow-through electrolytic cell with internal recycle at voltage of 5, 7 and 9 V, NaCl concentrations of 1%, 2% and 4%, recirculation rates of 0.4 and 0.62 L/s and initial chemical oxygen demand (COD) concentrations of 1475, 3060, 5180 and 6545 mg/L. The conversion of total phenols and COD as well as the extent of decolorization generally increased with increasing voltage, salinity and recirculation rate and decreasing initial concentration. In most cases, nearly complete degradation of phenols and decolorization were achieved at short treatment times up to 60 min; this was accompanied by a relatively low COD removal that never exceeded 40% even after prolonged (up to 240 min) times. The consumption of energy per unit mass of COD removed after 120 min of treatment was found to be a strong function of the operating conditions and was generally low at high initial concentrations and/or reduced salinity. The acute toxicity to marine bacteria Vibrio fischeri decreased slightly during the early stages of the reaction and this was attributed to the removal of phenols. However, as the reaction proceeded toxicity increased due to the formation of organochlorinated by-products as confirmed by GC/MS analysis. The toxicity to Daphnia magna increased sharply at short treatment times and remained quite high even after prolonged oxidation.