Ozone treatment and biodegradation of industrial wastewater containing thioethers.

Wastewater from an Akzo Nobel production site contains refractory sulfur components (cyclic thioethers). These components end up in the effluent of the existing biological wastewater treatment plant. The possibilities of (partial) oxidation and increasing biodegradability by ozone treatment for these types of compounds have been investigated. Ozonation experiments were carried out at laboratory scale, varying pH, temperature and catalysts. Biodegradability was evaluated from BOD measurements and BOD/COD ratios. Ozonation experiments at pH = 10 without catalyst addition or heating were found to give the best results with the simplest means: COD was removed for 80-90% and biodegradability increased from BOD/COD = 0.2-0.3 to > 0.7. These results could not be improved any further by adding catalysts like iron and manganese (hydr)oxides or activated carbon, by heating or by further increasing pH. Aftertreatment at pH = 10 required about 3 kg ozone per m3 for about 80-90% removal of the total COD. This results in high annual operating costs and is therefore less suitable in practice. Biodegradability already increased to 60% at COD removal of 50% and ozone consumption of 1.4 kg/m3 wastewater. On the basis of these findings, it was calculated that ozonation has potential as pretreatment for a side stream containing the major fraction of the cyclic thioethers mixture.