Degradation of Pesticides by Ozonation and Advanced Oxidation

In the Netherlands many water supply companies are upgrading their surface water treatment plants in order to guarantee the water supply and water quality in the coming years. The Water Supply Company North West Brabant (WNWB) has plans to upgrade their treatment plant at Zevenbergen. In the retrofit plant chlorination will be abandoned and probably ozonation will be the major barrier against microorganisms. Pesticide concentrations will be decreased by three barriers: storage, ozonation and activated carbon filtration.

If the ozone dosage is restricted just to reach the required disinfection level at pH 7.2, ozonation is a poor barrier against pesticides. Out of 23 selected pesticides, only 6 were effectively degraded: dimethoate, chlortoluron, diuron, isoproturon, metoxuron and vinclozolin (O₃/DOC = 0.55 g/g). Application of an (O₃/DOC ratio of 1.0 g/g results in an effective barrier for roughly 50% of the tested pesticides (also for diazinon, parathion-methyl, linuron, methabenzthiazuron, metobromuron, MCPA and MCPP). Pesticides were degraded more effectively at high pH and high temperature.

For additional degradation of high concentrations of persistent pesticides, advanced oxidation may be applied. Atrazine, propazine, simazine, chlor-fenvinphos, tetrachlorvinphos, 2,4-D, 2,4-DP and 2,4,5-T were degraded by O₃/DOC = 1.4 g/g and H₂O₂/O₃ = 0.5 g/g. Dicamba and dikegulac were most persistent. pH has a minor effect on the degradation of pesticides by advanced oxidation. Higher hydrogen peroxide dosages showed no improvement in degradation. After ozonation and advanced oxidation, about 50% of totally reacted atrazine and propazine was converted into desethylatrazine. No desisopropylatrazine formation was observed.