Ozonation is expected to improve the removal of pesticides by GAC filtration not only via the well-known effect of oxidation of pesticides, but also due to oxidation of Background Organic Matter (BOM) present in filter influent. Namely, an important part of BOM compounds will be partially oxidized because of ozonation. This will increase their biodegradability, and lower their adsorbability and molecular mass. Thus, enhanced biodegradation and reduced adsorption of BOM are expected in filters receiving ozonated rather than non-ozonated influent. Both biodegradation and adsorption of pesticides can be improved because of this.

Pilot plant experiments with pretreated Rhine River water spiked with about 3 μg/l of atrazine showed that ozonation results in incomplete oxidation of atrazine: about 25%, 45% and 65% of atrazine is oxidized when ozone doses of 0.5 mg/l, 1.0 mg/l and 1.5 mg/l are applied. For ozone doses up to 4 mg O3/l, desethylatrazine and desisopropylatrazine (the two by-products monitored) are formed at concentrations of up to 20% and 5% of the initial atrazine concentration, respectively. By-products of atrazine oxidation are expected to be more biodegradable but less adsorbable than atrazine. This makes uncertain whether they are removed in GAC filters better or worse than atrazine.

Improved atrazine removal was observed in the pilot plant GAC filter that received ozonated (0.8 mg O3/l) rather than non-ozonated pretreated Rhine River water. This improvement is thought to be due to ozone-induced oxidation of BOM, because atrazine was spiked after complete depletion of ozone and its concentration was the same (2.2±0.2 μg/l) in both ozonated and non-ozonated influent.

Research is defined to verify which of the anticipated processes and relationships underlie the improved atrazine removal observed in filters receiving ozonated influent.