More efficient atrazine removal was observed in the Granular Activated Carbon (GAC) filter that received ozonated rather than non-ozonated pretreated Rhine River water. Various pilot and bench scale experiments were done to verify that such improved removal is due to better biodegradation and/or better adsorption of atrazine, and that both are increased by enhanced biodegradation of Background Organic Matter (BOM) after ozonation. No indication of atrazine biodegradation in the GAC filters operated was found in either of the four different experiments conducted. Therefore, biodegradation of atrazine in GAC filters and its better biodegradation in filters receiving ozonated influent were not confirmed. However, the results obtained do not entirely exclude the possibility that atrazine was biodegraded in the GAC filters operated. If atrazine biodegradation in these filters was indeed negligible, it is possible to assume that the conditions for the growth of atrazine degrading bacteria were inadequate.

The enhanced biodegradation of BOM in filters receiving ozonated influent improves adsorption of atrazine in GAC filters. This can be concluded because atrazine was found better adsorbed onto GAC preloaded with ozonated water that passed through Non-Activated Carbon (NAC) filters, than onto GAC preloaded directly with ozonated water. Because of negligible adsorption of BOM in NAC filters, only biodegradation of BOM in NAC filters could account for the improved adsorption of atrazine that was observed. However, reduced adsorbability and molecular mass of ozonated BOM compounds can also contribute to the improved adsorption of atrazine in GAC filters.