Preozonation of biologically or physically–chemically treated wastewater effluents, followed by passage through granular activated carbon (GAC) for tertiary wastewater treatment was studied at the Duck Creek Wastewater Treatment in Garland, Texas. Whereas the average period of operation for the GAC before exhaustion without ozone pretreatment was 70 days, pretreatment with ozone or with oxygen alone extended GAC operation to at least 480 days, withoutexhaus–tion. Effluent streams consistently metapplicable discharge standards during this period of time, without the necessity of regenerating the GAC.