Abstract: An aqueous stream such as pulp mill condensate containing one or more sulfur compounds is treated by (a) reacting an aqueous feed stream containing TRS contaminant selected from the group consisting of hydrogen sulfide, methyl mercaptan, dimethyl sulfide, dimethyl disulfide, and mixtures thereof, with ozone to produce a first liquid product stream and a first gaseous product stream, wherein the total amount of said one or more contaminants in said first liquid product stream is less than 1% of the amount thereof in said feed stream, and wherein said first gaseous product stream contains ozone and one or more of said contaminants; and (b) treating said first gaseous product stream with an alkaline aqueous stream to remove essentially all of said one or more contaminants from said first gaseous product stream. Preferably, a second gaseous stream produced in step (b) and containing ozone is reacted with an aqueous stream containing one or more of said contaminants, to consume all the ozone in said stream.