A gas treatment system has a housing defining an interior space adapted for holding liquid therein. The housing has an inlet port and an outlet port spaced from the inlet port so that gas passing through the interior space flows through the liquid therein. A gas delivery system directs gas to be treated into the interior space of the housing at the inlet port. An ultrasonic waveguide assembly is disposed within the interior space of the housing and includes an ultrasonic horn disposed at least in part intermediate the inlet port and the outlet port of the housing. The ultrasonic horn is operable at an ultrasonic frequency to ultrasonically energize liquid within the housing. An ultraviolet light source is for emitting ultraviolet light into the interior space of the housing to treat liquid flowing from the inlet port to the outlet port.