A vertically-oriented scrubber apparatus for removing a plurality of distinct contaminants from a contaminated gas stream, comprising: a) a scrubber vessel having a ceiling, a floor, a cylindrical wall connecting the ceiling to the floor, a plurality of vertically-spaced heads (12, 14), a volume above each head (2, 4), a gas inlet, an induced draft fan, and a gas outlet; b) a first scrubbing fluid reservoir disposed within the bottom end of the scrubber vessel to a desired level, the first scrubbing fluid selected to remove a first group of contaminants from the contaminated gas stream; c) a first flooded head extending horizontally across the scrubber vessel at a position above the gas inlet, defining a first volume between the first scrubbing fluid reservoir and the first flooded head, wherein the first flooded head comprises a plate having a plurality of narrow slots extending throughout; d) a first scrubbing fluid reaction zone volume disposed to a desired level above the first flooded head, the reaction zone volume in fluid connection with the reservoir via one or more overflow tubes, each extending from the first scrubbing fluid reservoir through the first flooded head to a desired level above the first flooded head; f) a first scrubbing fluid inlet extending through the wall into a second volume above the first flooded head, first spraying means in fluid connection with the first scrubbing fluid inlet for spraying the first scrubbing fluid into the first scrubbing fluid reaction zone volume, and a first scrubbing fluid outlet in the floor; g) a second flooded head extending horizontally across the entire cross-section of the scrubber vessel at a position above the first flooded head, defining a second volume between the first scrubbing fluid reaction zone volume and the second flooded head, wherein the second flooded head comprises a plate having a plurality of narrow slots extending throughout; h) a second scrubbing fluid reaction zone volume disposed above the second flooded head to a desired level, the second scrubbing fluid selected to remove a second group of contaminants from the contaminated gas stream; and i) a second scrubbing fluid inlet extending into a third volume above the second flooded head, second spraying means in fluid connection with the second scrubbing fluid inlet for spraying the second scrubbing fluid into the second scrubbing fluid reaction zone volume, and a second scrubbing fluid outlet above the second flooded head passing through the wall of the scrubber vessel. a) a scrubber vessel having a ceiling, a floor, a cylindrical wall connecting the ceiling to the floor, a plurality of vertically-spaced heads (12, 14), a volume above each head (2, 4), a gas inlet, an induced draft fan, and a gas outlet; b) a first scrubbing fluid reservoir disposed within the bottom end of the scrubber vessel to a desired level, the first scrubbing fluid selected to remove a first group of contaminants from the contaminated gas stream; c) a first flooded head extending horizontally across the scrubber vessel at a position above the gas inlet, defining a first volume between the first scrubbing fluid reservoir and the first flooded head, wherein the first flooded head comprises a plate having a plurality of narrow slots extending throughout; d) a first scrubbing fluid reaction zone volume disposed to a desired level above the first flooded head, the reaction zone volume in fluid connection with the reservoir via one or more overflow tubes, each extending from the first scrubbing fluid reservoir through the first flooded head to a desired level above the first flooded head; f) a first scrubbing fluid inlet extending through the wall into a second volume above the first flooded head, first spraying means in fluid connection with the first scrubbing fluid inlet for spraying the first scrubbing fluid into the first scrubbing fluid reaction zone volume, and a first scrubbing fluid outlet in the floor; g) a second flooded head extending horizontally across the entire cross-section of the scrubber vessel at a position above the first flooded head, defining a second volume between the first scrubbing fluid reaction zone volume and the second flooded head, wherein the second flooded head comprises a plate having a plurality of narrow slots extending throughout; h) a second scrubbing fluid reaction zone volume disposed above the second flooded head to a desired level, the second scrubbing fluid selected to remove a second group of contaminants from the contaminated gas stream; and i) a second scrubbing fluid inlet extending into a third volume above the second flooded head, second spraying means in fluid connection with the second scrubbing fluid inlet for spraying the second scrubbing fluid into the second scrubbing fluid reaction zone volume, and a second scrubbing fluid outlet above the second flooded head passing through the wall of the scrubber vessel.