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, a volume above each head, a gas inlet, an induced draft fan, and a gas outlet; b) a submerged lower head positioned horizontally across the lower end of the scrubber vessel, wherein the submerged lower head includes a horizontal plate having a plurality of narrow slots extending throughout; c) a first scrubbing fluid reservoir disposed within the bottom end of the scrubber vessel below the submerged lower head, and a first reaction zone volume to a desired level above the submerged scrubbing head, the first scrubbing fluid selected to remove a first group of contaminants from the contaminated gas stream; d) a first scrubbing fluid inlet extending into a first volume above the submerged 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; e) a first flooded head extending horizontally across the entire cross-section of the scrubber vessel at a position above the first scrubbing fluid inlet, wherein the first flooded head comprises a plate having a plurality of narrow slots extending throughout; f) a second scrubbing fluid reaction zone volume disposed to a desired level above the first flooded head, the second scrubbing fluid selected to remove a second group of contaminants from the contaminated gas stream; and g) a second scrubbing fluid inlet extending into a second volume above the first 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 first flooded head passing through the scrubber vessel wall. a) a scrubber vessel having a ceiling, a floor, a cylindrical wall connecting the ceiling to the floor, a plurality of vertically-spaced heads, a volume above each head, a gas inlet, an induced draft fan, and a gas outlet; b) a submerged lower head positioned horizontally across the lower end of the scrubber vessel, wherein the submerged lower head includes a horizontal plate having a plurality of narrow slots extending throughout; c) a first scrubbing fluid reservoir disposed within the bottom end of the scrubber vessel below the submerged lower head, and a first reaction zone volume to a desired level above the submerged scrubbing head, the first scrubbing fluid selected to remove a first group of contaminants from the contaminated gas stream; d) a first scrubbing fluid inlet extending into a first volume above the submerged 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; e) a first flooded head extending horizontally across the entire cross-section of the scrubber vessel at a position above the first scrubbing fluid inlet, wherein the first flooded head comprises a plate having a plurality of narrow slots extending throughout; f) a second scrubbing fluid reaction zone volume disposed to a desired level above the first flooded head, the second scrubbing fluid selected to remove a second group of contaminants from the contaminated gas stream; and g) a second scrubbing fluid inlet extending into a second volume above the first 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 first flooded head passing through the scrubber vessel wall.