Abstract: An apparatus for removing a contaminant from a gas effluent. The apparatus comprises an absorption tower in which is placed a suitable packing where the gas effluent and contaminant-absorbing liquid contact one another. A recirculation path transfers spent contaminant-absorbing liquid to the top of the tower allowing the liquid to effect a new contaminant-removal cycle. Reagent is introduced in the recirculation path to refresh the contaminant-absorbing liquid. A dual-controller system dynamically varies the amount of reagent introduced in the recirculation path in accordance with the amount of contaminant in the gas effluent. A master controller calculates a reagent concentration set-point. The set-point is communicated to a slave controller that meters reagent in the recirculation path according to the set-point value.