The invention relates to a fluid storage tank having an interior volume and a floor and having an internal spill containment chamber comprising a vent gas scrubbing apparatus or scrubber. The chamber may be defined by a containment wall which completely separates the chamber from the tank interior volume. The containment wall may comprise an exterior portion which extends beyond the tank wall. The scrubber is disposed within the chamber and comprises a gas inlet passing through the containment wall into the tank interior volume, a gas outlet for venting scrubbed gases, and a gas scrubbing chemical storage and circulation system.