Abstract: A gaseous stream is passed from an upstream channel through a bubbling washing liquid into a downstream channel whence it passes through a horizontal channel and the purified gas is sucked out by a fan. A partition wall extending below the normal level of a washing liquid in a sump separates the upstream from the downstream channel and the horizontal channel is defined between two walls an upper one of which has an upstream end connected to the upper end of the partition wall while the lower wall is shorter and has an upstream end spaced from the partition wall. A vertical wall has an upstream end connected to the downstream end of the upper horizontal wall and is spaced from the downstream end of the lower horizontal wall, and an inclined bottom wall connects the downstream end of the vertical wall to the sump for returning collected washing liquid carried by the gaseous stream to the sump. The fan circulates the gaseous stream through the channels while bubbling it through the liquid in the sump.