Abstract: A generally columnar chamber for uniformly mixing a gas, such as ozone, into a liquid, such as water. The mixing chamber is assembled from a series of round tray sections each having an outer and an inner helical conduit sections and a central vertical tube section. The trays are stacked and secured together to form a continuous outer helix, inner helix and central column. A port at the bottom of the column directs liquid into the outer helical conduit spiraling up to the top of the column where a crossover port directs the liquid into the inner helical conduit downwardly to a cylindrical sump. The outlet of the inner helix into the sump is angled so as to create a vortex in the sump, forcing heavy particles outwardly to the sump wall. Heavy particles are allowed to migrate down the wall to a collection region at the bottom of the sump for later removal.