<p>A vertical container with a substrate such as sewage or sewage sludge which is to be treated with a gas is designed to maintain an ascending spiral flow by inlets for the gas/liquid dispersion in an eccentric arrangement at the bottom. Partitions break this flow at higher levels but continue the spiral flow after each break. On top, the flow is inverted and descends through a central tube. In one ascending section, the flow passes through carrier material for microorganisms.</p>
