Abstract: The coalescence and release of bubbles from packing that supports the growth of microorganisms is promoted by mechanical action on the packed bed arrangement, in particular by continuous or intermittent vibrations, rotations, or by rocking motions. With particular advantage, a reactor vessel which is typically tubular is filled with packing upon which microbial growth is sustained, preferably leaving a free space of 10% to 35%, in particular 15% to 20%. The reactor vessel has an axis that is substantially horizontal but preferably inclined by at least about 3.degree., and is set continuously or intermittently, for example, by an axle drive or by means of friction wheel drive, into rotational or rocking motions about that axis. The gas can be discharged, for example, via valves and a cam mechanism. Alternatively, a vent pipe, taken through the hollow shaft on the discharge side and upwards, can be provided behind a screen plate.