The biogas streams upwards through the regularly moistened trickle filter along film strips colonised by sulphuroxidising microorganisms which form a biofilm, in the process of which the hydrogen sulphide is converted into elemental sulphur. This sulphur is deposited at the bottom of the vessel with the flushing liquid and is then spread together with fermentation residues on crop land, thereby completing the cycle for this important fertiliser. This process is used as an effective addition to aerobic desulphurisation and only employs fermentation residues and water as input materials. The desulphurisation plant achieves an average purification capacity of over 94%. This means a reduction of 250 ppm to 15 ppm, for example.