(EP642471-A1)      Small amt. of highly viscous substrate in a fermentation tank is mixed with a large amt. of air so that a foam contg. air bubbles is formed. The foam is distributed and mixed into the substrate in a mixing zone close to the bottom of the tank so that very small air bubbles are formed.    Foam, is pref. formed by introduction of air via tube (1) into the gap between relatively rotating discs (3,4) provided with radial ribs on their facing surfaces with substrate entering the gaps between the ribs via apertures (6,7) in the discs and the foam being distributed into the substrate by mounting the discs near the bottom of the fermentation tank.    USE/ADVANTAGE -   In the fermentation of highly viscous material such as manure obtd. from holding animals. Two-stage process produces bubbles of air of very small size, which rise very slowly through the substrate providing a high degree, e.g. 20 percent or more, of utilisation of the oxygen in the air, giving efficient fermentation and reducing ammonia prodn.    (EP642471-B1)      Small amt. of highly viscous substrate in a fermentation tank is mixed with a large amt. of air so that a foam contg. air bubbles is formed. The foam is distributed and mixed into the substrate in a mixing zone close to the bottom of the tank so that very small air bubbles are formed.    Foam, is pref. formed by introduction of air via tube (1) into the gap between relatively rotating discs (3,4) provided with radial ribs on their facing surfaces with substrate entering the gaps between the ribs via apertures (6,7) in the discs and the foam being distributed into the substrate by mounting the discs near the bottom of the fermentation tank.    USE/ADVANTAGE -   In the fermentation of highly viscous material such as manure obtd. from holding animals. Two-stage process produces bubbles of air of very small size, which rise very slowly through the substrate providing a high degree, e.g. 20 percent or more, of utilisation of the oxygen in the air, giving efficient fermentation and reducing ammonia prodn.