Stirring, or agitating, in Danish biogas systems varies considerably in character even with regard to systems which receive similiar compounds of biomass and are fitted with similiar stirring equipment. In some systems stirring uses a great deal of energy, here the rearrangement of the stirring equipment, or supplementing it, can improve the situation. It is stated that there seems to be a need for the determination of a common energy-economical goal for stirring equipment in biomass conversion plants. On this basis new designs can be developed. New sets of criteria for such designs for mixing systems are presented and the theories on which they are founded are explained. Information is also given on the evaluation of layer stratification in the liquid manure tank, of the mixing in the reactor tank and of the effectivity of the mixing in the fore-tank and in the storage tank. Matters of economy are also dealt with. Diagrams and tables illustrate the detailed text. (AB).