NOVELTY - Two parallel, counter-rotating, driven stirrer-shafts (10, 11) operate as a pair (9), immersed in the fermentation fluid (8). These induce turbulent flow (18) around the shafts, causing intensive localized mixing (17) of the biomass (7) with the fermentation fluid. In addition, a horizontal recirculating flow (19) is induced in the tank, affecting its entire volume. The flow passes through the passage (15) between the shafts, and leaves the pressurized region (20). It is swept away, later to return by recirculation.    USE - The agitation equipment is useful for a biogas-producing digester.    ADVANTAGE - With a relatively simple construction, the agitation system achieves uniform mixing and distribution in the tank. Floating mats are a common problem in digesters, and require special measures for their detection. The new design eliminates them. Similarly sunken stratifications are destroyed. Efficiency of the entire fermentation process is increased.    DETAILED DESCRIPTION - In a variant arrangement, the shafts are located centrally in the tank, inducing diametral flow with splitting and recirculation. Further variants of the arrangement are detailed; more than one shaft pair may be included, for example. A common drive and transmission is provided for the shafts. Various conventional drive systems are applicable, including hydraulic and pneumatic. Speed and direction can be changed. Stirrers are modular and removable as such. Baffles may be included in the tank. Various conventional paddles or propellers are used.    An INDEPENDENT CLAIM is included for the corresponding method of biomass distribution in the tank.    DESCRIPTION OF DRAWING(S) - A plan view illustrates the tank, stirrers and flow paths.    Biomass (7)    Fermentation fluid (8)    Pair of shafts forming unit (9)    Parallel, counter-rotating, driven stirrer-shafts (10, 11)    Passage between shafts (15)    Region of turbulent flow (18)    Region of intensive localized mixing (17)    Horizontal recirculating flow (19)    Pressurized region (20)