NOVELTY - Converting biomass into methane involves fermenting part of biomass to form at least one intermediate product (p1) selected from hydrogen, carbon dioxide and acetate, carbon monoxide, volatile fatty acids, formate, alcohols, pyruvate, (di)methylamine, (di)methyl sulfide, trimethylamine, methyl mercaptan and incompletely reduced carbon compounds, and fermenting part of product (p1); and also generating at least one base and acid using at least one electrochemical cell, adding base to biomass during first fermenting step and adding acid to product (p1) during second fermenting step.    USE - For converting biomass into methane (claimed).    ADVANTAGE - The successive administration of a base and an acid, avoids the addition of buffer to the biomass, as a result undesirable increase in the salt concentration in the biomass is prevented. The use of a base and acid instead of a buffer is also advantageous from a financial and logistical viewpoint. The addition of at least one base to the biomass for fermenting prevents the pH falling below a critical pH of 5, and provides a favorably high pH for methanogenic micro-organisms this helps in retaining sufficient activity of the fermentive and acetogenic microorganisms. The generation of the base and the acid takes place at a distance from (although usually in the vicinity of) the biomass and the intermediate product, as a result the electrolysis process is made more readily manageable and safe as the formed oxygen gas and the hydrogen gas, which together form a flammable mixture are collected in facilitated manner. The use of two electrochemical half cells which are mutually coupled by a conductive salt bridge and each filled with water enables mutual separation of the base and acid, and thus used to store formed base and acid; and also structurally simplifies the device for converting biomass to methane.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for device (1) for converting biomass to methane comprising: at least one first compartment (2) for receiving biomass to be converted into at least one intermediate product, at least one base-delivering source connected to the first compartment, at least one second compartment (5) for receiving the at least one intermediate product to be converted to methane, at least one acid-delivering source connected to the second compartment, where the base-delivering source and the acid-delivering source form part of at least one electrochemical cell.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a device.    Device (1)    First compartment (2)    Closing valve (3)    Intermediate conduit (4)    Second compartment (5)    Electrolytic cell (14)    Power source (15)    Conductive separating wall (19)