TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Device: In the device, the electrochemical cell is formed by an electrolysis cell (14) which is provided with water. The electrochemical cell comprises two mutually coupled electrochemical half cells, where a base-generating half cell is arranged integrated into the first compartment, and acid-generating half cell is arranged integrated into the second compartment. The first and second compartments are integrated with each other; and are mutually coupled in series. A closing valve (3) is arranged between the first and second compartment. The first compartment is provided with at least one first acidity meter for detecting the pH in the first compartment. The device is provided with a control unit for determining the quantity of formed intermediate product at least on the basis of the pH detected by the first acidity meter and/or the quantity of base administered. The control unit is adapted to determine the quantity of base to be administered to the first compartment on the basis of the pH detected by the first acidity meter. The control unit is adapted to actuate the closing valve at least subject to the pH detected by the first acidity meter and/or subject to the quantity of base administered. The second compartment is provided with at least one second acidity meter. The control unit is adapted to determine the quantity of acid to be administered to the second compartment on the basis of the pH detected by the second acidity meter. The control unit is adapted to determine the quantity of formed methane on the basis of the pH detected by the second acidity meter and/or the quantity of acid administered; and is adapted to actuate the heating unit subject to the temperature detected by the thermometer. The first and/or second compartments are provided with heating unit to enable heating of respectively the biomass and the product (p1). The first and/or second compartments are provided with at least one thermometer; and transport unit in order to enable respectively the biomass and the product (p1) to be kept in motion.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The molar quantities of the base supplied to the biomass and the acid supplied to the intermediate product are substantially identical. At least one acid generated during the first fermenting step is additionally added during the second fermenting step to the intermediate product for fermenting. At least one base generated during the second fermenting step is additionally added during the first fermenting step to the biomass for fermenting. The step of generating the base and acid is carried out by electrolysis of water with formation of H+ ions and OH- ions. The method also involves detecting the pH of the biomass during the first fermenting step, where the quantity of base to be administered as according to the step of adding base to the biomass is at least determined on the basis of the pH detected. The pH during the first fermenting step is 5-7; and during the second fermenting step is 6.5-8. The method also involves detecting the pH of the intermediate product during the second fermenting step, where the quantity of acid to be administered as according to step of adding acid to the product (p1) is at least determined on the basis of the pH detected. The method also involves guiding the biomass into a first compartment prior to fermenting of biomass according to the first fermenting step; and displacing a part of the product (p1) from the first compartment to a second compartment prior to fermenting of the product (p1), where a maximum of 95 (preferably 30-80)%, of the product (p1) present in the first compartment is displaced to the second compartment. At least a part of the product (p1) is displaced from the first compartment to the second compartment as according to the displacing step after detection of a varying pH progression during the step of detecting pH of biomass. The temperature during the first and second fermenting step is 20-80 (preferably 30-50) degrees C. The biomass is actively heated during the first fermenting step and/or the product (p1) during the second fermenting step. The number of micro-organisms required for the fermentation according to the first and second fermenting step is 107 to 1012 per gram of respectively biomass and the product (p1). The mass of the micro-organisms required for the fermentation according to the first and second fermenting step is maximum of 15 mass%. The biomass is kept in motion during the first fermenting step, and the intermediate product is kept in motion during the second fermenting step. The method also involves separating the methane from a residual fraction during or after forming of methane as according to the second fermenting step. The biomass is subjected during the first fermenting step to a hydrolysis followed by an acidogenesis or an anaerobic oxidation. The acidogenesis or anaerobic oxidation is followed by an acetogenesis. The product (p1) is subjected during the second fermenting step to a methanogenesis. EXAMPLE - No suitable example given.