NOVELTY - A portion of biomass-containing liquid is subjected to batch anaerobic digestion of organic material from reactor (12) to vessel (14), in which methane is produced in reactor (12) and vessel (14). Portion of liquid from vessel (14) is recycled to reactor (12). A portion of liquid from reactor (16) is supplied to vessel (14) or reactor (12). Direction of liquid to reactor (16) is allowed to control level of volatile fatty acid in liquid to be recycled to reactor (12) by providing additional anaerobic methanogenic microorganisms in reactor (16).    USE - Method of controlling process conditions in microbiological digestion of organic waste material process used for production of biogas.    ADVANTAGE - The low volatile fatty acid level in the method of controlling process conditions has thermodynamically favorable propionate degradation.    DETAILED DESCRIPTION - A portion of a biomass-containing liquid is subjected to batch anaerobic digestion of organic material from reactor (12) to a liquid storage vessel (14), in which methane is produced in both the reactor (12) and the liquid storage vessel (14). A portion of the liquid from the liquid storage vessel (14) is recycled to the reactor (12). A portion of the liquid from reactor (12) and/or the liquid storage vessel (14) is supplied to a reactor (16). A portion of the liquid from the reactor (16) is supplied to the liquid storage vessel (14) or the reactor (12). A direction of liquid to the reactor (16) is allowed to control the level of volatile fatty acid in the liquid to be recycled to the reactor (12) by providing additional anaerobic methanogenic microorganisms in the reactor (16). An INDEPENDENT CLAIM is included for apparatus (10), which is used for controlling of process condition in the microbiological digestion of organic waste material process.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the apparatus for controlling of process condition in process for the microbiological digestion of organic waste material.    Apparatus (10)    Reactors (12,16)    Liquid storage vessel (14)    Pump (18)    Valve (20)