NOVELTY - The system has separator (104) which is provided for separating liquid fraction into liquid waste container (102) and solid fraction into solid waste container from waste stream (101b) so that solid waste container having solid content than liquid waste container. A communication unit (106) is provided between liquid waste container and mixer (202) for introducing liquid from liquid waste container to waste stream portion having solid fraction.    USE - System for producing biogas such as methane, hydrogen, and carbon dioxide from waste stream such as cattle, pig or poultry farms waste, industrial or municipal waste water streams, and waste foodstuffs or organic waste water such as sewage, livestock manure, faeces and urine.    ADVANTAGE - The system can produce effective and fast biogas from biomass containing waste streams and organic feedstock in efficient manner. By introducing liquid and/or inoculum material into the solid waste stream or into the solid waste container, the heavy liquid can be extracted from the overall content of the waste stream into the liquid container, and starting of the fermentation process can triggered. Thus, powerful and faster fermentation and biogas production can be achieved efficiently.    DETAILED DESCRIPTION - The system has mixer which is provided before separator so as to introduce liquid from liquid waste container and/or inoculum material into waste stream having biomass and to extract liquid from waste stream having biomass. The mixer is provided between separator and solid waste (103b) container so as to introduce liquid from liquid waste container into solid waste container and to extract liquid from solid fraction contained in solid waste container. The second separator is provided in connection with solid waste container for separating liquid fraction into liquid waste container from solid fraction of solid waste container. The communication units (109,301) are provided between second separator and liquid waste container to introduce liquid fraction separated from solid fraction to liquid waste container. The waste stream input is provided with slurry waste stream input (101a) and dry manure waste input (101b). The ripper (302) is provided for ripping dry manure waste into smaller portions before introducing into separator or liquid waste container. The mixer is provided for introducing liquid (201b) from liquid waste container, slurry (303) from slurry waste stream input, and/or third liquid (304) such as water, and/or inoculum material into dry manure waste stream. The heating means is provided in connection with communication means transferring liquid and/or solid portions. The liquid waste container is arranged to function as plug flow reactor for concentrating introduced liquid through anaerobic digestion without blending of liquid concentrations between different phases and input and output ends of plug flow reactor. The ripper is provided with gaps of diameters between 0.1-1.0 mm, elliptical gaps with minor axis between 0.1-0.3 mm, and with major axis between 0.5-0.8 mm. The pH-sensor is provided for measuring pH-value of liquid in liquid waste container and controller for controlling introduction of liquid from liquid waste container to waste stream portion having solid fraction so that introduction is performed when pH- value of liquid from liquid waste container is certain predetermined value. The sensor is provided for measuring dry content of incident waste stream. The controller is provided for controlling volume of liquid to be introduced from liquid waste container to waste stream through mixer so that liquid percentage of waste stream. The mixer or introduction unit is provided for introducing acid such as sulfuric acid, sodium hydroxide, calcium hydroxide, enzyme or biocatalyst into solid fraction. The surface is provided with anaerobic micro-organisms/microbes in liquid waste container configured to interact with liquid fraction thus producing biogas. The surface with anaerobic micro-organisms is arranged into units having maximal surface area for micro-organisms. The liquid fraction is introduced to surfaces of units. The controller is configured to discharge liquid fraction from liquid waste container as inoculum after 2-6 days, 2-4 days and 2-3 days from moment when liquid fraction is separated into liquid waste container. The mixer or introduction unit is provided for introducing additional organic waste material into the solid waste container such as grass, reed, and creed canary grass. The liquid is discharged from the previous liquid waste container into the subsequent liquid waste container after certain time period and/or when the liquid in said previous liquid waste container having essentially certain pH-value. The dry-type-reactor and communication unit are provided between solid waste container and dry-type-reactor to introduce solid material from solid waste container into dry-type-reactor. The mixer is provided before dry-type-reactor so as to introduce liquid from liquid waste container into dry-type-reactor. The heat treatment unit is provided for heating content of liquid waste container and/or solid waste container. INDEPENDENT CLAIMS are included for the following:    (1) a module for producing biogas from waste stream; and    (2) a method for producing biogas from waste stream, involves introducing liquid from liquid waste container to waste stream portion having solid fraction so as to extract liquid from solid fraction. The dry manure waste is ripped by ripper into smaller portions before introducing into separator or liquid waste container.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the system for producing biogas.    Slurry waste stream input (101a)    Waste stream (101b)    Liquid waste container (102)    Solid waste (103b)    Separator (104)    Communication units (106,109,301)    Liquid (201b)    Mixer (202)    Ripper (302)    Slurry (303)    Third liquid (304)