Abstract: A system (100, 200, 300, 400) for producing biogas from waste stream (101) including organic material includes a liquid waste container (102) and a solid waste container (103). In addition, it includes a separator (104) for at least partially separating liquid fraction into the liquid waste container (102) and solid fraction into the solid waste container (103) from the waste stream (101) so that the solid waste container includes more solid content than the liquid waste container. Furthermore, the system includes a communication member (106, 111) between the liquid waste container (102) and a mixer (105, 202) for introducing liquid from the liquid waste container to the waste stream portion containing solid fraction in order to extract more liquid from the solid fraction.