Abstract: A liquid substrate tank for a biogas plant includes an interior fillable with a liquid substrate and a bottom wall defining the interior on the bottom and being particularly at least regionally flat. A trough-shaped recess extending over a partial region of the bottom wall is formed in the bottom wall, to which and/or into which recess an extraction line of an extraction device is guided. The extraction device has an open-loop and/or closed-loop control device actuating the extraction device for extracting a substrate/sand mixture accumulating in the recess during operation from the recess and thus from the interior through the extraction line. The extraction device has an accommodation and/or sedimentation tank, or separator, connected to the extraction line relative to flow for accommodating or separating the substrate/sand mixture extracted through the extraction line into substrate and sand phases.