The present invention is directed toward a method and a system for reducing the amount of fluid in a biological mass comprising solids and fluid in the form of liquid and gas. The system includes a temporary storage container for receiving and temporary storing a biological mass, at least one separator arranged to receive the biological mass from the temporary storage container and separate at least a portion of the fluid of the mass from the solids of the mass, a solids container for receiving the solids fraction separated from the biological mass, and a liquid-purifying device arranged to purify the liquid which is separated from the mass by means of the separator. The system is substantially fluidtight towards the surroundings and the separator includes a screen cloth and a device for inducing a fluid flow. The fluid flow is arranged to carry a portion of the liquid out of the mass and through the screen cloth, the essential part of the fluid flow has been recirculated in the system (1).
