NOVELTY - Apparatus (I) for the conversion of fermentation stillage arising as a waste product of ethanol production into biogas, comprises: a separation unit (2) for the separation of the fermentation stillage into a thin fraction and thick fraction, where the thin fraction has a lesser dry weight content than the thick fraction; and a biogas reactor (4) to ferment the thin fraction, which is referred as the thin sludge fermenter (3).    USE - (I) is useful for the conversion of fermentation stillage arising as a waste product of ethanol production into biogas.    ADVANTAGE - (I) efficiently operates the process for ethanol production. (I) rapidly converts the thin fraction into biogas, hence a reactor with very low volume is sufficient, which reduces the energy needed for controlling the reactor temperature, and makes the whole process very efficient in terms of energy. (I) has an altogether smaller reactor volume for the same throughput, in comparison with conventional apparatus and requires a lower temperature, which is sufficient in the fermenters, bringing a further rise in energy efficiency. In the nitrogen sink, a low-nitrogen process water flow is produced from the reactor effluent, hence it is possible to direct the low-nitrogen process water flow back into the biogas reactor and/or into the reactor for ethanol fermentation, to dilute the reactor contents without at the same time increasing the pH value, which saves the costs of fresh process water and it avoids the concentration of too much ammonia in the reactor, since ammonia is toxic to the micro-organisms in the reactor. In the process: large amounts of acid gases (carbon dioxide and hydrogen sulfide) are dissolved, where the stripping out of gases generates an auxiliary flow assisting the stripping out of ammonia and this auxiliary flow is already present and does not need to be added; the removal of the acid gases raises the pH value, shifting the balance towards ammonia; and a greater amount of low-temperature waste heat is present, with a temperature too low for heating the reactors and this low-temperature waste heat is however highly suitable for heating the digested sludge and assists the stripping out of ammonia.    DETAILED DESCRIPTION - Apparatus (I) for the conversion of fermentation stillage arising as a waste product of ethanol production into biogas, comprises: a separation unit (2) for the separation of the fermentation stillage into a thin fraction and thick fraction, where the thin fraction has a lesser dry weight content than the thick fraction; and a biogas reactor (4) to ferment the thin fraction, which is referred as the thin sludge fermenter (3), where the separation unit (2) is designed with a single separation stage unit and the thin sludge fermenter is a heavy-duty biogas reactor through which the thin fraction flows, and preferably, the flow path in the thin sludge fermenter is pore-free so that the thin fraction may be fermented in the thin sludge fermenter with a low proportion of solids. An INDEPENDENT CLAIM is included for a process for the conversion of fermentation stillage arising as a waste product of ethanol production into biogas, comprising: separating fermentation stillage by a separation unit into a thin fraction and thick fraction, where the thin fraction is fermented in a first biogas reactor, and the thick fraction is fermented in a second biogas reactor (4) and is subsequently referred to as the thick sludge fermenter (4), and the biogas together with the effluents are collected in a storage tank (5).    DESCRIPTION OF DRAWING(S) - DETAILED The figure shows the structure of the apparatus.    Bioethanol plant (1)    Separation unit (2)    Thin sludge fermenter (3)    Thick sludge fermenter (4)    Storage tank (5)