TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: (I) comprises: at least two biogas reactor (3,4); and a storage tank for collection of the biogas and the reactor effluent produced in the biogas reactors (3, 4). The separation unit comprises a decanting centrifuge, membrane filter unit, press screw separator, drum filter and/or drum screen. The thin sludge fermenter (3) has a reactor vessel and a solids separator (3.8) downstream of the reactor vessel to which the reactor effluent of the reactor vessel is fed, where the solids separator (3.8) is so designed that the reactor effluent in the solids separator (3.8) is put under pressure so that the expulsion of gases is minimized and solids may settle and there is provided between the solids separator (3.8) and the reactor housing a pipe section (3.9) to return the solids settling in the solids separator (3.8) to the reactor housing. (I) has a digested sludge separation unit (16) to separate gaseous nitrogenous compounds from the reactor effluents. The thin fraction has a dry weight content of 5-12% and the thick fraction has a dry weight content of 20-35%. Preferred Process: In the process, where in a separation unit (6) in the form of a nitrogen sink (6), process water containing little or no nitrogen is separated out from the reactor effluent and the process water is fed via a return pipe (7) into the biogas reactor to dilute the fermentation stillage, and/or into a bioethanol plant (1). The gaseous nitrogenous compounds, solid portions and liquid portions of the reactor effluent are separated, in a digested sludge separation unit (16) (which is a part of nitrogen sink (6)). In the process: a separation aid is fed into the digested sludge separation unit (16); biogas remaining in the liquid portion of the reactor effluent is separated out, in a flash boiler (18); process water with low nitrogen content is obtained from the liquid reactor effluent, in a thickening unit (19) downstream of the flash boiler (18); the biogas is desulfurized in a desulfurization unit using a process chemical; a mineral trace element solution is added to the biogas reactor (3, 4), with the trace element solution comprising at least one trace element and one complexing agent.