TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Plant: The heater comprises heat exchanger arranged on sewage recirculation line comprising outlet piping to let sewage out of the digestion chamber and inlet piping to let sewage into the digestion chamber, where the heat exchanger is being positioned between the outlet piping and inlet piping. The inlet piping lets sewage in on a free surface of the sewage in the digestion chamber. The inlet piping comprises main tube and several branches, where the main tube is arranged around the perimeter of the bag-like casing and each branch introduces sewage into the digestion chamber in parallel with respective flank of the bag-like casing. The branches have inlet directions concordant with each other. The plant further comprises second bag-like casing of flexible material and fluid-tight, where the second bag-like casing defines in internal zone, chamber for sewage, and second bag-like casing is hydraulically connected with the first bag-like casing of the digester to transfer sewage from the first bag-like casing to the second bag-like casing. The second bag-like casing carries out second digestion stage of the sewage contained in the chamber, and the first bag-like casing and second bag-like casing comprising digester that works based on two digestion stages in series. The plant comprises a return line for returning sewage from the second bag-like casing to the first bag-like casing. The return line has length that is in common with the recirculation line, and the heat exchanger is arranged on the common length. The heater is positioned inside the digestion chamber. The heater comprises coil pipe and connection piping to connect the coil pipe to a heating system outside the bag-like casing, where a length of the connection piping inside the digestion chamber comprises at least one flexible portion. The bag-like casing has a bottom face resting on first surface of a compartment for at least partially receiving the bag-like casing, where the plant comprises a heater positioned between the bag-like casing and the first surface and/or the lateral surface of the compartment, and the heater comprises coil pipe peripherally arranged around the lateral face of the bag-like casing. The digester comprises at least one stirrer for stirring the sewage, where the at least one stirrer is submerged stirrer positioned inside the digestion chamber. The digestion chamber has a volume of 1000-5000 m3.