TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Method: The particulate carbon-rich compounds of the ascending or ascended reaction mixture are removed and the wastewater is discharged in the remaining process. The at least part of the process water can be reused for the preparation of the starting mixture. Only a portion of the ascending reaction mixture is added with oxygen-containing fluid. The part of the ascending reaction mixture, which is added with oxygen containing fluid, substantially the amount of water corresponding to that, is released during the hydrothermal carbonization of the biomass. The part of the reaction mixture, which is added with oxygen-containing fluid, after wet oxidation, is fed for biological purification stage. The particulate carbon-rich compounds are substantially separated from the portion of the ascending reaction mixture, which is added with oxygen-containing fluid, before the oxygen-containing fluid is added. The molecular oxygen (O2), preferably hydrogen peroxide is used as the oxygen source. The heat released during the hydrothermal carbonization and/or the wet oxidation is used for heating the starting mixture to flow downward and/or to dry the separated particulate organic compounds. The downstream is brought down to the depth of at least 200 m, before the rise of the upstream begins, preferably to the depth of at least 350 m. The ascending reaction mixture is added with the oxygen-containing fluid at the depth, where the reaction mixture has a temperature of at least 200 degrees C and/or a pressure of at least 1.6 Mpa. Preferred Components: A sampling line for removing reaction mixture from the upstream is included in the up-comer. The device for introducing oxygen-containing fluid and is positioned, in such a manner that the oxygen-containing fluid can be supplied to the ascending reaction mixture only in the sampling line. The inlet of the sampling line is a separating device, which is provided and is arranged in such a manner that transfer of the reaction mixture suspended particulate carbon-rich compounds from the up comer into the interior of the sample line is substantially under bondable, while a transfer of the reaction mixture dissolved organic compounds into the interior of the sample line is substantially is not bondable. EXAMPLE - No suitable example given.