NOVELTY - Treatment of process water from a hydrothermal organic material conversion process involves anaerobically fermenting a biomass in a fermenter; mechanically dewatering the fermented biomass obtained in the fermenting step; subjecting the mechanically dewatered, fermented biomass obtained in the dewatering step to the hydrothermal organic material conversion process; separating the process water from a conversion product; separating a feed material; and returning at least a part of the second component to the fermenter.    USE - The method or apparatus is useful for treatment of process water from hydrothermal organic material conversion process (claimed).    ADVANTAGE - Method is provided for the efficient and inexpensive treatment of process water from hydrothermal organic material conversion process.    DETAILED DESCRIPTION - Treatment of process water from a hydrothermal organic material conversion process involves an anaerobic fermenting step of anaerobically fermenting a biomass in a fermenter; a dewatering step of mechanically dewatering the fermented biomass obtained in the fermenting step; a hydrothermal conversion step of subjecting the mechanically dewatered, fermented biomass obtained in the dewatering step to the hydrothermal organic material conversion process; a first separation step of separating the process water from a conversion product obtained in the hydrothermal organic material conversion process; a second separation step of separating a feed material, which includes at least the process water obtained in the first separation step, into at least a first component and a second component so that a level of chemical oxygen demand in the first component is lower than a level of chemical oxygen demand in the second component and less than 10 g/L; and a yield of the first component relative to the feed material is more than 30% by weight; and a recirculation step of returning at least a part of the second component to the fermenter. An INDEPENDENT CLAIM is included for an apparatus for treatment of process water from a hydrothermal organic material conversion process, comprising a fermenter for anaerobically fermenting a biomass, a dewatering unit connected to the fermenter, where the dewatering unit is arranged to receive the fermented biomass from the fermenter and to mechanically dewater the fermented biomass, a hydrothermal conversion unit connected to the dewatering unit, where the hydrothermal conversion unit is arranged to receive the mechanically dewatered, fermented biomass from the dewatering unit and to subject the mechanically dewatered, fermented biomass to the hydrothermal organic material conversion process, a first separation unit for separating the process water from a conversion product obtained in the hydrothermal organic material conversion process, a second separation unit connected to the first separation unit, where the second separation unit is arranged to receive the process water from the first separation unit and to separate a feed material, which includes at least the process water from the first separation unit, into at least a first component and a second component so that a level of chemical oxygen demand in the first component is lower than a level of chemical oxygen demand in the second component and less than 10 g/L, and a yield of the first component relative to the feed material is more than 30% by weight, and a first recirculation unit for returning at least a part of the second component to the fermenter.