A reactor for treating sludge containing organic matter, particularly by hydrothermal carbonization, including: - a vessel comprising an inner chamber arranged to receive the sludge and to form a circulation path for the sludge adapted to allow the sludge to circulate, - a sludge inlet arranged to introduce the sludge into a sludge introduction area of the inner chamber, - a sludge outlet arranged to discharge at least part of the sludge contained in the inner chamber, and - a steam inlet arranged to inject steam in a steam injection zone of the inner chamber along a steam injection direction, the steam injection direction being against the sludge circulation direction in the steam injection zone along the circulation path, the steam injection zone being separated from the sludge introduction zone, - a circulator arranged to circulate the sludge in the inner chamber along the circulation path, said reactor being characterized in that - the vessel comprises a wall forming a tube and being arranged to separate two parts of the circulation path having opposing sludge circulation directions (2, 4), - the inner chamber comprises an inner space located inside the tube and an outer space located outside the tube, the reactor being arranged: - so that the steam injection zone is located in a lower region of the inner space of the tube, the lower region comprising a first end of the tube, - so that the circulator is located, at least in part, in an upper region of the inner space of the tube, the upper region comprising a second end of the tube opposite the first end, and - so that the sludge introduction zone is located between the lower region and the upper region of the inner space of the tube, and in that the circulation path comprises at least a closed loop.going through the inner space and through the outer space of the tube. - a vessel comprising an inner chamber arranged to receive the sludge and to form a circulation path for the sludge adapted to allow the sludge to circulate, - a sludge inlet arranged to introduce the sludge into a sludge introduction area of the inner chamber, - a sludge outlet arranged to discharge at least part of the sludge contained in the inner chamber, and - a steam inlet arranged to inject steam in a steam injection zone of the inner chamber along a steam injection direction, the steam injection direction being against the sludge circulation direction in the steam injection zone along the circulation path, the steam injection zone being separated from the sludge introduction zone, - a circulator arranged to circulate the sludge in the inner chamber along the circulation path, - the vessel comprises a wall forming a tube and being arranged to separate two parts of the circulation path having opposing sludge circulation directions (2, 4), - the inner chamber comprises an inner space located inside the tube and an outer space located outside the tube, the reactor being arranged: - so that the steam injection zone is located in a lower region of the inner space of the tube, the lower region comprising a first end of the tube, - so that the circulator is located, at least in part, in an upper region of the inner space of the tube, the upper region comprising a second end of the tube opposite the first end, and - so that the sludge introduction zone is located between the lower region and the upper region of the inner space of the tube, - so that the steam injection zone is located in a lower region of the inner space of the tube, the lower region comprising a first end of the tube, - so that the circulator is located, at least in part, in an upper region of the inner space of the tube, the upper region comprising a second end of the tube opposite the first end, and - so that the sludge introduction zone is located between the lower region and the upper region of the inner space of the tube,