Disclosed is a reactor for treating, particularly by hydrothermal carbonization, sludge containing organic matter, including, with: a vessel (100) including an inner chamber arranged to receive the sludge and to form a path of travel for the sludge adapted to allow for circulation of the sludge, a sludge inlet (1) arranged to introduce the sludge into a sludge introduction area of the inner chamber, a sludge outlet (11) arranged to discharge at least part of the sludge contained in the inner chamber, and a steam inlet (3) arranged to inject steam in a steam injection zone of the inner chamber along a steam injection direction, the steam injection direction being different from a sludge circulation direction in the steam injection zone along the circulation path, the steam injection zone being separated from the sludge introduction zone.

This specification discloses a continuous flow reactor comprising a reaction vessel enclosing a rotating shaft which bears multiple impellers. A removable insert comprising baffles separates the interior of the reaction vessel into multiple interconnected chambers each enclosing at least one impeller, and a gap between the outer edge of the baffle and the vessel's interior wall facilitates insert removal for maintenance but avoids excessive backmixing.