An apparatus for aftertreatment of exhaust gas comprising: a housing configured to define an inner cavity (310-314, 410-413); an exhaust inlet arranged to the housing for entering exhaust gas to the inner cavity (310-314, 410-413); a first flow guide (420, 430) in the inner cavity (310-314, 410-413) configured to guide at least part of the exhaust gas to a first direction to form a first direction flow comprising a center flow and an edge flow circulating and advancing around the center flow in a first direction; a reactant inlet being arranged to the housing for dispensing reactant to the first direction flow in the inner cavity (310-314, 410-413) to mix the reactant with the exhaust gas; the apparatus further comprising a second flow guide in the inner cavity (310-314, 410-413) located downstream from the first flow guide (420, 430); the second flow guide being configured to: guide at least a part of the first direction flow to a second direction to form a second direction flow, wherein the second direction being opposite to the first direction; and guide the second direction flow to a third direction to form a third direction flow, wherein the third direction being downstream to the second direction and parallel to the first direction; the apparatus further comprising an exhaust outlet arranged to the housing for exiting output exhaust gas from the inner cavity (310-314, 410-413); the exhaust outlet being downstream to the second flow guide; mixing of the reactant and the exhaust gas occurring within the first direction flow, the second direction flow and the third direction flow, characterized in the first flow guide (420, 430) comprising a perforated tubular element; wherein at least part of the exhaust gas is configured to enter an interior of the perforated tubular element through apertures to form the center flow advancing inside the perforated tubular element; the perforated tubular element being configured to guide the exhaust gas to circulating and advancing movement in the first direction to form the edge flow circulating and advancing around the perforated tubular element. a housing configured to define an inner cavity (310-314, 410-413); an exhaust inlet arranged to the housing for entering exhaust gas to the inner cavity (310-314, 410-413); a first flow guide (420, 430) in the inner cavity (310-314, 410-413) configured to guide at least part of the exhaust gas to a first direction to form a first direction flow comprising a center flow and an edge flow circulating and advancing around the center flow in a first direction; a reactant inlet being arranged to the housing for dispensing reactant to the first direction flow in the inner cavity (310-314, 410-413) to mix the reactant with the exhaust gas; the apparatus further comprising a second flow guide in the inner cavity (310-314, 410-413) located downstream from the first flow guide (420, 430); the second flow guide being configured to: guide at least a part of the first direction flow to a second direction to form a second direction flow, wherein the second direction being opposite to the first direction; and guide the second direction flow to a third direction to form a third direction flow, wherein the third direction being downstream to the second direction and parallel to the first direction; guide at least a part of the first direction flow to a second direction to form a second direction flow, wherein the second direction being opposite to the first direction; and guide the second direction flow to a third direction to form a third direction flow, wherein the third direction being downstream to the second direction and parallel to the first direction; the apparatus further comprising an exhaust outlet arranged to the housing for exiting output exhaust gas from the inner cavity (310-314, 410-413); the exhaust outlet being downstream to the second flow guide; mixing of the reactant and the exhaust gas occurring within the first direction flow, the second direction flow and the third direction flow, characterized in the first flow guide (420, 430) comprising a perforated tubular element; wherein at least part of the exhaust gas is configured to enter an interior of the perforated tubular element through apertures to form the center flow advancing inside the perforated tubular element; the perforated tubular element being configured to guide the exhaust gas to circulating and advancing movement in the first direction to form the edge flow circulating and advancing around the perforated tubular element.