An aftertreatment system connected downstream an internal combustion engine arrangement for receiving combustion gas exhausted from the internal combustion engine arrangement during operation thereof, the aftertreatment system comprising: - a primary aftertreatment system comprising a primary reduction system, the primary reduction system comprising a first catalytic reduction arrangement, wherein the primary aftertreatment system further comprises an oxidation catalyst and a particulate filter upstream the first catalytic reduction arrangement; - a secondary reduction system comprising a second catalytic reduction arrangement, the secondary reduction system being positioned in a first conduit and arranged in fluid communication between the internal combustion engine arrangement and the primary aftertreatment system; - a bypass conduit connected between the internal combustion engine arrangement and the primary aftertreatment system, wherein the bypass conduit and the first conduit are connected in parallel with each other; - a first valve arrangement connected to the bypass conduit, the first valve arrangement being operable in a first state in which combustion gas exhausted from the internal combustion engine arrangement is directed through the first conduit to the primary aftertreatment system via the secondary reduction system, and second state in which combustion gas exhausted from the internal combustion engine arrangement is directed through the bypass conduit to the primary aftertreatment system bypassing the secondary reduction system; and - a second valve arrangement arranged in the first conduit in fluid communication between the second catalytic reduction arrangement and the primary aftertreatment system. - a primary aftertreatment system comprising a primary reduction system, the primary reduction system comprising a first catalytic reduction arrangement, wherein the primary aftertreatment system further comprises an oxidation catalyst and a particulate filter upstream the first catalytic reduction arrangement; - a secondary reduction system comprising a second catalytic reduction arrangement, the secondary reduction system being positioned in a first conduit and arranged in fluid communication between the internal combustion engine arrangement and the primary aftertreatment system; - a bypass conduit connected between the internal combustion engine arrangement and the primary aftertreatment system, wherein the bypass conduit and the first conduit are connected in parallel with each other; - a first valve arrangement connected to the bypass conduit, the first valve arrangement being operable in a first state in which combustion gas exhausted from the internal combustion engine arrangement is directed through the first conduit to the primary aftertreatment system via the secondary reduction system, and second state in which combustion gas exhausted from the internal combustion engine arrangement is directed through the bypass conduit to the primary aftertreatment system bypassing the secondary reduction system; and - a second valve arrangement arranged in the first conduit in fluid communication between the second catalytic reduction arrangement and the primary aftertreatment system.