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 . <EOS>