An aftertreatment system for a diesel engine is disclosed. The aftertreatment system may include a first flow conduit configured to convey an exhaust gas stream, and a first diffuser assembly positioned fluidly downstream of the first flow conduit and be configured to increase flow uniformity of the exhaust gas stream. The first diffuser assembly may include a first disperser having a honeycomb structure surrounded by a first outer frame. The first disperser may have a number of cells per square inch defining a first cell density. The aftertreatment system may also include a first selective catalytic reduction cassette positioned fluidly downstream of the first diffuser assembly, and the first selective catalytic reduction cassette may include a selective catalytic reduction catalyst.