Abstract: An apparatus for treating an exhaust gas stream and having removable module(s) is disclosed. The apparatus comprises a housing, providing fixed boundaries at least between an inlet and an outlet and at least two sections arranged within said housing, the housing defining an interior exhaust passage that extends sequentially through said sections, wherein one or more treatments are to be performed upon a gas flowing through said sections. At least one of said sections is removable from within the fixed boundaries of the housing in a direction essentially perpendicular to said flow direction.