Method for restoring performance capabilities of exhaust gas treatment apparatus

A method for recovering performance of a discharge gas processing apparatus, which apparatus includes a honeycomb catalyst having gas conduits for feeding a gas to be treated, the catalyst being provided in a discharge gas conduit of the apparatus and, in use, performing gas treatment on the sidewalls of the gas conduits, characterized in that the honeycomb catalyst is in the form of a single layer of a flue gas NOx removal catalyst, and that the method comprises rearranging the honeycomb catalyst for recovering performance thereof such that a deteriorated portion of the honeycomb catalyst is transferred from the inlet side of the discharge gas conduit so that a predetermined range of the discharge gas conduit from the inlet side represents a portion other than the deteriorated portion, wherein the deteriorated portion is on the upstream side in terms of the flow of the gas to be treated, extends to cover the predetermined range of the honeycomb catalyst, and is determined on the basis of a sustained turbulent flow distance which is the distance from the inlet to a site where turbulent flow energy is lost in the course of transition from turbulent flow to laminar flow.