Abstract: An apparatus and method for increasing the mass transfer of a treatment substance into a liquid flowing in a pipe in a full pipe flow regime has a diversion conduit which receives a portion of the liquid. The portion of the liquid is mixed with a treatment substance and then reintroduced into the pipe at a downstream location through an injection structure. Between the diversion conduit, on the upstream side, and the injection structure, on the downstream side, there are a plurality of flow vanes disposed circumferentially about a cylindrical inner wall of the pipe, where each flow vane extends radially inward toward a central axis of the pipe, extending into the main stream flow of the liquid. Another embodiment of the invention has a flow grid located downstream of the injection structure.