A system of equipment allowing addition of solid materials to a pressurized pipeline wherein said solid material is conveyed in such a way as to be readily dissolved by the liquid in said pipeline. The system includes a solid material transfer device that is used to transfer solid material to the point of intake in a pressurized pipeline, without allowing liquid from the process pipeline to access the solid material in the solid material feeder. The system of equipment has been found to be particularly useful in feeding pellets containing resazurin.