Tying up Resins and Activated Carbon in Permeable Bags in Water Treatment

What are the advantages and disadvantages of tying up powdery or crystalline resins and/or activated carbon in permeable bags before using to charging water treatment composite filter cylinders in drinking water treatment plant? It is a common practice, by some professionals and others alike, to tie up powered or crystalline resins and activated carbon in permeable bags before charging them into the water treatment composite filter cylinders. Citing the advantages of not losing some into the pores of sand and gravel beds and narrowing the pores in such beds. Has anyone done a research on this? What happens to the "total surface areas" of the resins and activated carbon, active sites contacts and contact time, in this case? Is the functional efficiency of resin or activated carbon reduced or improved?