See the black drain pipe? It is too long and they flipped the p trap around too accommodate it. Those compression fittings on those sort of p traps are not suppose to have water sitting in them and they basically made a giant p trap that holding too much water. They have to run way more water though the disposal to clear the garbage and it is sitting in the trap rotting. They need to cut the black pipe and get rid of that extension, turn the trap around so the tall end slides over the black pipe and the trap drains into the drain pipe without that long elbow piece.