Answer
Before starting the work, have both the septic tank and the pumping chamber pumped out. At this point, you may be able to snake out the line between the two, which will be much easier than replacing it. If it does need to be replaced, dig up the area, including about 6 inches below the pipe. The pipe will likely need to be cut to get it out. Make 2 cuts, take out the center piece, then pull out the other two pieces from the tanks. Do not make the hole in the tanks any larger than you need to. Add about 2" of 3/4" gravel to just below the tank openings. Replace the old cast iron pipe with schedule 40 PVC pipe. Be SURE the pipe drains the correct direction! Seal the pipe into the tanks with Quikrete and allow to cure according to the package directions. Fill around the pipe with 3/4" gravel - should have at least 2" below and 2" inches above the pipe. Then cover the pipe back up with soil - you'll want to mound the cover dirt slightly to allow for a bit of settling.