Cast iron pipi from septic tank to collection tank plagged shut with hard as a rock black deposit. I cut 6' plagged pipe, but pressure washer could not cut through remaining one foot pipe to the tank

Can you take another shot are describing the issue? The septic tank is the collection tank. Did you mean a distribution box or between the house and the tank? In any case it is not likely that you canclean this out with a pressure washer. Use a sewer auhger with a 2" or large cutter, rent an electric auger, this is not a small job. Drop down to the bottom of this link Sewer Product Rentals - Sewer Augers - Rooters - Columbus, Georgia I'd suggest the 3/4" 75' auger for a septci cast drain line.