A fellow man of ingenuity! I considered many things I could dump down, including poisons, cleaners and yes boiling water... but I think ultimately I want to get the guy closer to me, so I can be sure of his fate. Maybe I've already gone to the Dark Side. Hmm.
Can you snag him with a set of barbecue tongs and toss him in a plastic storage container and just take him outside?
Shit, thought he was a small one. burn the house down, piss on the ashes and start over. The boiling water idea bothers me for whatever reason. Guess I’m turning into a pussy or some shit.
Do you have a pressure washer? Use it with this and it might send him down the pipe a bit and encourage him never to return. https://www.amazon.com/dp/B085WJ4GWG/
Where is this located? The scales are not highly keeled like pit vipers, so that rules out most venomous snakes ( rattlers, cottonmouth, copperheads) except possibly corals, and its not a coral for sure. Im 90% convinced its just a rat snake. Ive been bitten by them quite a few times and its like the rough side of velcro rubbed you, totally harmless and kind of funny. You could try the Ice bucket challenge but that will likely make it retreat back into the deep. Put a glove on if your worried, but just grab it outta there next time you see it. I had a similar experience not too long ago but this one was working its way up thru and electrical hole above my breaker box in an unfinished room.
Had a buddy of mine years ago that used a C02 fire extinguisher and basically cooled a rattlesnake down to where it was temporarily frozen. Used a snow shovel and scooped it up and released it way away from his shop.
I think trying to scare, freeze, heat, chemically treat it might just make him run back into your septic system, where it could lay dormant for months. You need to know he's gone, or constantly be wondering if he's starring up at you.
Fill the tank and bowl with ice? But then, I'd suspect it would just slither away and come out in a different shitter.