Three years ago my septic tank and alarm was hit by lightning and I was gone on vacation. When I cam home the smell all most knocked me over. Went down stairs and stepped in about a foot of crap that overflowed from the toilet. Over 500 gallons of  the tank backed up thru the toilet. Good thing for home owners insurance because it cost over $39,000 to rip out walls, doors, all wood workings, linoleum, walls, cabinets carpets. Took a couple of weeks for these machine to get all of the bacteria removed. Never ever let your toilet overflow  cause it ain't pretty!