I am fairly certain this is no real help at all since I am certainly no expert on this sort of subject. It is simply my thoughts on the matter.
We recently had a very similar treatment for one of our cats. The veterinarians told us that for human beings the waste is no big deal at all. It simply goes right down into the sewer. But for a cat you get to collect and store the waste for 80 days. After that it can go out in the garbage. I have forgotten what the half-life of the isotope was but it is just a matter of a few days (four days, eight days, something like that) As with humans, the cat is radioactive enough to set off sensors at airports, border crossings, etc. so they give you paperwork to explain the radioactivity.
I think I can understand your concern. In a large sewage system the tiny contribution of one individual gets lost in the mix. (A famous saying I like: "The solution to polution is dilution.") But in your own private septic system there is no such mixing. It is sort of like the kitty litter thing.
Maybe it would be prudent to literally use a chamber pot and store the waste, both solid and liquid, for about three half-lives. At that time the level of waste radioactivity will be only 1/8th of what it was in the beginning, pretty weak. Store that waste for the same 80 days as they do for cats and then dump it in the septic system a bit at a time. Your septic system will never have to deal with any significant amount of radioactivity at all. The vast majority of the material will have been expelled in those first three half lives.