Okay. I had a thought, earlier. So, I reread the story for confirmation. Confirmation having been achieved, I then did a search on the thread to see if this has been mentioned before. It doesn't seem so.
As such!
It is my conclusion that Anna doesn't actually use the toilet.
See, it's not uncommon for that to be kinda glossed over in stories. Like, you don't generally say "and then they used exactly 13 squares of toilet paper," and so on. But generally, you have narrative gaps where that
could have happened.
With Anna, though, frequently we have large swaths of her day that are told in great detail, with no gap in which she might have used the facilities. And that's more telling than any gaps that are left, because if you can find one day where she doesn't use the bathroom, then obviously she doesn't need to; it doesn't matter If there are other days where she might have but we can't prove it.
So, for example, when she goes into her quarters for the first time and makes the adorable blanket nest? That's fully detailed from start to finish, and she conks out to sleep there without ever using the bathroom- she doesn't even go in to look at it until the following morning, where she has her terrifying encounter with the shower- whereupon she scampers out without, again, leaving any time to use the actual toilet. (And really, given the smart shower and smart tub, I imagine that this bathroom also contains a smart toilet which would be its own brand of terror for her- so it'd definitely be worth a mention)
And the thing is, it's
completely believable that this is the case. We know it's been months or even years since she last turned off her impeller field, which prevents things from touching her and vice versa. Expelling waste inside her own force field would... probably be awkward. And extremely messy. But neither would it have been a good idea for her to turn it off, ever, when defending the beleaguered Unknown Canadian Town. Similarly, dealing with a period in a besieged town that probably ran out of tampons and other supplies before she even hit puberty? She pretty much would've
had to have Durga taking care of that for her. So why not other wastes as well? Durga can reprocess them, helping keep the water split out for anything that might need hydration, hydrogen, or oxygen. The solid wastes can be consumed for other materials, such as sulphur and the like. Helpful for making more ammunition, or refining other compounds. Waste not want not, after all!
Plus, this gives yet another aspect of her alien, almost inhuman nature. None of her classmates have noticed this, yet- but if she ever goes camping or has a sleepover, they might. It's a neat little thing that I almost didn't think of before I started really looking back... but I like it. Gives a nice touch of 'other' to her!