Toilet catching on fire

My sim put the Composting Container upgrade on her toilet. The description says: "Transforms waste material into a compost and reduces water usage. May spontaneously combust if compost is not used."

And it's spontaneously combusted multiple times while she's been using it. I finally had to delete it and put in a new un-upgraded one. I've never had any option on the toilet (and I was checking frequently) to empty or use the compost, but there was never anything! Is there something I'm missing?