Huh? Why on earth would anyone plumb their toilet into the grease trap? There’s probably some cross-contamination from sewage getting into the grease trap outlet, but that doesn’t mean that toilet waste flows through it. There’s also lots of contamination from people washing their hands, cleaning meat and fish, washing dirty vegetables [0], etc in the kitchen sink.

Certainly one ought not to eat anything extracted from the grease trap, but one also ought not to eat anything extracted from the grease collection barrel.