BFing poo is that liquid that it can just go through the wash. I just put my nappies in on their own ad do an extra rinse. If you were that worried you could rinse the insert quickly under a tap to get the bulk of it off but I wouldn't worry with it being so fine. If you did want to put them in with other clothes i'd put them through a rinse cycle first (on their own) to dilute the urine.