There were some Delta Airlines Aircraft (the original DC-8 61 with the engines that would "whine" when you GPU'ed them) where it was just a septic tank and did not flush. It had the "blue chemical" in it. I found that out the hard way. In one of the rare occasions when I flew UNAM (at age 8), I had to call an FA back to the lavatory, having just dropped a perfect deuce, because I couldn't find the "handle." "Oh, don't worry, it's a special chemical that dissolves it..." the FA replied. Embarrassing (well, for a 8 yo, at least).
Skip to 01:39 for the GPU start.