While I don't want to argue with an illustrious poster that post about the 727 is wrong. The 727, like all of it's era sends "grey" water out via drain masts. Human waste is stored in tanks under the toilet and dumped on arrival. The problem with that particular incident, and I'm sure others, was a leaking dump valve on the fwd toilet. This in turn led to the fitting of a secondary bung to contain leaks and to those such as myself to carry out regular pressurised aircraft leak checks.