ID black water tank outlet?

Pipe to flange is standard ABS pipe cement (not technically a glue, but a solvent welding process--the plastics are dissolved by the cement a little and resolidify together as the solvent evaporates). Pipe flange to tank is also probably solvent welded, as the tank appears to be ABS, but it could also be a friction or heat weld (and would definitely be if the tank is, say, polyethylene).