Early on there was mention of Elsan emptying. In the 1980s the airman detailed for Elsan duties was paid 50p per day, taxed, not per Elsan but per day. In order not to saddle one poor soul with the duty and to spread the bountiful largess the duty was spread around.
I once had to do it, liquids only, and I didn't know where there was a sluice. There was a handy drain nearby.