OP, without knowing how much you know, or want to know, about toilet systems, let me get into a discussion based on some of your symptoms.   First, as the sanitary system goes from the top deck to the engine room, more and more toilets join the system.  Any one of these toilets can contribute to a problem, and everyone "upstream" of the blockage will not have a working toilet.  So, if I went to a crew cabin (typically the lowest on the ship), and purposely introduced something to block
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