Usually the big issue in apocalyptic fiction is the power going out.
But some, if only lip service, attention is given to water turning off, and then a tiny bit to the sewer backing up.
People poop a lot, and getting rid of it all safely takes a lot more effort than is generally noted:
From A Sewer Catastrophe Companion (pdf) (hat tip: No Tech Magazine)
The guide is only 24 pages.