After the collapse of the Roman Empire in 476, the sewer system and the public toilet system also collapsed.

Of course it did. There were no valves; water just flowed continuously, from remote source, via aqueduct, through cities, and then out of the city (the ‘cloaca maximus’ in Rome, for example). Homes and business attached to the water system paid a fee based on the diameter of their inlet from that system.

Loss of empire, meant loss of control of the sources — and aqueducts were long and vulnerable to attack. The loss of continuous water flow meant the end of the sever system and of toilets that used that water to carry waste away.

As a side note, that loss of water flow also was an issue for milling grain (although much milling was done by slaves using hand mills, there was a good bit processed for the lower classes). Enter Belisarius, a general under Justinian who, in process of attempting to reconquer the Western Roman territories, hit on moving the mills to rafts in the Tiber, using the river flow to drive the mills. For water power nerds, this meant converting from overshot to undershot or (more likely) side wheels. Much less efficient, but if the overshot wheels have no water…