Synth toilets are exactly what the name implies. They are toilets for Synths.
Synths have to use specially suited toilets, since toilets used by organics typically are not suited for flushing down pellets of tungsten, lead or uranium. The synthetic diet is very different from the typical organic diet, so the end result needs to be recycled. Synth toilets therefore tend to simply be containers that safekeep hazardous materials until they can be transported off to recycling centers, either automatically or through the use of helper drones. Toilets for synthetics are widely available and are clearly labeled as such, to avoid confusion or situations like that one time when that one Synth ate 3 packs of potassium jelly beans, used a toilet for organics and discovered how potassium reacts with water.