Mop Water Usually Belongs in the Toilet
The toilet is generally the best place to dump dirty mop water at home because it connects to the sanitary sewer or septic system and is built to carry waste away. That is also why several cleaning and plumbing guides point people toward the toilet rather than a kitchen sink or bathtub drain.
Dirty mop water is not just water. It may contain soil, food crumbs, pet hair, bathroom dust, detergent, disinfectant, and whatever mystery grit was hiding under the edge of the fridge. A toilet handles suspended waste better than a narrow tub drain, and flushing sends the mess through the waste line quickly.
There is a hygiene reason too. If I pour mop water into a bathtub, I now have floor dirt sitting in the place where someone stands barefoot to get clean. Yes, you can rinse and disinfect the tub
