You should always pour the dirty mop water into a drain. If you do have not the facility to pour directly into a drain, then discharge the dirty mop water into a floor drain, and another option is toilet to discharge it.

correct drain to handle a bucket of dirty mop water
Floor Drain

Note it, never pour dirty mop water into a basin. Dirty mop water consists of a variety types of contamination and may also consist some pieces of solid material. This will contaminate and clog your basin.

Correct way to handle a bucket of dirty mop water:

You should wear hand gloves to handle dirty mop water. After pouring the dirty mop water into a drain the mop should be properly cleaned with cleaning solution then allow it to dry.

What is the correct way to handle a bucket of dirty mop water?
Mop cleaning near a drain hole

Strip the gloves, rinse off your hands with handwash. Don’t forget to wash your legs.