When I pressure wash my driveway (4,000psi / 3.5gpm / turbo nozzle) I also clean my property's ~60 gallon City dual wheeled bins interior and exterior.
This is a messy operation.
The bin must be located against something immobile, like a tree, building or vehicle, so it doesn't scoot away from the water pressure.
A downward sloping work area is preferred for draining and visual interior observation.
There is always splashing when cleaning the interior, getting the operator wet and dirty.
It is virtually impossible to get the interior completely clean because of the container shape.