Not sure if it's the same in Canada, but where I am from the problem is the oil (lubricant and fuel) from the car and at car washes the runoff gets treated by letting it go through an oil trap.

The trapped oil gets disposed of separately, the treated oil-free water goes to the sewer.

Hell if I know.

AFAIK the regulations were put into place to prevent untreated water from going into storm drains and into the water system, so presumably the cities are fine if you pour it down into the bathtub drain and let it go to the waste water treatment plants.

You can allow the liquid to evaporate off in an evaporation pond, or combine it with an absorbent gel. Then the solids can be put in trash.