Cleanness of decanting water and oil

[Leffe123]

I live in sweden where we have a rule in the municipality that says if you have a floor well in the garage, you must have an oil separator that cleans to maximum 5 ppm of oil for discharged water. It is possible to buy ready-made oil separators with coalescer, but they are large and expensive.
Since my flow only comes from melted snow from the car, it will be very low.
Can I then collect the water from the floor well in a small tank, and then let it decant, and then be sure that it has separated after a certain time?