Based on my research in the subject the best way of filtering this water is by using wetlands after having passed the water through an initial filtering stage to get rid of any plastics in it, however although I have been looking around the web I haven't found any equation I could use to calculate the flowrate of water through a wetland.

You can find an image of the wetland filter I'm thinking of using here: Schematic of the Horizontal Subsurface Flow Constructed Wetland (Wikipedia).

Just in case it's not clear, I am not attempting to make the water drinkable I just want to make the water clean enough so it can be used to wash cars again. The water will be stored at most 12 hours during the night so it can be used again in the morning.

Any advice in the matter would be greatly appreciated.