First, and foremost, is the question of your sewer service area. Is your facility serviced by a municipal sanitary sewer service provider? If not, you be be able to get away with some type of ground recharge system provided you can get out most of the contaminants, as described below. You may need a permit or approval from your local or regional government to discharge into ground (this is guaranteed if you discharge to a surface water).
Second, grit. Grit is your biggest source of pollutant. Make sure you install a grit capture system that is large enough to cope with your flows and provides enough detention/retention to settle out most of the fines. Recommended detention time: is about 2 hours. This is often calculated as an overflow rate L/min/m2 or gal/day/ft2.
Third, dissolved salts and detergents. No interceptor will rid you of these substances, so careful selection of detergents (i.e, phosphate free) may be important to your system.
Next, oil and grease. If your facility is a 'detailing shop' these contaminants will be ranked more important but for most car wash sites, this is not a big issue.
Last but not least, is the issue of water 're-use.' Any amount of wash water you can recycle that you can build into your system is a bonus. Most sites that re-use water will direct that treated water to a pre-rinse or a wash cycle. Final rinse is usually clean water. Your treatment system can be as sophisticated as you can afford. A full treatmetn Reverse Osmosis unit will rid you of the salts in the recycled water but will concentrate the salts in your waste brine.
Hope that helps. fs