Did the inspector say something about too little water standing in the pit in proportion to the dirt-mud-sand? That seemed to be a concern by one of our city's inspectors. I wonder if there is "some grandfathering in" when it comes to pit traps? An older former employee who use to lease service stations was familiar enough ... so just before it goes out to the sewer ... we made a stainless steel framework ... so there is about a 4'x2' area with a stainless cover over it for the 90° downward facing pipe that goes out to the city is isolated & only adjacent water at a higher level enters & almost zero anything but water. We have signs & watch carefully (mostly attended) for anyone releasing oil that will float on top.
It seems from public announcements etc. grease from restaurants & even residences is something they watch even closer. I have seen city crews use the fire hydrant water to flush the storm sewer system sometimes. Being this far north in North Dakota & in a city ... the city crews & highway dept uses massive amounts of sand ... but again it appears that would be specific to storm sewers.
I am sure someone who recently developed a car wash with all the correct permits can share the most current federal requirements ... a specific city may have add'l requirements though. The quality of the drawing(s), video, &/or animation certainly could be a factor!