well, this is an unusual question but i'll take a stab at it :oD
having been in the forklift industry for many years now and have worked for the big dealers i can say this:
-they generally have a wash pit that has a water/oil separator system.
-the pit they have is accompanied by a hot water steam washer like a 'hotsy' or other brand pressure washer system.
-they either use a soap injection system in the pressure washer and use something like purple power or some other solution that can dissolve grease.
-and they usually hire a company to come and collect and dispose of the waste that is generated from this system, usually caught in a catch basin/basket in the drainage.
-if you check with your state environmental agency they will give you all the information you need to make sure you are in compliance with all the usual regulations.
As far as stats on this subject? i can't really answer that particular aspect of it as i don't deal with that side of it directly.