If it's the latter, is there a way to filter it through a grease trap in order to divert it to the grey water line? If not, then why would I pay $2000 for a composting toilet,and still have to install a septic system? Advice please!

Grey. Grease traps only apply to commercial kitchens.

Thanks! I was getting different information from different sources.

I'm sorry the ICC and the UPC mention lavs, clothes washers, showers and bathtubs and laundry drains but does not list kitchen sinks. Kitchen sinks are another matter, though I am would not argue it.

This is a rational source on the matter.

http://oasisdesign.net/greywater/law/improve/cpcupc.htm

A septic tank is likely in your future but should be sized considerably smaller using all grey water allowed.

Gotta believe that its considered grey water. I know the sinks in RV's drain to the grey water tank. Not a good idea to be disposing of grease down the drain anyway, asking for a plug no matter how much hot water to use to flush it away. The little bit of grease from dish washing and such is miniscule compared to everything else.