b. Brown Grease/Trap Grease
Brown grease is an emulsion of vegetable and animal oil, fat, grease, solids and water. It is separated from the wastewater in a grease interceptor (grease trap) from where it can be collected for different purposes. In many countries these interceptors are obligatory for canteens, restaurants, supermarkets etc. As the brown grease is still a water/oil emulsion with an undefined amount of impurities and (usually) high free fatty acid content, its use is very limited. Some countries allow the use as feed. Other options are to use it as a substrate for biogas or wastewater treatment plants.
The websites of two big American companies, Darling International, Inc. and Sanimax, give good overviews on the service and collection systems.