The City of Boston ran an antiquated sewage disposal system until about 1990-this was gravity fed line that directed raw sewage into a holding vat, then released it into the harbor. this sewage tunnel had a vent stack that was about 50 foot high. A local historian (maybe) once told me that the stack would get filled with grease, which a local guy collected-what he used it for , i have no idea. Would such gease be usable? I assume it would have all kinds of bacterial contamination-. Ever heard of such a scheme?
Probably rendered to produce usable grease for some purpose. Now large producers of grease like restaurants have to have their own grease traps, but there will be one at the sewage processing plant also.