Most municipalities have a rule of a MINIMUM of a 55 gal. per minute grease interceptor for any restaurant kitchen. It's not magic, just a big box with an inlet and an outlet and two or three screens and baffles. Greasy water goes in to the box, grease rises to the top, dirty-albeit grease free water goes down the drain. Depending on usage the crust of grease/crud needs to be removed every 3-4 mths. All sinks--including hand sinks, and mop sinks as well as floor drains and any other equipment and (kettles, tilt-skillts) MUST be hooked up to this as well--but NOT the dishwasher.
Many municipalities around here "suggest" that you open it and clean it out yourself one a month and have the "honey wagon" suck it out 2-3 times a year. And there's method to this madness: If you clean it out yourself, you know what is going down your drain, and might possibly, actually pay more attention to the dish pit. What do you do with the crud? Scoop it into a mayo bucket with a tight fitting lid and put into the dumpster.. Don't like mucking it out? Take a little more care, wipe out greasy sheet pans and cookware with wadded up newspaper first to get rid of most of the crud, and better yet have filter system in the dish pit to remove most of the solids before they even hit the grease trap.
ABOVE ALL do not use a garbage disposal unit and do not hook up a potato peeler directly to the grease trap, these things can wreak some serious damage to the whole sewer system.. And believe me, you do NOT want a city sewer worker showing up at your door with a "snake", a tiny video camera mounted on a cable that gets shoved down your lines for an inspection....
The whole thing grosses you out? There is the "Little dipper". It's a little box that has a series of rotating discs mounted in a platform that sits on the surface of the water in a smaller version of a grease trap. The little discs get "combed through" and the pure grease gets siphoned off into a container that you can sell or have picked up for free. This is about 3-4 times more expensive than a regular grease interceptor, but 3 times as more intelligent, and you don't need to pay the $2-$300 "honey dipper" charge every couple of months that you would for a regular grease interceptor
Deep fryer oil is a commodity. Bio-diesel is one use, soap is another.