The grease interceptor is the most expensive single item in a restaurant plumbing scope and the one most often sized wrong. The confusion comes from the fact that two different departments care about it.
Who decides the size
The plumbing code gives a formula based on fixture units and flow. The local wastewater authority (the town or city) usually has its own minimum, often larger, based on seating and the type of kitchen. The larger number wins. In Gilbert and Chandler that is usually the municipal minimum.
Interior vs. exterior
An exterior in-ground interceptor is the default for new construction: bigger, cheaper per gallon, pumped from outside. Interior hydromechanical units fit a TI where the slab cannot be cut, but they are small and need frequent service. Decide this before the underground is laid out.
What goes to the interceptor
Kitchen sinks, floor sinks under equipment, dishwashers (depending on jurisdiction) and mop sinks. Not restrooms. Getting a restroom line into the interceptor is a common error that fails final.
What we do
We size to the stricter of the two codes, confirm it with the wastewater authority in writing before we order the tank, and show the owner the pumping cost so there are no surprises in year one.
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Send the drawings or describe the building.
A project manager or Carol replies the same business day. Contract clients on the 24-hour line get a plumber, not a service.