Plumbing Codes can vary from state to state. Most western states start with the Uniform Plumbing Code (UPC), adopting the UPC in-all or in-part with amendments. Gravity grease interceptor requirements were contained in Appendix H of the UPC prior to 2006. Starting with the 2006 UPC, these requirements were moved to Chapter 10. The 2006 UPC also changed the term “grease trap” to “hydro-mechanical grease interceptor” to eliminate confusion. A trap provides a liquid seal to prevent the emission of sewer gases into a structure through the connected fixture. A grease interceptor intercepts free-floating fats, oils and grease from waste water discharges and whether or not it provides a liquid seal, it is not intended to serve the purpose of a trap.
Resources
State plumbing codes:
Insurance Institute for Business Home and Safety interactive map with plumbing codes by state.
Green Drain's comprehensive list of plumbing codes by state.