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How to Size a Grease Interceptor for NYC DEP- New York City

We have covered grease traps and grease interceptors in this blog before however, there is a lot of confusion on how to appropriately size an interceptor for permitting in New York City. As a quick review, grease traps are basic draining systems for fat, oils, and grease that are small enough to sit under a sink; while a grease interceptor is a large device that is buried underground to intercept thousands of gallons of fats, oils, and grease before they reach the city sewer system.

If your architects and engineers have done their job correctly, then it should be easy for your licensed master plumber to figure out the correct sizing for your grease interceptor. First, they must comply with the criteria in the applicable tables found in Title 15 of the Rules of the City of New York “Best Management Practices for all Non-Residential Discharges of Fats, Oils, and Grease into the Public Sewer System.

Sizing a Grease Interceptor

The first step in sizing a grease interceptor is to review the kitchen layout. The kitchen layout will determine where and how much grease will flow through the interceptor.

After the layout has been determined you must calculate the aggregate volume that flows from each unit. The aggregate volume is the maximum volume that can flow through the interceptor simultaneously.

For reference, the NYC Department of Environmental Protection (NYC DEP) has two tables, each with interceptor capacity values (in pounds) corresponding to aggregate volume from kitchen fixtures (in cubic inches). Kitchen fixtures in Table 1 include fixtures such as pot sinks, prep sinks, and scullery sinks, as Table 2 includes fixtures with more aggregate volume such as automatic dishwashers, scraper sinks, and any fixtures where soup or stock kettles are discharged.

The retaining capacity is calculated separately for the fixtures in each table and the two values are then added. The tables can be viewed directly on the NYC Department of Environmental Protection website.

  • For commercial kitchens with floor drains where grease may be discharged, Table I still applies. However, the aggregate volume must be increased by 1,540 cubic inches for each floor drain.

  • Soup and stock kettles that discharge into a floor drain count towards the Table II aggregate volume, even if floor drains are Table I fixtures.

  • Table I reaches up to 123,000 cubic inches, while Table II reaches up to 61,600 cubic inches. If these values are exceeded, the grease interceptor capacity must be calculated by an NYC Registered Design Professional, based on data extrapolation.

  • The retention capacity of grease interceptors (in pounds) must be at least twice the numerical value of flowrate through the unit (in gallons per minute). For example, you need at least 40 lb for 20 gpm.

A final requirement is that water flowing through a grease interceptor must not have a temperature above 180°F.

Not a Simple Task

As with any major new build, renovation, or demo project, every aspect of the design process is crucial. Specific requirements from the NYC DOB and the NYC DEP should be expertly followed by Professional Engineers, Registered Architects, and in this case specifically, Licensed Master Plumbers. Sizing a grease interceptor is no simple task so let us help you today!

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