Grease interceptors are used in plumbing systems to trap greases, oils and solid particles before they can reach the public sewage. For this reason, they are also known as grease traps. A certain oil content is normal in wastewater, and it forms a thin floating layer once it reaches septic tanks and water treatment facilities. However, wastewater from food preparation contains more oil than in other applications, and the waste from multiple commercial kitchens can overwhelm water treatment systems and cause spillage. Lard and other high-viscosity fats can also solidify as they cool down, trapping other solids and blocking sewage pipes.
Grease interceptors should be installed for any commercial kitchen equipment where grease may be released into drain pipes. Consider that some plumbing fixtures that do not normally receive grease may be exposed to large amounts occasionally: for example, floor drains are not normally exposed to large volumes of grease, but there may be cases where it is spilled on the floor in large amounts and the floor drains must handle it. Of course, there are also cases where the use of grease interceptors is obvious. For example, scullery sinks and meat preparation sinks produce large amounts of grease, oils and solids.
For projects in New York City, only a Registered Design Professional (RDP) can submit commercial kitchen plumbing designs for approval by the NYC Department of Buildings, and only a Licensed Master Plumber (LMP) can carry out the actual installation. Also consider that the NYC Department of Environmental Protection (DEP) performs routine inspections to ensure that grease interceptors receive proper maintenance and cleaning.
In addition to NYC-specific requirements, grease interceptors are subject to national standards such as:
The first step to specify grease interceptors is reviewing the kitchen layout and determining which equipment must be routed to grease traps. Interceptors may be specified individually or for multiple fixtures at once, as allowed by the available space.
After a grease interceptor layout has been proposed, the next step is calculating the aggregate volume from the fixtures connected to each unit. Aggregate volume is the maximum volume that can flow through the interceptor simultaneously, determined by adding the wastewater volume of individual fixtures plus any required margins.
For calculation of grease interceptor retaining capacity, the NYC Department of Environmental Protection provides two tables, each with interceptor capacity values (in pounds) corresponding to aggregate volume from kitchen fixtures (in cubic inches).
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Kitchen Fixtures Covered by Table I |
Kitchen Fixtures Covered by Table II |
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Pot sinks |
Scraper sinks |
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 in the NYC Dept. of Environmental Protection website.
Assume the aggregate volume is 3,000 cubic inches for Table I fixtures and 2,500 cubic inches for Table II fixtures. With these values,
This value applies if all the kitchen fixtures in the example discharge into a common interceptor. If multiple grease traps are used, the procedure is carried out separately for each one. Although the procedure is straightforward, keep in mind that some exceptions apply:
A final requirement is that water flowing through a grease interceptor must not have a temperature above 180°F.
A recommendation not only for grease interceptors but for equipment in general is adhering to manufacturer instructions. A good product can have poor performance if it used in the wrong application or installed incorrectly.
Grease interceptors are available in recessed, semi-recessed and on-floor versions. They must be equipped with vented flow control fittings to keep discharge below the flow capacity specified by the manufacturer. Using manual valves to reduce flow to a grease interceptor is not allowed.
After initial installation, grease interceptors must be kept under optimal operating conditions. Cleaning and grease removal must a part of routine maintenance, and the units should be accessible for inspection by the NYC Dept. of Environmental Protection.
The use of alternative devices is allowed, as long as the following requirements are met:
Grease interceptors perform a fundamental role in commercial kitchens, as well as other applications where significant amounts of grease or oil are discharged. They are very important for public hygiene, and the NYC Dept. of Environmental Protection places stringent requirements regarding their use, while carrying out routine inspection for units in operation.
By working with qualified design engineers, you can ensure your grease traps are specified properly. This ensures code compliance, offers hygiene for your staff and customers, and prevents the uncontrolled discharge of grease and oil into the environment.