Grease Interceptor Sizing and Installation Guidelines

Michael Tobias
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    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.

    Make sure your grease interceptors are specified correctly.

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    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:

    • PDI-G101: Testing and Rating Procedure for Hydro Mechanical Grease Interceptors (Plumbing and Drainage Institute)
    • ASME A112.14.3: Grease Interceptors (American Society of Mechanical Engineers)
    • ASME A112.14.4: Grease Removal Devices

    Grease Interceptor Sizing Procedure

    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).

    Kitchen Fixtures Covered by Table I

    Kitchen Fixtures Covered by Table II

    Pot sinks
    Food preparation sinks
    Scullery sinks
    Floor drains used only for washdown

    Scraper sinks
    Automatic dishwashers
    Any fixture where soup and 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 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,

    • Table I requires 14 lb of retaining capacity
    • Table II requires 20 lb of retaining capacity
    • The total grease interceptor capacity is 34 lb.

    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:

    • 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.

    General Guidelines for Grease Interceptor Installation

    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:

    • An NYC Registered Design Professional must file a formal petition.
    • The proposed system must deliver equivalent or superior performance compared with a grease interceptor.
    • Equivalency must be demonstrated with detailed documentation and calculations.

    Final Recommendations

    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.

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    Tags : Commercial Kitchen Design Commercial Kitchen Plumbing Grease Interceptors

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