Grease interceptors are used in plumbing systems to trap grease, oils and solid particles before they reach the public sewer. For this reason, they are also known as grease traps. A certain oil content is normal in wastewater and forms a thin floating layer when it reaches septic tanks and water treatment plants. However, wastewater from food preparation contains more oil than other applications, and waste from many commercial kitchens can overload water treatment systems and cause spills. Lard and other high-viscosity fats can also solidify as they cool, trapping other solids and blocking drain pipes.
Grease interceptors must be installed on any commercial cooking 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 on occasion: for example, floor drains are not normally exposed to large volumes of grease, but there may be cases where it is spilled onto the floor in large quantities and the drains of the floor must deal with it. Of course, there are also cases where the use of grease interceptors is obvious. For example, pantry sinks and meat preparation sinks produce large amounts of fat, oils and solids.
Make sure your grease interceptors are specified correctly.
For projects in New York City, only a Registered Design Professional (RDP) can submit commercial kitchen plumbing designs for approval to the New York City Department of Buildings, and only a Licensed Master Plumber (LMP) can perform the actual installation. Also consider that the New York Department of Environmental Protection (DEP) performs routine inspections to ensure that grease interceptors are properly maintained and cleaned.
In addition to New York-specific requirements, grease interceptors are subject to national standards such as:
- PDI-G101: Testing and Classification Procedure for Hydromechanical Grease Interceptors (Institute of Plumbing and Drainage)
- 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 in specifying grease interceptors is to review the kitchen layout and determine which equipment should be routed to the grease traps. Interceptors can be specified individually or for multiple pieces of equipment at the same time, as available space allows.
After proposing the grease interceptor layout, the next step is to calculate the aggregate volume of equipment connected to each unit. Aggregate volume is the maximum volume that can flow through the interceptor simultaneously, determined by adding the volume of wastewater from individual facilities plus any required margins.
For calculating grease interceptor holding capacity, the New York Department of Environmental Protection provides two tables, each with interceptor capacity values (in pounds) corresponding to the aggregate volume of cookware (in cubic inches).
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Kitchen utensils covered by Table I |
Kitchen Utensils Covered by Schedule II |
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Pan sinks |
Scraper sinks |
The retention capacity is calculated separately for the equipment in each table and the two values are then added together. The tables can be viewed directly on the New York Department of Environmental Protection website.
Assume the aggregate volume is 3,000 cubic inches for Table I fittings and 2,500 cubic inches for Table II fittings. With these values,
- Table I requires 14 lb of holding capacity
- Table II requires 20 lb of holding capacity
- The total capacity of the grease interceptor is 34 lbs.
This value applies if all cookware in the example discharges to a common interceptor. If multiple grease traps are used, the procedure is carried out separately for each one. Although the procedure is simple, remember that some exceptions apply:
- For commercial kitchens with floor drains where grease can be discharged, Table I still applies. However, the aggregate volume must be increased by 1,540 cubic inches for each floor drain.
- Soup and broth boilers that discharge into a floor drain count toward Table II aggregate volume, even if the 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 a New York registered design professional based on data extrapolation.
- The holding capacity of grease interceptors (in pounds) must be at least twice the numerical value of the flow rate through the unit (in gallons per minute). For example, you need at least 40 pounds 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 to follow the manufacturer's instructions. A good product can perform poorly if used in the wrong application or installed incorrectly.
Grease interceptors are available in built-in, semi-recessed and floor-standing versions. They shall be equipped with vented flow control fittings to maintain discharge below the manufacturer's specified flow capacity. It is not permitted to use manual valves to reduce flow to a grease interceptor.
After initial installation, grease interceptors must be maintained in optimal operating condition. Cleaning and degreasing must be part of routine maintenance, and units must be accessible for inspection by the New York Department of Environmental Protection.
The use of alternative devices is permitted, provided that the following requirements are met:
- A New York Registered Design Professional must file a formal petition.
- The proposed system must provide equivalent or superior performance compared to a grease interceptor.
- Equivalence must be demonstrated with detailed documentation and calculations.
Final Recommendations
Grease interceptors play a key 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 New York Department of Environmental Protection imposes strict requirements on their use while also conducting routine inspections of operating units.
By working with qualified design engineers, you can ensure your grease traps are specified correctly. This ensures code compliance, provides hygiene for your employees and customers, and prevents uncontrolled discharge of grease and oil into the environment.
