8.2 Interceptors and Separators

Purpose and Sizing of Grease Interceptors

In commercial food preparation facilities, grease-laden waste poses a significant threat to municipal sewer infrastructure. Fats, oils, and grease (FOG) entering the drainage system cool and solidify, adhering to pipe walls and eventually causing complete blockages. Under IPC Section 1003, grease interceptors are required to separate and retain FOG before the wastewater discharges into the sewer system. There are two primary types of grease interceptors, each with distinct sizing and installation criteria:

Hydromechanical Grease Interceptors

These are smaller units, typically installed indoors near the kitchen equipment (such as under-counter or floor-recessed units). They use a combination of baffling, flow control, and air entrainment to separate grease.

• Sizing Basis: Sizing is based on the drainage flow rate in gallons per minute (gpm) of the connected fixtures, in accordance with ASME A112.14.3 or PDI G101 standards (IPC 1003.3.4.1).
• Flow Control: Hydromechanical interceptors must have an approved rate-of-flow control device installed on the inlet side. The flow control device must be vented to prevent air-binding and ensure that the flow rate does not exceed the interceptor's rated capacity.
• Sizing Calculation Example: Sizing based on fixture volume utilizes the formula $V = L \times W \times D / 231$ to find the volume in gallons. For example, a three-compartment sink with compartments measuring 20" x 20" x 12" has a volume of $3 \times (20 \times 20 \times 12) / 231 = 62.3$ gallons. Applying a 75% load factor gives $62.3 \times 0.75 \approx 46.7$ gallons. Sized for a 2-minute drain-down time, the required flow rate is $46.7 / 2 = 23.4$ gpm, meaning a 25 gpm rated interceptor is required.

Gravity Grease Interceptors

These are large underground tanks, typically located outdoors. They rely on gravity separation, allowing wastewater to remain in the tank long enough for FOG to float to the top and heavy solids to settle to the bottom.

• Sizing Basis: Sized based on liquid volume capacity (gallons) and retention time calculations (IPC 1003.3.4.2). They must be designed to provide a retention time of at least 30 minutes during peak flow periods. Gravity grease interceptors are typically sized between 500 and 2,000+ gallons depending on the restaurant seating capacity and meal volume.

Commercial Kitchen Drainage Rules

Food Waste Disposers

Per IPC Section 413.3 and 1003.3.2, commercial food waste disposers must discharge through a separate trap and bypass grease interceptors.

Inspector Critical Rule: If food solids are routed into a grease interceptor, they rapidly fill the separation chamber. This reduces the effective liquid volume, cuts retention time, and causes grease to escape into the sanitary sewer. Additionally, decomposing food waste produces acidic conditions that can damage the interceptor. Thus, food waste disposers must bypass grease interceptors.

Dishwashers and High-Temperature Wastes

Commercial dishwashers discharge water at temperatures exceeding 140°F (60°C) along with emulsifying detergents. When grease is emulsified or kept hot, it will not float or separate. It remains in suspension, flows through the grease interceptor, and solidifies further downstream in the public sewer. Therefore, dishwasher waste lines should bypass grease interceptors or be routed to a temperature-reducing system unless the grease interceptor is specifically designed and rated to handle high-temperature flows.

Oil Separators: Garages and Vehicle Services

IPC Section 1003.4 requires the installation of oil separators (oil/water interceptors) in repair garages, car washes, motor fuel-dispensing facilities, vehicle service areas, and any place where oil or flammable liquids enter the drainage system.

Design and Sizing

Oil separators must meet the following structural requirements:

1. Depth: The separator must have a depth of not less than 2 feet (610 mm) below the invert of the discharge drain.
2. Water Seal: The outlet must have a water seal of not less than 18 inches (457 mm). This deep water seal prevents flammable vapors and oils from passing into the main sewer line.
3. Sizing Capacity: Sizing is based on the number of service stalls. Garages with 1 to 4 stalls require a minimum oil separator capacity of 6 cubic feet (45 gallons). For each additional stall, 1 cubic foot (7.5 gallons) of capacity must be added.

Car Wash Interceptors

Commercial car wash facilities generate significant sand, dirt, and grit along with oil. In these applications, a sand interceptor must be installed upstream of, or integrated with, the oil separator. The sand interceptor captures heavy solids, preventing them from filling and blocking the oil separator.

Sand and Grit Interceptors

Per IPC Section 1003.5, sand and grit interceptors are required for commercial laundries, car washes, bottling plants, and any facility where heavy sediment or sand is present in the waste stream. These interceptors must have a water seal of at least 2 to 6 inches and a large settling chamber to allow sand to fall out of suspension. The interceptor must be easily accessible for cleanout.