August 23, 2024
Some plumbing codes contribute to FOG-related pretreatment problems by using vague language about which fixtures should be routed through an interceptor. Field inspections show that FOG (fats, oils, and grease) is introduced through every kitchen fixture, underscoring the need for precise regulations. Despite this, misconceptions persist about the necessity of routing floor drains, sinks or other fixtures to an interceptor, even though spills often end up being dumped into these drains during busy times. While multi-compartment sinks are recognized for producing significant FOG, fixtures like bar sinks or hand sinks are often overlooked, even though they handle items like cappuccino, café drinks, milkshakes, and mixed beverages that contain milk fat. Schier recommends that all kitchen fixtures be connected to grease interceptors, ensuring a more comprehensive approach to FOG management.
Different plumbing codes provide specific guidelines regarding which fixtures must be connected to a grease interceptor. Here's a summary of the requirements from the major codes:
1. Uniform Plumbing Code (UPC)
Kitchen sinks
Dishwashers (gravity grease interceptors only)
Floor drains in kitchens
Pre-rinse sinks
Mop sinks
2. International Plumbing Code (IPC)
Pot sinks
Scullery sinks
Kettle sinks
Soup kettles
Woks
Floor drains or sinks in areas where grease-laden waste might be introduced
Automatic dishwashers without pre rinse sinks
Pre-rinse sinks
3. National Standard Plumbing Code (NSPC)
Sinks (pot, scullery, pre-rinse)
Dishwashers
Floor drains
Floor sinks
Mop sinks
Any other fixtures where FOG is introduced
Dishwashers
One of the most contentious fixtures in the kitchen is the dishwasher. The thinking is that it somehow interferes with the functionality of a grease interceptor. However, Schier has designed interceptors to accept the flow of dishwashers through engineering and testing. Below are some decision factors jurisdictions use when considering whether or not to connect the dishwasher.
1. Temperature - If the dishwasher is discharging at a temperature below 140 degrees Fahrenheit - the max temperature for PVC - it will not affect the interceptor. Hot water actually aids in separation, and studies have shown that as temperature increases with oils, the specific gravity decreases and thus the oil separates more quickly. Hydromechanical units are tested at 160 degrees, and Schier lists a 150-degree maximum for the lifetime warranty. Typically, high temperature dishwashers will have a bracket on the back for a tempering device that infuses cold water to bring the temperature within the limits of PVC. These devices are readily available in the industry. All manufacturers certify hydromechanical interceptors to the ASME A112.14.3, so they should all be able to handle 140 degrees.
2. Flow Rate - If the interceptor is rated for flow above the discharge of the dishwasher, which is typically 25 GPM, then flow rate is not an issue.
3. Soap, Water and Grease Mix - Dishwashers are turbulent and cause the mixing of soap, water and grease to create bonds using a shearing force. Hydromechanical Interceptors have a calm, quiescent environment without turbulence so any soap entering the interceptor doesn't have the opportunity to bond as there is no shearing force. Studies have shown that there is no conclusive evidence that dishwashers cause a scouring of the interceptor when in use. We have seen issues with the swamping of units and a dishwasher is blamed as the culprit. This is typically a result of high flow rates in smaller under sink units where there is an issue with the flow control device (it's not installed), not necessarily the fault of the dishwasher.
4. Design – Properly designed grease interceptors can meet all of the above requirements and therefore the dishwasher can connect.
Food Waste Disposers
There are inconsistencies regarding the discharge of food waste disposers (also known as garbage disposals), sometimes even within the code. When a jurisdiction requires a food waste disposer to discharge through the grease interceptor, Schier recommends installing a solids interceptor (SI) upstream of the grease interceptor. A typical sizing recommendation for the solids interceptor is one size smaller than the grease interceptor being utilized. For example, an SI-250 is recommended for use with our GB-500 grease interceptor or an SI-75 with a GB-250.
What do the codes say?
IPC- A food waste disposal shall not discharge to a grease interceptor.
UPC- No food waste disposer or dishwasher shall be connected to or discharged into a grease interceptor.
Exception- Food waste disposers shall be permitted to discharge to grease interceptors that are designed to receive the discharge of food waste.
NSPC- A solids interceptor is required when food waste disposal discharges through a hydromechanical grease interceptor (HGI) but is not required when discharging through a gravity grease interceptor (GGI).
Food waste disposers are becoming used less frequently, as jurisdictions are removing them for use in commercial kitchens, because studies show that they add both solids and FOG to the waste stream. This creates a build-up that needs to be removed at some point from the sewer system.
Kitchen fixtures in the overall project design call for engineers to use fixture calculations to determine the flow rate needed for a hydromechanical interceptor. Connecting all kitchen fixtures to a grease interceptor is crucial for preventing sewer blockages. Removing the food waste disposer from the system further protects against issues with FOG. While the codes all agree on some fixtures, stating that all kitchen fixtures should connect would further assist all of the stakeholders with grease management when sizing and installing these devices.