Trapping and Venting for Grease Interceptors

From time to time I will get an email that contains a schematic of a grease interceptor installation with the question, "does this look right?"

Usually the email-sender wants to know if the trapping and venting is shown correctly.

The answer requires a knowledge of both the applicable plumbing code and the installation requirements of the specific grease interceptor.

Model plumbing codes require each fixture discharging into a grease interceptor be individually trapped and vented, and require the installation of a vent downstream of the grease interceptor.

Standards also come into play when it comes to determining the type and location of vents upstream of a grease interceptor.

PDI G101 mandates the installation of an vented external flow control. The vent on the flow control is an air intake.  As the waste stream flows through the orifice in the flow control device negative pressure is created, drawing in air from the air intake which is intended to mix with the waste water as it enters the grease interceptor aiding in separation efficiency.

ASME A112.14.3 allows four types of ratings as follows:
  Type A - units with external flow control, with air intake (vent): directly connected
  Type B - units with external flow control, without air intake (vent): directly connected
  Type C - units without an external flow control: directly connected
  Type D - units without an external flow control: indirectly connected

Manufacturer's are required to identify which Type their interceptor is rated to when certifying to the ASME standard. 

PDI G101 and ASME A112.14.3 Type A certified interceptors are required to have a vented external flow control installed upstream of the interceptor.

Okay, so lets take a look at some drawings and see what works and what doesn't.

What's wrong with this installation?

It depends on which model plumbing code applies to the installation.

The International Plumbing Code (IPC) 1002.1 (exception 3) allows a grease interceptor to serve as a fixture trap - where it is intended by the manufacturer to serve as a trap - for a single fixture or a combination sink of not more than three compartments so long as the vertical distance from the outlet of the fixture to the inlet of the interceptor is not more than 30 inches and the developed length of the waste pipe from the most upstream fixture outlet to the inlet of the interceptor does not exceed 60 inches.

This drawing for a PDI G101/ASME Type A grease interceptor appears to be compliant with the IPC.  Assuming the interceptor is intended to serve as a fixture trap, there is no requirement to install an additional trap and vent between the fixture and the interceptor. 

The Uniform Plumbing Code (UPC) does not permit a grease interceptor to serve as a fixture trap and also prohibits the installation of a vent between the air intake on the flow control and the grease interceptor. The above diagram would not be compliant with the UPC. There needs to be a trap and vent between the fixture and the vented flow control fitting.

What's wrong with this installation?

Actually, nothing!

The drawing shows a PDI G101/ASME Type A grease interceptor connected to a the three compartment sink that is trapped and vented, an external flow control with air intake and a vent on the downstream side of the grease interceptor.

This installation would be compliant with both the IPC and the UPC.

What's wrong with this installation?

The drawing shows a PDI G101/ASME Type A grease interceptor with an external flow control with air intake, however it also shows a trap on the fixture but no vent for the trap.

This is not compliant with the IPC since the code does not allow double trapped fixtures. The code does allow the interceptor to serve as a trap - assuming that this interceptor is intended to serve as a fixture trap - thus adding a trap to the fixture upstream of the interceptor creates a double trapped fixture installation. Either adding a vent to the trap or removing the trap on the fixture altogether would solve the problem.

The only solution for compliance with the UPC is to add a vent for the trap on the fixture upstream of the vented flow control fitting.

What's wrong with this installation?

This drawing shows a semi-automatic draw-off type grease interceptor but without a vented external flow control.

Since all semi-automatic draw-off grease interceptors are certified to either PDI G101 or ASME A112.14.3 Type A (that I am aware of) an external vented flow control must be shown for compliance with the IPC.

For compliance with the UPC this drawing would have to show both a vented external flow control and a trap and vent for the fixture.

What's wrong with this installation?

Again, there is nothing wrong with this installation, though it may not be immediately obvious as to why.

The unit shown is certified to ASME A112.14.3 Type C (without external flow control) with a built-in or integral flow control and does not require an air intake.

That being the case the interceptor does not require a vented external flow control - jurisdictions unfamiliar with this type of interceptor often question drawings like this and understandably so.

This installation is compliant with both the IPC and the UPC since it shows a trapped and vented fixture discharging through an approved interceptor without external vented flow control with a vent installed downstream.

Air Admittance Valves (AAV)

The UPC does not include provisions for AAVs except as a part of an "Engineered Vent System" under section 912.0, much to the chagrin of engineers around the country. Many states that adopt or adapt the UPC have added an allowance for AAVs so you will have to check with your state to see if they are approved.

The IPC approves the use of and installation requirements for AAVs under section 918.0.

What's important to remember about AAVs is that they only allow for the relief of negative pressure in the drainage system. Therefore, when using an AAV to vent a grease interceptor, it's important that the drainage system has provision for the relief of positive pressure to ensure proper flow.

Boilerplate drawings can be trouble

To be honest, much of the confusion over whether a drawing shows a code compliant installation of a grease interceptor or not, can be the result of boilerplate drawings provided by manufacturers in submittals or installation instructions. Manufacturer's want to provide guidance for a broad range of installations without providing detailed drawings for every single installation variable that can exist. 

When it comes to traps and vents for grease interceptors and the fixtures discharging to them, it's incumbent upon an engineer or contractor to identify and comply with local code requirements regardless what a manufacturer's drawing shows. 

Hopefully after reading this post you will find it easier to identify a drawing that is right for your installation or one that needs to be corrected. 

Kitchen fixtures and grease interceptors; does it matter?

Some plumbing codes have exacerbated FOG related pretreatment problems by incorporating vague language regarding which fixtures should be routed through an interceptor.  Extensive field inspections have proven that FOG is being introduced through every fixture in the kitchen at one point or another.

For some reason not everyone thinks that floor drains or floor sinks need to be routed to an interceptor.  Yet spills like the one pictured here are very common.  We would advise that the kitchen staff isolate a spill like this and use absorbent materials to clean it up carefully.  Yet when it gets busy its more likely that someone with a mop and bucket will rush to the area and "do their best" to get it mopped up or at least liquid enough to squeegee into a nearby floor drain or floor sink.  

We know that dish ware that ends up in a multi-compartment sink is going to produce a significant amount of FOG, but fixtures such as bar sinks get over looked. Yet some common sources of FOG are cappuccino, cafe' breva, cafe' macchiato, cafe' latte, cafe' mocha, frappuccino, hot chocolate, iced cafe's with milk or cream, milk shakes, mixed alcoholic beverages with milk or cream such as white russian, irish coffee, kahlua and cream and so on.  The point being that milk fat is very common and found in many bar drinks where associated glass ware is typically also rinsed and washed.

The best way to prevent a restaurant from discharging FOG to the collection system is to connect all of the following fixtures to a properly sized, installed and maintained grease interceptor:

• sinks used for washing pots, pans, dishes, cutlery, kitchen utensils, including pre-rinse sinks
• drains serving self-cleaning exhaust hoods installed over commercial cooking equipment
• drains serving commercial cooking equipment that discharges oil and grease (i.e. woks, soup kettles, tilt kettles, etc.)
• drains serving garbage compactors used to compact waste that may contain, or be contaminated with food waste
• floor drains
• floor sinks
• mop sinks

There are two remaining fixtures which should be carefully considered:

• Dishwasher discharge is a high-temperature mixture of FOG, solids, water and surfactants from excess detergent.  Testing has shown that higher temperatures actually assist in separation performance.  However, interceptors are designed to separate free floating FOG not FOG that has been emulsified by surfactants.  Some jurisdictions believe that it is better to route the dishwasher through the grease interceptor and hope that excess surfactants do not emulsify previously captured FOG, rather than guarantee that the FOG and solids in the dishwasher effluent are delivered directly to the collection system by not routing it through the grease interceptor.
• Many FSEs scrape dirty dishes into their food waste disposal unit increasing FOG-laden solids that can be discharged to the collection system.  This is a good reason to route these fixtures to a grease interceptor.  However, extra solids loading from a food waste disposal unit can be problematic. To help prevent the interceptor from prematurely filling up with food waste, jurisdictions can require a solids interceptor after a food waste disposal unit or, ultimately, consider eliminating food waste disposal units altogether to improve interceptor performance and reduce the amount of total suspended solids (TSS) entering the collection system.