Tuesday, March 4, 2014

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.


  1. Ken:

    I agree about the dump sinks behind the mocha-cappa-frappa coffee bars. They rinse the containers used to make these fat ladden drinks and dump the extra down the drain. Back bar sinks are often overlooked. This is where the extra product is dumped in higher concentration that most wash sinks.

    We also require dishwashers to drain to an interceptor. Hydromechanical interceptors are tested for grease removal with 150-degree water so tempature shouldn't be a problem.

  2. I also agree with connecting the dump sinks for those coffee places and bars, unfortunately we have Starbucks and similar coffee places that have managed to get by without interceptors.

    Are there any studies on the effects of temperature vs grease/oil separation as it pertains to interceptors? I would think that high temperature water, such as from a temperature sanitizing dishwasher, would keep grease and oil in suspension much longer and would greatly reduce separation and removal efficiency.

  3. We require any FSE that has a 3-Compartment sink to install a grease protection device based on plumbing fixture count. The best way to address food disposal units is for the POTW to administratively prohibit them. My belief is that dishwashers can be connected to an interceptor, but not to the smaller indoor grease traps.