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When engaging in technical discussions with municipal officials regarding the impact of food waste disposers, what often comes up is a concern about FOG — fats, oils and grease. Wastewater treatment managers, especially those overseeing sewage collection systems, deal with FOG blockages as part of routine maintenance. These deposits are listed by the US EPA as a major contributor to blockages that cause sanitary sewer overflows, a public health risk. Incidentally, FOG deposits are seen in sewers around the world, including countries where disposers are not as widespread as in the US.

The above picture shows a FOG deposit 60 feet downstream of a grease interceptor, a device required by many municipalities to control FOG from food service establishments. Deposits also often appear in lift station wet wells near the high level mark, as shown below.

The FOG deposit phenomena

In 2008, the Water Environment Research Foundation released a study titled “Fats, Roots, Oils and Greases in Centralized and Decentralized Systems,” also known as the FROG study, which detailed three important facets of the FOG deposit phenomena. These included grease interceptor performance, FOG deposit characterization (chemical make-up), and the interaction of roots and FOG.

The characterization portion of the study, headed up by Dr. Kevin Keener of Purdue University, determined that FOG deposits collected from around the U.S. were similar in composition, containing very high concentrations of Calcium and saturated fats; both much higher than what would normally be found in sewage. Subsequent information appeared in US Today last summer from another FROG team member, Dr. Joel Ducoste of North Carolina State University.  His study called the deposits “soaps,” and at least partially implicated grease interceptors as contributing to the problem. The actual research suggests that free fatty acids (broken down FOG) reacts with Calcium, which could come from concrete grease interceptor tanks, as well as other sources, to form calcium soaps.

Soap we use every day is sodium based, and is water soluble, while calcium soaps are insoluble, thus making their removal difficult and time consuming. But what is interesting about this latest research is that grease interceptors – the very devices intended to control FOG – may actually be contributing to the problem by acting as a reservoir for bacteria to hydrolyze FOG into precursors for the hard deposits to form downstream.

Sewer blockages from FOG deposits are not caused by food waste

FOG deposits are the result of a chemical reaction where calcium and free fatty acids combine to form insoluble calcium soaps. Calcium has many potential sources including, but not limited to, urine, concrete, water hardness, and dairy products.  Free fatty acids are the result of the hydrolysis of fats, oils and greases, which occurs in grease interceptors.  But since we also digest food in our bodies, including FOG, this indicates that possibly the human digestion system is a source of the bacteria that facilitate the breakdown of FOG.

But sanitary sewage is not supposed to flow through interceptors, so what is the source of the bacteria and enzymes that break down FOG? More importantly, how can we control FOG deposits?


6 Replies to “CLEAR the FOG

  • Tim Evans on said:

    That’s a nice summary, Michael. When I saw his data, I was struck by Keener’s finding that FOG deposits are dominated by saturated fatty acids, irrespective of the proportion of saturated and unsaturated fats in the local cuisine and that the mechanical strength of the FOG was correlated with its calcium content. Samples of FOG collected in the UK are consistent with this. We are calling it FOGc (compound) because it’s not just fat/wax that has cooled to below its melting point.

    You ask where the hydrolysis bacteria come from. Create a niche and microorganisms will colonise it somehow. They’ve been around 4 billion years! We now realise that sewers are complex microbial ecosystems and DNA profiling of biofilms (slimes) on the walls of sewers shows the population makeups differ depending on the composition of the sewage. The composition of the soluble fraction of wastewater changes as it passes through sewers becuase of this interaction with the biofilms. We know it happens but we yet don’t know enough to predict outcomes.

    So, microbial populations in passive grease traps/interceptors hyrolysing FOG so that soluble free fatty acids are released in the effluent, why not, it seems quite reasonable, with the benefit of hindsight. I think that’s the reason San Francisoc Water and others are favouring mechanical grease recovery units and skimmers over passive tanks – they get the lipids away from the water as quickly as possible and thus minimise the time for hydrolysis and release to the effluent. It’s also consistent with the cities in Germany that have ratcheted down the permitted PET/n-hexane extractable in grease tap effluent because even though there are traps they still get FOGc in sewers.

    All this should be music to the ears of people who want to innovate better separators.

    • kelemanm on said:

      Thanks Tim! Undoubedtly a very interesting subject. The concept of biofolims and the interaction occurring within them with several different wastes and bacteria is certainly something in need of further research.I too agree that mechanical grease removal devices seem to have the most promise, but I often hear concerns of the challenge of necessary oversight and maintenance. Any thoughts?

  • Yi Wang on said:

    Dear Michael and Tim, I am part of the FOG research group at North Carolina State Univeristy and I’m very glad to know people are really talking and concerning about FOG.

    We have found out the mechanism of FOG deposit formation — a step-wise hydrolysis followed by soapnification (a soap making process), and have succefully re-produced them under laboratory environment.

    As Michael mentioned above, more challenges come from the FOG removal at the pre-treatment stage. Mechanical removal devices require extra $$$ and maintenance, and only remove “freely” FOG. Those emulsified/suspended/soluble FOG will flow to sewer system and form deposit. One of my current research is to find a physical/chemical process to remove these soluble FOG. The idea is to find some non-hazdous chemical addtive to detergent that traps FOG globules and prevents them from contacting with calcium down the road.

    I’d like to hear from you for all aspects of FOG related thoughts and suggestions on FOG removal. Thanks!!

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