FOG overflow: IR clues to drainage problems

Soap story

It is perhaps no real surprise to any chemist who has unblocked a drain clogged with white lardy deposits, but new FTIR spectroscopic evidence confirms that degraded fat, oil and grease (FOG) reacts with calcium compounds in the murky water of drains to form hardened deposits that are, to all intents and purposes, composed of soap, leading to blockages and overflows.

One of the earliest memories of truly hands-on chemistry many of us have is of the soapy feeling one gets when skin comes into contact with an alkali. The soapy feeling is, of course, due to the saponification of fatty acids in the oils on the skin and is a great tactile demonstration of how soap is made: fatty acid + alkali. In other words, soaps are salts of fatty acids. Any source of fatty acids can be treated with alkali to form such salts and caustic drain cleaners exploit this fact to clear blockages caused by the build up of fats from kitchen sinks. However, in the subterranean depths of public sewers, the chemistry is a little more complex with a concoction of calcium, and other mineral ions, countless types of fat, oil and grease, and plenty of other contaminants to slurry the reaction.

Down in the sewer

Now, researchers at North Carolina State University in Raleigh have investigated how fat, oil and grease (FOG) can create hardened, soapy deposits in sewer lines leading to stalactite-like blockages and ultimately contributing to the risk of a sewer overflowing when water levels rise. "We found that FOG deposits in sewage collection systems are created by chemical reactions that turn the fatty acids from FOG into, basically, a huge lump of soap," explains Joel Ducoste, a professor of civil, construction and environmental engineering at NC State. Such hardened FOG deposits reduce the cross sectional area of pipes, reducing the flow of wastewater and raising environmental and public-health problems when sewers overflow, leading also to potentially expensive repair work and restoration.

As the numbers and density of commercial food preparation and serving facilities increase, so do the amounts of fats, oils, and grease (FOG) that are routinely discharged into sewer collection systems," the team says. This has in part led to the fact that almost half of sanitary sewer overflows each year in the US are due to blockages caused by FOG deposits.

The team, which includes Francis de los Reyes, an associate professor of civil engineering, food scientist Lisa Dean, chemist Simon Lappi and graduate students Xia He and Mahbuba Iasmin, used Fourier transform infrared (FTIR) spectroscopy in their study of deposits from sewers in 23 US cities. FOG itself does not create these deposits, Ducoste explains. The fats must be first broken down into their component parts, the free fatty acids and glycerol. These free fatty acids - specifically, saturated fatty acids - can react with calcium in the sewage collection system to form the hardened deposits. "Until this point we did not know how these deposits were forming - it was just a hypothesis," Ducoste adds. "Now we know what's going on with these really hard deposits."

The researchers are now investigating the various sources of the calcium in the sewage collection system and also the rate of soapy deposit formation. Once they've resolved those questions, Ducoste says, they will be able to create numerical models to predict where a sewage system may have "hot spots" that are particularly susceptible to these blockages. Ultimately, Ducoste explains, "if we know how - and how quickly - these deposits form, it may provide scientific data to support policy decisions related to preventing sewer overflows."

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.