Researchers find fat turns into soap in sewers and contributes to overflows

RALEIGH, N.C. — Researchers from North Carolina State University have discovered how fat, oil, and grease (FOG) can create hardened deposits in sewer lines: it turns into soap! The hardened deposits, which can look like stalactites, contribute to sewer overflows.

“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,” said Joel Ducoste, Ph.D., a professor of civil, construction and environmental engineering at NC State and co-author of a paper describing the research. These hardened FOG deposits reduce the flow of wastewater in the pipes, contributing to sewer overflows.

The research team used a technique called Fourier Transform Infrared (FTIR) spectroscopy to determine what the FOG deposits were made of at the molecular level. FTIR spectroscopy shoots a sample material with infrared light at various wavelengths. Different molecular bonds vibrate in response to different wavelengths. By measuring which infrared wavelengths created vibrations in their FOG samples, researchers were able to determine each sample’s molecular composition.

Using this technique, researchers confirmed that the hardened deposits were made of calcium-based fatty acid salts — or soap.

“FOG itself cannot create these deposits,” Ducoste said. “The FOG must first be broken down into its constituent parts: glycerol and free fatty acids. 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 said. “Now we know what’s going on with these really hard deposits.”

The researchers are now focused on determining where the calcium in the collection system is coming from, and how quickly these deposits actually form. Once they’ve resolved those questions, Ducoste said, 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 said, “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 paper, “Evidence for Fat, Oil, and Grease (FOG) Deposit Formation Mechanisms in Sewer Lines,” is forthcoming from Environmental Science & Technology. The paper was co-authored by Ducoste; Francis de los Reyes, Ph.D., an associate professor of civil engineering at NC State; Lisa Dean, Ph.D., an assistant professor of food science at NC State; Simon Lappi, Ph.D., from NC State’s chemistry department; and NC State Ph.D. students Xia He and Mahbuba Iasmin. The research was funded by the Water Resources Research Institute and the U.S. Environmental Protection Agency.