Researchers from North Carolina State Univ. 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,” says Dr. Joel Ducoste, a professor of civil, construction and
environmental engineering at NC State and co-author of a paper describing the
research. Collection systems are the pipes and pumping stations that carry
wastewater from homes and businesses to sewage-treatment facilities.
These hardened FOG deposits reduce the flow of wastewater in the pipes, contributing
to sewer overflows—which can cause environmental and public-health problems and
lead to costly fines and repairs.
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 says. “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 says. “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 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 says, “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.
