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Fat turns into soap in sewers, contributes to overflows

Date:
April 21, 2011
Source:
North Carolina State University
Summary:
Researchers have discovered how fat, oil and grease can create hardened deposits in sewer lines: it turns into soap! The hardened deposits, which can look like stalactites, contribute to sewer overflows.

Sewer pipe. Researchers have discovered how fat, oil and grease (FOG) can create hardened deposits in sewer lines: it turns into soap.
Credit: iStockphoto/John Mroz

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.

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"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 research was funded by the Water Resources Research Institute and the U.S. Environmental Protection Agency.


Story Source:

The above story is based on materials provided by North Carolina State University. Note: Materials may be edited for content and length.


Journal Reference:

  1. Xia He, Mahbuba Iasmin, Lisa O. Dean, Simon E. Lappi, Joel J. Ducoste, and Francis L. de los Reyes, III. Evidence for Fat, Oil, and Grease (FOG) Deposit Formation Mechanisms in Sewer Lines. Environmental Science & Technology, (in press)

Cite This Page:

North Carolina State University. "Fat turns into soap in sewers, contributes to overflows." ScienceDaily. ScienceDaily, 21 April 2011. <www.sciencedaily.com/releases/2011/04/110421104501.htm>.
North Carolina State University. (2011, April 21). Fat turns into soap in sewers, contributes to overflows. ScienceDaily. Retrieved November 28, 2014 from www.sciencedaily.com/releases/2011/04/110421104501.htm
North Carolina State University. "Fat turns into soap in sewers, contributes to overflows." ScienceDaily. www.sciencedaily.com/releases/2011/04/110421104501.htm (accessed November 28, 2014).

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