If you put something down a drain you shouldn't have and the drain gets blocked, it's usually not much more than annoying. But for the people who manage the sewers, blockages in the pipes that go from our homes and businesses to treatment facilities cause bigger problems---sewage spills called "sanitary sewer overflows." (Yuck!)
One of the major causes of sewer pipe blockages is the formation of hardened, insoluble deposits of fats, oils and grease (FOG); these formations look something like the stalactites in a cave, they have a grainy texture like sandstone, and they adhere strongly to a pipe wall. But little has been known about how they form or even what they are made of.
A group of scientists from North Carolina State University analyzed the contents of these deposits with a technique that uses infrared light to determine the molecular composition of a substance. They were fatty acids, of course, but also calcium. "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 N.C. State engineering professor Joel Ducoste, a co-author on the paper, which will be published in Environmental Science & Technology.
The fat and grease break down into glycerol and free fatty acids that then chemically react with calcium present in the sewage system to form these hard deposits. The researchers hope to determine how quickly the deposits form and where the calcium is coming from so that they can then create a model to predict where the blockages might occur and prevent these messy sewer overflows.