Lose the fat: Targeting grease to curtail sewer overflows

Nov 23, 2009

Sewer overflows are a nasty business, posing dangers to human health and the environment. North Carolina State University is launching a new project with funding from the U.S. Environmental Protection Agency (EPA) that targets the fat and grease that contribute to millions of overflows every year, and will give urban planners new tools to further reduce the risk of sewage spills.

At issue are overflows from "sanitary" sewer systems, as opposed to "combined" systems that also handle stormwater. EPA estimates that 40 percent of the 3 to 10 billion annual sanitary sewer overflows around the country are caused by hard deposits made up of fat, oil and grease (FOG) that clog sewage pipes. In other words, the FOG buildup is directly responsible for hundreds of millions of sewer overflows, as well as untold costs in health effects, environmental harm, cleanup expenses, and damaged public perception for the utilities.

Dr. Joel Ducoste, an associate professor of civil engineering at NC State, is launching a research effort to address the problem under a prestigious Science To Achieve Results grant from EPA.

"Our goal is to discover the fundamental chemical reactions that lead to these deposits," Ducoste says, "because if we can understand it, we can figure out how to reduce it - via enhanced treatment technologies, more cost-effective maintenance and by setting appropriate regulatory standards.

"We think a reaction takes place in the sewage collection system when FOG interacts with calcium or other metal ions to form these hardened deposits," Ducoste says, "similar to the that are used to make household soaps. We'll be testing this as part of the grant and, if this is the case, we will explore ways to remove the substances that contribute to these deposits.

"Also," Ducoste says, "once we understand the chemistry behind these deposits, we can develop models to identify potential 'hot spots' where FOG deposits may form." Identifying these hot spots will allow utilities to use preventative maintenance to avoid overflows. Perhaps more importantly, the models could serve as useful tools for urban planners - helping them determine whether an existing sewer system can sustain population growth, or if the system needs to be modified in order to accommodate future growth.

The urban planning component is essential, Ducoste says, because significant amounts of FOG get into sewage systems from high-density residential areas, such as apartment blocks and condominiums. Local governments need to be aware of potential sewer overflow problems that could stem from population growth in a concentrated area. Currently, sewage systems try to keep out FOG by using grease interceptors - but they are found only at restaurants.

The EPA grant is for approximately $570,000 over three years.

Source: North Carolina State University (news : web)

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