NC State Researchers Study FROG Problem in Sewer Systems From Grease Traps

Team Conducts Tests on Fat, Roots, Oil and Grease Blocking Sewer System

There is a FROG problem in the sewer systems. No, it’s not the croaking kind but the slick, matted kind. The accumulation of fat, roots, oil and grease (FROG) in sanitary sewer collection systems can lead to sanitary sewer overflows (SSOs). Ongoing sewer system maintenance by public utilities is required to prevent wastewater overflows that end up in creeks and rivers. Better ways of removing FROG during a pretreatment stage can help reduce maintenance costs.

Dr. Joel J. Ducoste, associate professor of civil, construction, and environmental engineering at NC State University, and a multidisciplinary team have received funding from the Water Environment Research Foundation to study aspects of FROG pretreatment. In particular, they will investigate ways to improve grease interceptor performance, analyze the surface chemistry of pipe surfaces to determine if one surface over another favors fat, oil and grease (FOG) accumulation and study treatments designed to reduce the regrowth of roots in pipes.

Joining Ducoste, who is leading the two-year project, are Dr. Kevin M. Keener, associate professor of food science at Purdue University; Dr. John W. Groninger, associate professor of silviculture in the Department of Forestry at Southern Illinois University; Leon Holt, utility pretreatment manager with the Town of Cary; and Barbara Oslund, senior engineer at Solutions-IES in Raleigh.

The primary focus of the FROG project deals with two types of blockages that cause SSOs. Ducoste said, “According to the EPA [U.S. Environmental Protection Agency], blockages make up at least 40 percent of sanitary sewer overflows.” The first type of blockage is caused by food-related wastes, FOG, that get into the sewer systems from food preparation and cleaning activities at food service establishments and, to a lesser degree, at private residences. The second is caused by plants, particularly tree roots that have penetrated sewer pipes through cracks. Root hairs combined with hardened fat create dense mats that build up in the pipes and eventually constrict or block wastewater flow. The team’s multifaceted approach to tackling these issues includes surveys, field and lab work, pilot-scale testing and computer simulations.

One research facet involves sending surveys to wastewater pretreatment coordinators around the country. The surveys contain questions related to collection system characteristics and the nature of problems associated with FOG and roots. The surveys also ask for samples from problem areas. Soultions-IES is coordinating the surveys and compiling statistics based on the responses. They will collect the samples and forward them to a private lab for testing.

Another research facet deals with surface chemistry. Ducoste explained, “There is anecdotal evidence that material used to reline cracked pipes may be preferential for FOG.” Keener will analyze the surface chemistry of the relined pipe, as well as other pipe materials, such as clay, reinforced concrete and PVC, to determine if there are existing pipe surfaces that are more conducive to the accumulation of FOG. He also will examine root surfaces.

The remaining facets pertain to major portions of the project dealing with root and food-related blockages. Ducoste said, “As part of the overall scheme of the project, we are going to look at how to improve the removal of fat, oil and grease from the sewer systems through pretreatment. Grease interceptors are the frontline attack for food service establishments.”

Ducoste explained that a grease interceptor is essentially a holding tank. Greasy wastewater, detergents and food particles from food service establishments enter into an underground grease interceptor. The greasy wastewater is held in the interceptor until the FOG separates and rises to the top, where it accumulates until removed. The separated wastewater then flows through the interceptor into the sewer system.

Team members will take samples of FOG from active grease interceptors at various local food service establishments to get an idea of what is entering and leaving the interceptors. They will measure for chemical components in the samples, such as fatty acids, total oil and grease, solids concentrations, total suspended solids, chemical oxygen demand (COD) and biochemical oxygen demand (BOD). The team will then be able to develop synthetic wastewater so that they can run their pilot-scale grease interceptor tests with realistic concentrations. This information loop also includes computation of fluid dynamic simulations to find possible ways of improving an interceptor’s performance.

Ducoste said, “First, we will do computer simulations to try to predict what is going on in the pilot-scale tests. If we get a good correlation between the simulations and the experimental results from the pilot-scale, we will do some simulations based on “what if” scenarios — what if we change the influent configuration, what if we change the location of the baffle, what if we change the location of the effluent.” If a simulation configuration reveals a possible improvement, they will seek confirmation by modifying their pilot-scale interceptor according to the configuration and then test it.

At the end of these experiments, the team will have a scientific, fundamental understanding of what is happening inside grease traps and will have recommendations for improving performance. Ducoste admits that this fundamental understanding is a first step in tackling the issue of FOG removal. “It is an initial and important first step, but it is not the end.” More research will be needed.

In conjunction with the grease trap experiments, the team is studying the root regrowth in sewer systems. Operating out of a modified greenhouse at the Oxford Tobacco Research Station in Oxford, N.C., the team is growing Turkey Fig, Black Willow, Silver Maple, River Birch, and Ficus to test chemical and mechanical pretreatment methods. Groninger is an advisor on the project.

The plants, growing on top of PVC pipes slitted to mimic cracks, are currently in a winter dormant stage. In the growing seasons of spring and summer, the team will run a series of experiments that will compare different treatment methods of removing root growth in pipes. Two are common, but different, types of chemical compounds that are used across the country, and the third is a root saw. Ducoste said, “We’re interested in looking at the regrowth issue. How fast does regrowth occur when trying these different approaches.”

Although the FROG project is in its early stages, the team has already received preliminary data from surveys and field tests. They have started making synthetic wastewater for the pilot-scale interceptor and have done some preliminary computer simulations. Their root system is in place and awaits the growing season. At the end of the project, the team will pull all the data they have collected and turn it into a report with recommendations.

The FROG project is a large undertaking and an important first step in determining how to prevent sanitary sewer overflows that strain public utilities and threaten watersheds.

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