2017 Excellence in Environmental Engineering and Science® Awards Competition Winner

Honor Award - University Research

Fats, Oils, and Grease (FOG) Waste: Fate and Transport in Interceptors and Sewers, Energy and Recovery Through Anaerobic Co-Digestion

Entrant: North Carolina Research Team: Dr. Joel J. Ducoste, BCEEM, Dr. Francis L. de los Reyes, III,
and Dr. Tarek N. Aziz
Person in Charge: Joel D. Ducoste, Ph.D., BCEEM
Location: Raleigh, North Carolina
Media Contact: Joel Ducoste

Entrant Profile

Dr. Joel J. Ducoste is a Professor in the Civil, Construction, and Environmental Engineering (CCEE) Department at North Carolina State University. Dr. Ducoste is a national and international recognized expert in modeling water and wastewater treatment processes using Computational Fluid Dynamics (CFD). His current research interests include physico-chemical processes in water treatment, chemical and UV disinfection, advance oxidation, water/wastewater process optimization, wastewater sewer collection system sustainability, energy recovery from wastewater systems.

Dr. Francis L. de los Reyes III is a Professor in CCEE and a University Faculty Scholar. Dr. de los Reyes' research focuses on biological processes and combines modeling, bioreactor experiments, and molecular microbial ecology tools in addressing fundamental and practical issues in environmental biotechnology and environmental engineering. Another research focus is sanitation in developing countries. He is interested in two main areas: the interface between microbial ecology and environmental engineering, and global sanitation.

Dr. Tarek N. Aziz is an Assistant Professor in CCEE. Dr. Aziz is interested in the interface of biological and chemical processes and environmental fluid mechanics. His research has involved investigation of anaerobic digestion for enhanced energy production in wastewater treatment, removal of FOG from wastewater, and the impact of natural and artificial mixing in lakes on biological processes in those systems.

Drs. Ducoste, de los Reyes, and Aziz collectively worked on the project and planned out the set of experiments and modeling work performed by graduate students to help understand the physical and chemical makeup of FOG deposits, the FOG transformation and removal processes, and its biological conversion into biogas using anaerobic co-digestion in municipal wastewater treatment. Dr. Ducoste took the lead in FOG deposit characterization, CFD and design of grease interceptors, measurement of FOG using advanced methods. Dr. de los Reyes took the lead in microbial ecology of grease interceptors, impact of bioadditives, and anaerobic co-digestion bioreactor operation and microbial analysis. Dr. Aziz took the lead in techno-economic assessment of anaerobic co-digestion of FOG.

Project Description

When fat, oil, and grease (FOG) are released into the sewer collection system, they accumulate on pipe walls, which reduces their conveyance capacity. This reduced capacity results in the uncontrolled discharge of untreated sewage into the environment, which is referred to as a sanitary sewer overflow (SSO). According to the U.S. EPA, roughly 3–10 billion gallons of sewage are discharged illegally into the environment each year as a result of SSOs. FOG accumulation represents 47% of all blockage-based sanitary sewer overflows. For decades, the science and engineering of FOG deposit formation, grease abatement devices, and energy recovery from FOG wastes have been largely neglected.

Over the last 11 years, our research team has systematically performed novel and important research in FOG deposit formation in sewer lines, optimizing and characterizing grease interceptors (GIs), and recovering methane using anaerobic co-digestion of FOG wastes. Our research group is perhaps the leading research group in the world in these areas, and the collective efforts have led to many scientific and engineering research "firsts":

First to identify the chemical makeup and reaction process in the development of insoluble solids related to FOG discharge in sewers
First to recreate the formation of FOG deposits under laboratory conditions and provide evidence that concrete corrosion can lead to increased FOG deposit formation
First to develop and validate a numerical transport model of FOG in a grease interceptor
First to quantify the internal microbial ecology and performance of bio-additives in full scale grease interceptors
First to perform direct comparison of FOG removal performance between internal and external grease interceptors and the challenges of removing FOG droplet size suspensions
First to develop a saponification kinetics model to describe FOG deposit formation
First to develop a hydraulics sewer system transport model to predict high risk zones of FOG deposit formation in the collection system
First to develop an experimental approach to measure the concentration of both FOG and long chain fatty acids in FSE wastewater discharge
First to show the highest increase in biogas production in an anaerobic digester with the co- digestion of municipal grease trap waste due to step feeding
First to develop a techno-economic analysis of anaerobic co-digestion of FOG with biosolids, leading to a decision-support tool for industry and municipalities

Integration

Our research focus has two prongs: addressing the fundamental scientific questions in FOG fate, transport, and recovery of energy; and developing approaches and technologies for SOLVING the problem of FOG deposit formation and maximizing energy recovery. We consider the entire problem from when it is produced in food service establishments, to separation and degradation in grease interceptors, to what happens in sewer lines, to how it can be treated in a way that recovers energy, to how different municipalities with varying capacities can decide if anaerobic co-digestion is economical.

Quality

The research we performed has been reported in 15 peer-reviewed papers (in leading journals such as Environmental Science and Technology, Water Research, Applied Microbiology and Biotechnology), 30 conference presentations (oral and poster), 6 research reports (for EPA, WERF and NC WRRI). The research has funded 8 graduate students (all theses or dissertations). The quality of the research is also documented in the support letters from academics, consulting engineers, and municipalities, who are either experts in the field or end-users/beneficiaries.

Originality and Innovation

For decades, FOG research (fate in sewer lines, FOG measurement, microbial ecology and physical modeling of GIs, anaerobic co-digestion of FOG waste) has been fragmented and lacking. As outlined above, we have achieved a number of "firsts", all of which are the result of our focus on answering key fundamental questions (e.g., "How do FOG deposits form? What happens inside a grease interceptor?"), and aiming for solutions (e.g., "Where in a network will blockages likely develop? How can we re-design the grease interceptor? How can we recover energy from FOG?"). The NC State team is perhaps the leading research group in this field, and each of the published peer reviewed papers present original and innovative approaches and results.

Complexity

To accomplish the different goals of this project, the research team had to use methods and approaches from different spheres of science and engineering: physical processes (hydraulics, design of GIs), chemical processes (saponification reactions, measurement using FTIR), biological processes (molecular microbial methods such as 16S rRNA gene analysis, anaerobic digestion), and computational methods (CFD, numerical transport, techno-economic analysis). The scope of the project included analyzing multiple aspects of food waste chemistry, corrosion chemistry, microbiology, mathematical modeling, and process engineering. The approaches included lab-scale experiments, field observations, and numerical methods.

Contribution to Social and Economic Development

An NREL survey of thirty US metropolitan areas shows that FOG is generated at a rate of approximately 1.88 gallons FOG/person/year. This is an estimate of only the FOG portion of GIW and does not include food solids. A survey of a North Carolina metropolitan area determined that, including the entire contents of the GI, there is an average of 18.65 gallons/person/year. Based on these estimates, there are approximately 5.8 billion gallons of grease interceptor waste (GIW) in the United States. GIW has a high biochemical oxygen demand, and the dewatered GIW contains roughly 7,000-10,000 BTUs/pound. Based on the above estimates, the co- digestion energy yield would be approximately 22,000–32,000 GWh/yr or 60% of the energy demand for the entire water and wastewater industry. This potential is the reason why municipalities such as the City of Raleigh are investing in FOG co-digestion (see letter of support). Similarly, the problem of FOG deposits and sewer blockages is important to many municipalities in the US and in other countries such as the UK (see letter from Cranfield University). The research at NC State University has provided new areas of investigation in preventing FOG deposits, maximizing methane production, developing new measurement techniques for FOG, and developing new tools for predicting blockage hotspots. These advances contribute directly to environmental protection, sustainable energy production, and economic development.

Click images to enlarge in separate window.


Figure describes the first documentation of the chemical makeup of FOG deposits in sewer systems. The data clearly identifies three clear long chain fatty acids in FOG deposit with the primary being Palmitic. The data also revealed a high degree of moisture content and total fats. This initial work led to further more discriminatory investigations using fast fourier infrared analyses that proved that FOG deposits were fatty acid salts of calcium produced from a saponification reaction.	Figure displays a schematic of the chemical constituents and their source that are involved in the saponification reaction leading to FOG deposits. It shows that these constituents can come not only from the wastewater stream but also from additional other processes such as the corrosion of concrete structures that release calcium, the biological breakdown of FOG that leads to higher concentrations of palmitic acid, and the role that excess FOG has on transporting long chain fatty acids to the pipe wall to continue to saponification reaction. This is the first mechanistic model of FOG deposit formation involving saponification reactions.

Figure displays a schematic (left) of how a FOG deposit solid can be developed from the aggregation of smaller FOG deposits that also contain intermittent regions of debris from wastewater. A Figure of a cored sample from a FOG deposit displays the debris layer along with layers of these fatty acid salts of calcium.	Figure displays a photographic image of Lab Based FOG deposits created from different long chain fatty acids. These results show that FOG deposits can be created from the major long chain fatty acids found in oils and not just from saturated fat sources as was first thought by industry professionals prior to this research being performed. In addition, the results shown in this figure also proved that corrosion from these concrete samples lead to the formation of these FOG deposits as no calcium was added to the surrounding oil/water solution.

Figure displays the computational fluid dynamics simulation of fluid velocities and FOG separation in a standard grease interceptor. CFD simulations like these were used to improve the separation performance by modifying the inlet and outlet geometry as well as internal baffle configurations.	Figure displays the internal microbial ecology in a full scale grease interceptor. It demonstrates the community of organisms that plays a significant role in the production of volatile fatty acids and the low pH environment found in GIs. It also displays those organisms that likely grow well in an oily wastewater rich environment. Determining the community organism that can grow in this environment was important determine organisms that may help degrade FOG and provide a maintenance strategy for grease interceptors.
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Figure shows a schematic of a full-scale grease interceptor (GI) and the sampling points in determining the physical, chemical, and microbiological characteristics of the contents. This is the first comprehensive characterization of the GI microbiology and chemistry.	Figure shows the effect of bioadditives on GI performance. In one case, the bioaugmented GI had better performance, while the effect was insignificant in another case. This shows that the effect of bioaugmentation is highly variable.

Figure shows how anaerobic codigestion of FOG and GI waste can lead to sustainable bioenergy production, help with biosolids management at wastewater treatment facilities, and provide treatment for wastes. We achieved the highest reported increases in methane yield (~400%) compared to biosolids digestion alone.	Figure shows how full-scale GIs are sampled for characterization, or for obtaining samples for feeding anaerobic digesters.

Figure shows how our results compare with previous research. We achieved the highest methane yields per mass of volatile solids added.	Figure shows the changes in microbial communities (as analyzed using 16S rRNA gene sequencing, and depicted in a principal component analysis plot) as anaerobic bioreactors adapt to changing loadings. We discovered that changing the pulsing of loads led to more robust microbial populations, and that higher loadings of adapted bioreactors converged to the same microbial populations.

Figure displays the separation performance of different types of internal grease interceptors compared to an external grease interceptor. When the FOG source water contained different oil/water emulsion strengths (i.e., increasing emulsion strengths would decrease the oil droplet size), the internal grease interceptors performed poorly in removing FOG compared to an external grease interceptor. This study called into question the benefit of using internal GIs for the removal of FOG.	Figure displays a numerical model of FOG Deposit formation based on alkali driven hydrolysis and saponification. The model was able to predict the kinetics of lab based FOG deposits.

Figure displays the performance of the FOG deposit model . Results show that it was able to predict of experimental FOG deposits at different pH conditions. The model could then be used in a system wide sewer collection model that also predicts the spatial distribution of these deposits in sewer pipes.	Figure displays the incorporation of the FOG deposit kinetics model into a Sewer collection system model of FOG Deposit Formation using CITYDRAIN 2.0.3. platform. The model provided a simple approach to connecting different portions of the collection system using a graphic user interface and describing how FOG is transported through the sewer and reacts with calcium to create FOG deposits

Figure displays the results of predicting the FOG deposit accumulation in a sewer collection system. Results showed that 65 percent of the high accumulation zones reported by the municipality matched the experimental results. Deviations between the model and the municipal report of high FOG deposit accumulation zones were likely due to not accurately replicating the sewer hydraulics. In addition, some of the pipes in the sewer system had mechanicals deformations due to age that were not reflected in the sewer model.	Figure displays measurements of long chain fatty acids in the effluent of a grease interceptor as a function of the pump out frequency. Results clearly show that higher maintenance frequency can result in reducing the hydrolysis of FOG that is necessary for FOG deposit formation. Prior to these results, it was not known whether there are additional benefits to higher pump out frequency on the GI effluent quality especially if the thickness of the grease and solids layer did not violate municipal requirements.