Fats, Oils, and Grease
Metabolic Pathways of FOG Removal w/ In-Pipe Technology
Fats are esters of fatty acids and glycerol which are normally solid at room temperature. Oils are esters of fatty acids and glycerol which are normally liquid at room temperature. Grease is a general term used to describe a soft or melted animal fat or a lubricant (Campbell, 1999). Collectively they are referred to as fats, oils, and grease (FOG).
Large amounts of FOG are disposed into wastewater on a continuous basis particularly where there are high numbers of restaurants or food processing facilities. When FOG enters the wastewater it tends to congeal within the piping system and at lift stations and the headworks, creating flow problems and odors due to the FOG hydrophobicity and poor solubility in water and from the oxidation of the organic FOG compounds (putrification). Some microbes are particularly useful in biodegrading most of the FOG discharged into the wastewater.
Degradation of FOG begins with the breakdown of the complex molecule by extracellular enzymes produced by microorganisms. Microorganisms produce many different classes of lypolytic enzymes including true lipases and esterases (e.g., carboxylesterase). Lipases display the most activity towards water-insoluble long-chain triglycerides while esterases degrade smaller molecules that are at least partially soluble in water. Lipase activity depends on the presence of a substrate/water interface while esterase activity follows the Michaelis-Menten kinetic reaction where maximum activity is reached long before the solution becomes substrate saturated (Jaeger et al., 1994). Bacteria as a group have great diversity in the activity levels of the lipases and/or esterases that they produce. Some of the lipases and esterases are very broad in their substrate activity while others have preferences for specific fatty acids. For example, the Bacillus subtilis lipase attacks fatty acids with chain lengths of 8 carbons found in the 1, 3-positions of a triglyceride while the Staphylococcus aureus lipase has a very broad range of substrate degradation capabilities (Thomson, 1999).
Other compounds produced by microorganisms and useful in the process of biodegradation of FOG are biosurfactants which are excreted into the environment surrounding the microorganism. The physiological role that biosurfactant production plays has been speculated to be facilitation of the growth of microorganisms on water-immiscible substrates by reducing the interfacial tension and making the substrate more bioavailable (Maier, 2003). Some of the bacteria genera reported to produce surfactants include Pseudomonas, Rhodococcus, Mycobacterium, Nocardia, Flavobacterium, Corynebacterium, Clostridium, Acinetobacter, Thiobacillus, Bacillus, Serratia, Arthrobacter, and Alcanivorax (Maier, 2003). The type of biosurfactant produced is genus and sometimes even species specific.
After the FOG has been exposed to biosurfactant and degraded by enzymes, the fatty acids and glycerol are consumed by other microorganisms that are capable of utilizing them (pseudomonads, Acinetobacter, various bacilli, and E. coli) (Gottschalk, 1986). The fatty acids are oxidized to acetyl-CoA via a pathway called β-oxidation. If the fatty acid has an even number of carbon atoms, then the entire chain is degraded to acetyl-CoA. If the fatty acid chain is an odd-chain fatty acid, then the last fragment is propionyl-CoA which is converted to acetyl-CoA through a variety of possible pathways. β-oxidation of fatty acids, in combination with the tricarboxylic acid cycle and respiratory chain, provides more energy per carbon atom than any other energy source (Zubay, 1996).
In some sections of collection systems, FOG is very difficult to metabolize. In these situations, a specialized proprietary mix of aerobic vegetative bacteria specialized for hydrocarbon digestion is added to the treatment process. Although these organisms require an aerobic environment to live, sufficient oxygen is in the wastewater in wet wells to allow them to assist in breaking down FOG at point of entry. Also, they can survive short periods of oxygen deprivation by using nitrate as an alternative electron acceptor. These organism are common soil bacteria that are adapted to living in low nutrient conditions, or r-strategists. An ‘r strategist’ relies on high reproductive rates for continued survival within the community. A ‘r strategist’ microorganism is one that, through rapid growth rates, takes over and dominates situations in which resources are temporarily abundant (Andrews and Harris, 1986). A ‘r strategist’ (e.g., Aspergillus, Penicillium, Pseudomonas, and Bacillus) can rapidly colonize and degrade large, readily available organic matter in a short period of time.