Don’t Get Trapped in Improper Grease Interceptor Design
By Max Weiss
What do you need to know about grease interceptor design? You just fabricate a tank, big or small, with some interior features to prevent direct inlet-to-outlet flow and install it underground outside a food service establishment (FSE) or in the floor inside the building. The job is done, right?
Unfortunately, it is not difficult to find examples, diagrams, and dialog reflecting such a cavalier attitude toward what is in reality a rather sophisticated application of fluid mechanics performing the important task of protecting collection systems from overflows due to grease blockages.
The primary purpose of a grease interceptor is to intercept fats, oil, and greases (FOG) to prevent that material from collecting in building drainage piping or municipal collection systems and fouling or passing through wastewater treatment plants. A growing secondary purpose of grease interception is harvesting the energy potential contained in FOG.
A grease interceptor is the single drainage system maintenance device installed to prevent certain sewer system failures and subsequent surface water contamination. Considering the current emphasis on water quality protection and water usage restrictions, you would presume similar dedication to sound engineering practices to prevent the nullification of protection and conservation efforts by chronic sewer overflows. Sadly, that presumption is becoming more the exception than the rule.
A properly designed, correctly sized, and precisely installed grease interceptor is essential to the operational reliability of FSE drainage systems and, subsequently, wastewater systems. To effectively design, size, and install a grease interceptor, you must be knowledgeable of the physics employed in the design of the device, which is the topic of this article.
Water in a drainage system is the conveyor by which materials are transported from the point of last beneficial use to the point of removal from the water conveyor or the next point in the carbon or nitrogen cycle. The velocity, viscosity, temperature, and chemistry of the water interact to facilitate or impair the efficiency of transport (suspension) within the water conveyor. Quantifying the properties of the water and the properties of the materials in the water is essential to the design of an effective grease interceptor.
On a scale of greater to lesser effect on the suspension of a fluid of dissimilar density (FOG) in water, velocity is number one, followed by chemistry and then viscosity/temperature (which are inseparable in this instance). If you believe velocity can be estimated and averaged in interceptor design or that cooler water accelerates the ascension of a less-dense fluid, further study of fluid mechanics is required.
A complete understanding of the nomenclature relating to grease interceptors is essential to the proper utilization of these devices. This statement may seem fundamental and superfluous; however, not long ago I was seriously reminded of how essential nomenclature is to the correct application of a grease interceptor.
I was requested to inspect a “grease trap” installation emitting odor into a building. Upon entering the portion of the machinery room housing the “grease traps,” the odor was unmistakable and nearly overwhelming. I observed two hydromechanical grease interceptors, complete with solids interceptors, installed on the floor, beautifully plumbed, and discharging to the sanitary sewer via direct no-hub connections. Bubbling could be seen at both solids interceptor covers, indicating vapor flow into the building. The building engineer, plumbing contractor, and city inspector all referred to the devices as “grease traps.” I inquired, “Where are the traps?” They looked at me incredulously and pointed to the beautifully installed interceptors, with no trap between them and the sanitary sewer. Because they referred to them as “traps,” the installation was designed and the interceptors were installed and inspected as if they were, in fact, gas traps. The result was the introduction of sewer gas, possibly containing pathogens, into a hospital!
The moral of the story is to know the nomenclature and use it properly. It is vitally important to protecting public health.
Following are some terms regarding grease interceptor design.
• Air-injecting flow control: This device has an orifice and vent to create venturi-effect air entrainment and modulated velocity to the inlet stream of a grease interceptor. It is distinguished from a flow restrictor.
• Grease: This polar hydrocarbon compound consists of FOG triglycerides with three fatty acids bonded to a glycerol.
• Grease interceptor: This plumbing appurtenance or appliance is installed in a sanitary drainage system to intercept nonpetroleum fats, oil, and greases from a wastewater discharge.1 Two subtypes follow:
• Gravity grease interceptor: Abbreviated GGI, this plumbing appurtenance or appliance is installed in a sanitary drainage system to intercept nonpetroleum fats, oil, and greases from a wastewater discharge and is identified by a 30-minute retention time, baffle(s), not less than two compartments, a total volume of not less than 300 gallons, and gravity separation. These interceptors comply with the requirements of Chapter 10 of the Uniform Plumbing Code or are designed by a registered professional engineer.2
• Hydromechanical grease interceptor: Abbreviated HGI, this plumbing appurtenance or appliance is installed in a sanitary drainage system to intercept nonpetroleum fats, oil, and greases from a wastewater discharge and is identified by flow rate and separation and retention efficiency. The design incorporates air entrainment, hydromechanical separation, interior baffling, and/or barriers in combination or separately, as well as one of the following:
1. External flow control, with air intake (vent): directly connected
2. External flow control, without air intake (vent): directly connected
3. Without external flow control, directly connected
4. Without external flow control, indirectly connected3
• Free oil and grease: These materials rise rapidly to the surface of water under calm conditions. The droplet size is 150 microns or greater.4
• Emulsion: This heterogeneous system consists of at least one immiscible liquid intimately dispersed in another liquid in the form of droplets with a diameter generally exceeding 0.1 micron.5
• Chemical emulsion: Oil is chemically emulsified in water when emulsifiers such as surfactants or soaps are present. The surfactants have hydrocarbon chains. The simplest ones are sodium laurel sulfate or stearic acid, which have a hydrophilic (water-loving) and a lipophilic (oil-loving) end. The lipophilic end enters the oil droplet, while the hydrophilic end remains in the water. Since this creates a charge on the otherwise neutral oil droplet, the droplets repel each other and disperse. This is called a chemically stabilized emulsion. The droplets are less than 20 microns.6
• Mechanical emulsion: Mechanically emulsified oil is due to shear that can result from the wastewater traveling through a pump, wastewater splashing into a tank, or anything else that can break up and disperse large oil droplets. These oil droplets range in size from 20–150 microns.7
Why “hydromechanical” and “gravity” as names for grease interceptors? As illustrated by the anecdote above, inaccurate nomenclature can lead to injury, damage, and lawsuits. More seriously, a lack of discipline in adhering to binomial naming conventions in codes, standards, and less formal publications has led to a need for correction. The usage of precise nomenclature is essential to the predictable outcome of a system.
Physics Pertinent to Separation
Gravity is utilized in the design of both GGI and HGI units. In the case of a GGI, it is gravity alone, as quantified by Stokes’ Law, which is a mathematical equation that expresses the vertical velocities of small spherical particles in a fluid medium. The law, first set forth by the British scientist Sir George G. Stokes in 1851, is derived by consideration of the forces acting on a particular particle as it sinks or rises through a liquid column under the influence of gravity.
For brevity and simplicity, see Figure 1 for a depiction of how Stokes’ Law affects droplets of FOG. The rates of rise are in still water. Horizontal water velocity in excess of vertical FOG velocity dictates pass through the interceptor. Vertical FOG velocity is mainly the function of droplet size and density in both HGIs and GGIs.
This function, simply stated, is:
Vp = [G/(18 5 μ) ] x (dp – dc) x D
Vp = Droplet settling velocity, cm/sec
G = Gravitational constant, 980 cm/sec2
μ = Absolute viscosity of continuous fluid, poise
dp = Density of particle (droplet), gm/cm3
dc = Density of continuous fluid, gm/cm3
D = Diameter of particle, cm
Since the equation was developed for solids falling, particle (or droplet) rise velocity is a negative number. Following are the assumptions Stokes made in this calculation.
1. Particles are spherical.
2. Particles are the same size.
3. Flow is laminar, both horizontally and vertically.
The single variable influencing the rate of vertical travel is the size of the sphere. The factors affecting FOG droplet size are mechanical and chemical emulsions. As noted, an emulsion is when the oil is broken up into droplets that disperse in water. The smaller the droplets, the more stable the emulsion. Emulsion droplets are between 0.1 and 20 microns in size. When the droplets eventually contact each other, they coalesce and rise to the surface.
Mechanical emulsions are a result of long runs—horizontal and vertical—between the FOG source and the interceptor, pumping drainage accumulation from sumps, and high turbulence near the FOG source such as may be produced by a dishwasher. Mechanical emulsions resulting in droplet size approaching 20 microns (0.00079 inch) will require approximately 13 minutes to rise 1 inch.
Chemical emulsions can be more problematic to effective separation because an emulsifier (usually a surfactant, detergent, or soap) is present. Surfactants consist of a hydrophilic/oleophobic (oil-repelling) end and a hydrophobic/oleophilic (oil-attracting) end and act as a coupling agent between the oil and water. Because the emulsifier is polar on one end (it has a charge) and is non-polar at the other end, it prevents the oil droplets from approaching and coalescing.
Combinations of mechanical and chemical emulsions, usually generated by commercial dishwashers employing chemical sanitizers in conjunction with other soap-laden discharges, are not appreciably (less than 15 percent) recoverable in either a GGI or HGI.
HGIs are somewhat less vulnerable to mechanical emulsification pass-through because the design incorporates air entrainment, counter-current flows, and internal baffles designed to create low pressure areas within the flow, known as the Bernoulli Effect, which is a statement of the relationship between flow speed and pressure in a fluid system. In essence, when the speed of horizontal flow through a fluid increases, the pressure decreases. The purpose of the design is to increase droplet collision to create larger droplets and accelerate ascension by attached entrained air bubbles.
Retention Design Elements
Once FOG is separated from flowing discharge, it is necessary to retain it until it can be removed from the interceptor either manually by pumping in the case of a GGI or by dipping in the case of an HGI.
Typically, retention is affected by the interactions between floating FOG, baffle placement, and outlet piping configurations (see Figures 2 and 3). No matter the hydraulic efficiency of a design, retention is effective for only a matter of days because FOG is not a stable compound once in contact with water. Hydrolysis (a chemical process in which a water molecule is added to a substance, resulting in the split of that substance into two parts) begins to break the bond between the glycerol and the three fatty acids (see Figure 4). Glycerol is denser than water with a specific gravity of approximately 1.22, so it cannot be retained in a grease interceptor by flotation and barriers designed to hold floating material. Hydrolysis also has the effect of decreasing pH to 3–4, thereby increasing corrosive effects in the interceptor and the collection system. Frequent FOG removal is the only remedy to the effects of time and hydrolysis.
Grease Interceptor Referenced Standards
Hydromechanical grease interceptors are referenced by the following design and performance standards:
• Military Specification MIL-T-18361
• ASME A112.14.3: Grease Interceptors
• PDI-G101: Testing and Rating Procedure for Grease Interceptors
• CSA B481: Grease Interceptors
These standards require construction qualities as well as separation and retention performance levels.
Gravity grease interceptors are referenced by these design standards:
• ASTM C1613: Standard Specification for Precast Concrete Grease Interceptor Tanks
• IAPMO/ANSI Z1001: Prefabricated Gravity Grease Interceptors (replaces IAPMO PS 80)
These standards require construction qualities and geometric configurations related to suitable performance. They do not reference separation and retention performance levels.
Design Comparisons and Observations
Grease interceptors from other countries exhibit certain design characteristics that in some respects seem to speak more closely to the challenges presented by the increasing specific gravities of FOG, detergent and surfactant efficiencies that maintain chemical emulsions, and mechanical emulsions produced by long runs to end-of-pipe interceptor locations.
For example, it is quite obvious that the rate of ascension of FOG droplets directly relates to the overall separation and retention effectiveness of either interceptor type. However, domestic designs perpetuate a dubious design feature of deeply submerged inlet discharge (see Figure 5). Conceivably, such a feature is to reduce inlet velocity agitation or horizontal transport of floating FOG. However, this supposition bears examination. Dissipation of inlet kinetic energy may be accomplished by means other than inlet submergence. Several examples of circular configurations in both GGI and HGI applications also are emerging in foreign and domestic markets.
In conclusion, it is not the case that grease interceptors have not evolved since their 19th century introduction. It is the case that grease interceptor evolution leaves much room for innovation to meet changing performance and administrative demands.
1. Uniform Plumbing Code, Chapter 2, International Association of Plumbing and Mechanical Officials, 2009.
4. Corbitt, R. A., The Standard Handbook of Environmental Engineering, McGraw-Hill, New York, 1990.
Max Weiss (firstname.lastname@example.org) is a consultant with 19 years of experience in systems and project design relating to pollution control and remediation. He is a past member of the Industrial Research Consortia and Technical Advisory Committee, Center for Biofilm Engineering, Montana State University, involved in biofilm research and application, FOG pollution control, system design and analyses, and engineering research. Max serves on ASME, IAPMO, ICC, PDI, and CSA committees having to do with grease interceptors and invented the Remediator™, an application of fixed biofilm to dispose of drain grease, holding patents in that technology. He also is a vice president of ASPE’s Research Foundation.