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By Michael Fankel, CIPE
Large precast concrete grease interceptors have a proven history of adequately separating grease from wastewater before being discharged into sewers at regulation quality. This is precisely what jurisdictions want a grease interceptor to do. But as more of these large precast grease interceptors are specified, questions arise about how to size and configure them. The Water and Wastewater Section of National Precast Concrete Association (NPCA) recently sponsored research on large-volume grease interceptors to develop sizing and geometry recommendations. This article contains the results of that research and is considered to be recommendations only-not standards-at this time.
During the research, it became evident that not enough information is available to develop a standard specification. Interceptor design testing and the effects of various grease concentration discharge regulations is limited. There are many design inconsistencies between various authorities and jurisdictions as well. This draws the conclusion that third-party comprehensive testing with controlled conditions and protocol will be necessary to develop a comprehensive standard design specification for large precast concrete grease interceptors.
Currently, there are proven standard methods for producing high-quality, watertight precast concrete grease interceptors. There are also a number of current standards available for sizing and configuring interceptors. Recent empirical data shows that large grease interceptors can be designed under these current standard methods and operate efficiently.
Large precast grease interceptors adequately separate grease from wastewater, greatly reducing the risk of sewer clogging, because they provide more storage volume and longer retention times. On the other hand, small grease traps located near grease sources in food facilities have been attributed to causing blockages in public sewer systems because of the lack of maintenance.
Maintenance for large precast interceptors, which are located outside the building, can be much more easily regulated by service contracts, thereby providing assurance that the grease-laden wastewater will be treated before entering the sewer system or the ground. Servicing costs can be low because large interceptors require less frequent cleaning.
Another benefit to the precast interceptor’s increased storage volume is that garbage grinders can be routed directly into the interceptor, eliminating the need for a separate solids interceptor. A grease interceptor constructed of precast concrete provides the added advantages of structural integrity, long service life, design variability, and easy inspection by public authorities.
Addressing Concerns, Inconsistencies, and Difficulties
Following are some reasons why a standard specification cannot be developed without testing being done first.
From the available research, the primary FOG (fats, oils, greases) test medium was animal fat (lard). This material is easier to capture and retain than typical vegetable oils, which have a specific gravity closer to that of water. The problem this presents is that, because most establishments typically use vegetable oils in cooking, the test data does not reflect actual situations.
Another FOG medium not tested is emulsified oils, which result from the use of detergents in a dishwashing machine. These emulsified oil particles are smaller in diameter than other types of FOG and require a longer retention time to separate from the wastewater. A report prepared by the city of Toronto, Ontario, indicates that light oils, such as olive and vegetable, are only a minor nuisance and not a problem. Because other jurisdictions have different opinions, there is no clear answer.
A controversial issue is the intermittent baffle (used to modify wastewater flow) arrangement inside an interceptor. The Washington, D.C., Suburban Sanitary Commission recommends two sets of baffles to provide three compartments. Austin, Texas, and other jurisdictions recommend a single baffle. Some recommend no baffles at all. The Uniform Plumbing Code (UPC) requires one baffle. However, each of the arrangements apparently yields effluent FOG concentrations acceptable to those authorities.
Geometry criteria for interceptors (length, width, and height ratios) are significantly different between various authorities. An Austin study concluded that the shape was of minimal importance. Some believe that the length of the interceptor is more crucial for separation than depth.
There has been no consideration of sizing an interceptor to retain sufficient FOG for a reasonable cleaning schedule.
General industry practice recommends cleaning when grease occupies 50 percent of the volume between the interceptor bottom to the outlet pipe invert. Another recommends cleaning when 25 percent of that space contains grease, and the Environmental Protection Agency (EPA) recommends cleaning at 75 percent. Another accepted practice is to clean interceptors between 60 and 120 days. There is no scientific basis for any of these figures.
The minimum retention time is another point of contention. Austin recommends a minimum of 12 minutes. In other correspondence, the same author recommends a 15-minute detention. Conclusions of the Toronto report recommend five minutes. Each of these times apparently meets the requirements of that jurisdiction.
A problem also exists with the lack of consistent effluent discharge criteria between various jurisdictions regarding the allowable level of FOG in the effluent discharge from an interceptor. Following is a short listing:
Since little has been done to study the amount of FOG produced by various establishments per day or meal served, it is difficult to establish the amount of grease storage actually needed. The following areas have varying capacity considerations: Massachusetts and Vancouver, British Columbia, both require a grease retention capacity of two pounds for each gallon-per- minute of flow; Austin requires a retention capacity of maximum flow rate multiplied by the required retention time of 12 minutes; Johnson County, Kansas, requires a grease retention capacity in pounds of grease based on the maximum flow rate (based on fixture size) multiplied by the required retention time of 30 minutes; and Toronto requires a minimum retention capacity of interceptors in pounds of grease based on the flow rate in liters per second multiplied by 300. This equates to a minimum retention time of five minutes.
Interceptor Design Criteria
The criteria mentioned throughout the remainder of this article are based on information available at this time and are only recommendations not meant to override local governing standards.
Large precast concrete grease interceptors have advantages that impact design. A key advantage is that they can receive discharge from fixtures, garbage grinders, and all other food preparation areas, therefore eliminating the need for any solids interceptors for kitchen fixtures and kitchen equipment. Since separation will occur by gravity there is no need to include flow control devices and other moving parts in the design.
One factor that appears most critical for performance is that separation efficiency is directly related to the retention time, which in turn is affected by storage capacity. A larger retention time provides more separation, providing there is adequate clear space. If the clear space becomes full of grease, the space needed for additional incoming wastewater is eliminated, and separation cannot occur.
Grease interceptors should be designed large enough to provide adequate retention time for all types of FOG, because longer retention time equals larger interceptors (emulsified oils require the longest retention times and vegetable oils require longer retention times than animal fats); provide enough grease storage capacity to meet cleaning requirements, which will vary for different situations; promote longer time intervals between cleaning to save money; manage various discharge and influent flow rates with larger interceptors; accommodate all loading levels using higher FOG concentrations for longer retention times.
Inlet baffles are necessary to increase the retention time and avoid short-circuiting, and each inlet and outlet baffle should extend at least four inches above liquid level to within 12 inches of the inside interceptor floor. There should be a four-inch minimum of freeboard above the top of the outlet pipe as a vent space. If intermittent baffles are used, the initial one should be the largest of the compartments. Commonly accepted compartment ratios are 4/5 to 1/5 and 2/3 to 1/3. Two or more single compartment interceptors can replace one large unit with intermittent baffles. An access manhole should be provided over the inlet and outlet, and have a watertight opening with a diameter of 30 inches.
A liquid depth range should be 36 to 72 inches and an approximate ratio of depth to width should be 1 to 1.5. Six to 12 inches on the bottom should be allowed for the accumulation of settled solids (the smaller figure applies to interceptors smaller than 1,000 gallons), while 50 percent of the wetted height of the whole interceptor (each compartment) should be allowed for the storage of grease.
The use of large precast grease interceptors will continue growing because they perform well and many jurisdictions prefer them. There are methods in use, like the UPC code, that work for designing quality precast interceptors. Where improvement lies is in consistency of all the different designing methods. Therefore, NPCA is investigating third-party testing that will determine optimum interceptor sizes and configurations to use as a standard specification. Until testing is complete, you should work with your local precasters, many of whom are currently manufacturing high-quality interceptors that will meet your needs.