Grease Removal Through Traps, Interceptors and Bioremediation
Grease is generated by food preparation and food service facilities. Significant quantities of that grease accumulate in building piping, municipal collection systems, wastewater treatment plants and onsite system drain fields. Grease frequently interferes with subsequent treatment plant sludge disposal or passes through the treatment plants, fouling receiving waters both surface and subsurface. Escaping grease in outflow and sludge can serve as transport and growth support for pathogenic organisms, resulting in contamination of lakes, rivers, beaches and groundwater.
Grease is a, if not the, principal element of collection system failure, whether through pipe-wall accumulation capacity reduction, misalignment point accumulation restrictions, fitting impact cementations, root penetration point accumulations, moving agglomerations, concentrated corrosion, or all of the above. The eventual and inevitable result of excess grease in drainage piping is sanitary sewer overflow (SSO).
As a significant example, an Orange County, CA, Grand Jury (2000-2001) convened for the purpose of surveying 35 wastewater collection and treatment facilities, 12,000 miles of sewer lines, and over 6,800 restaurants in Orange County, and their relationship to SSOs resulting in beach closures. The grand jury determined the leading cause to be grease-clogged sewage lines from restaurants and high-density residential areas. They further observed that only a few of the agencies had an inspection and enforcement program in place.(1)
The problems created by grease in drainage and collection systems are commonly known. Just as common is the application of grease traps or interceptors to remove grease from restaurant effluent. The Orange County Grand Jury listed in its findings the following "disposal" methods(2):
"Restaurants generally dispose of cooking grease/oil in the following three ways:
- 1. Grease traps--These are small devices hooked directly to the outgoing drains of sinks and dishwashers and are located inside the restaurant. Because they hold small quantities of captured grease, these traps must be emptied and cleaned on a regular basis, and the grease properly discarded to prevent grease overflow into the sewer system.
2. Grease interceptors--Interceptors are large underground devices usually located outside the restaurant and connected to the restaurant's outgoing sewer drainage system. These large tanks have heavy manhole-like covers that are difficult to access for inspection. When full, they must be emptied and cleaned by private waste pickup and disposal companies.
3. Large covered barrels--Barrels in which higher quality cooking grease is placed are kept covered and then collected by commercial companies who sell it for industrial reuse. These barrels are placed outside the restaurant. They are easily accessible and are usually not a contributing factor to the problem, unless they are accidentally spilled and the contents enter sewer or storm drains.
This example of terminology is not presented to criticize Orange County. Its extraordinarily formal assessment method is to be complimented. This is simply an example of how "commonly" accepted usage of terms, especially when appearing in a judicial document, may become fundamental elements of policy determination and implementation.
Apart from the obvious (storage in three kinds of containers is not "disposal," it is storage), formal mixed use or misuse of terms result in imprecise and sometimes unusable policy and impart an air of legitimacy to that usage. It is this writer's assessment that such is the case in the use of the terms "grease trap," "grease interceptor" and "disposal" in many publications.
Imprecision in the use of the terms "grease traps" and "grease interceptors" has become so prevalent that the statement "the terms can be used interchangeably" is accepted as fact.(3) That statement is just as incorrect as the conclusion that separation and retention of FOG in a grease trap or grease interceptor constitutes pretreatment or disposal.(4) Disposal is accomplished only when the pollutant can no longer enter the environment (water, soil or air) in a form or quantity as to be deleterious.
Observance of these distinctions is more critical as policies and codes become formalized in ordinance and statute, but is most critical because of the effect on the evolution of compliance technology and requirements of measured performance of that technology.
Imprecision in the application of the terms "grease traps" and "grease interceptors" is concurrent with imprecision in the application of the devices, frequently to the extent that there is regulatory perception of failure of one or both devices as suitable elements in a pretreatment strategy. Imprecision in the application of those terms is especially manifested in sizing and placement, as illustrated in the Orange County example.
Inconsistent use of terms or equivocation further impair development of concise and comprehensive grease abatement policy when we speak of "disposal" and "removal." Misapplication of these terms is becoming inextricably entangled with the evolution of an unexpressed premise that the removal (by whatever means) of FOG from a grease trap or grease interceptor constitutes the culmination of pretreatment, or disposal of, the pollutant. The terms are not synonymous. While disposal constitutes removal, removal does not constitute disposal.
Equating or misapplying the terms "grease trap" and "grease interceptor" is damaging to the acceptance of the intended use of the products. Equating the terms "removal" and "disposal" damages efforts to establish comprehensive grease pollution control programs, frequently leaving operators of FOG producing facilities to search for "proper disposal" methods on their own.
As we learn from economics, demand creates supply. The marketplace and operators are responding to the ever-increasing demand for "disposal" methods with both constructive and destructive solutions. Some of the more destructive are blending (trap/interceptor grease with restaurant grease(5) at the facility) for hauling to rendering or freezing trap/interceptor grease for dumpster deposit. Then there is the pump & dump, where an unethical pumper arrives with a full tank, places the hose, and discharges the contents of his tank, thereby displacing the interceptor or trap contents downstream. Then he extracts the contents of the trap or interceptor, leaving it nice and clean, and proceeds to the next account to repeat the process. Finally, there is the ubiquitous w.c., always close at hand, volume capable and easily cleaned with a single flush.
Some of the more constructive disposal options are: incineration; centralized receipt and treatment by sewage treatment works; composting; land filling; bio-diesel; bio-fuel and bioremediation. The options are myriad, each with greater or lesser relevance to the objective of preventing the pollutant from reentering the environment and food chain.
Many FOG producing facility operators are not aware of their "disposal" responsibilities, as expressed in this explanation of Florida regulation:
Did you know that if you have your restaurant grease trap serviced (pumped-out) by a waste hauler, it is your responsibility to ensure that your waste is properly disposed at a Septage Receiving Facility or a grease processing/recycling?(6)
Critically distinctive classifications of grease predetermine the suitability of various types of grease for re-use. Pertinent to this article are the official classifications "yellow grease," which can contain restaurant grease (see definition in footnote 5), and "brown grease," generally known as trap grease. Though the distinction has been maintained for over 80 years by responsible renderers and recyclers(7) for purposes of public health and safety and product quality, it is not uncommon to see policy statements and prominent publications mixing and misusing the terms, or just calling both "grease" with no distinctions as to source or content.
Ultimately, equating "grease trap" with "grease interceptor," "removal" with "disposal," and "restaurant grease" with "trap grease" leads to selection of equipment and processes which maximize ease and economy, frequently at the expense of environmental responsibility, regulatory compliance and public health and safety.
So, what can be done with the stuff? Brown grease disposal options are limited to methods which do not introduce the substance to feed stocks, surface or ground water transport, or products which may come in contact with or be consumed by animals or humans. The restrictions on brown grease disposal are operative once the grease is removed from traps, interceptors and grease removal devices. Blending brown grease with yellow grease to be recycled as yellow grease is not acceptable, and responsible renderers do not knowingly engage in the practice. Contamination, by solvents, cleaning chemicals and pathogenic organisms, is the principal reason for this restriction.
Brown grease disposal methods such as composting, caking for landfill deposit, land application, incineration and blending or treatment at municipal treatment works are expensive, ranging from 20 cents to 85 cents per gallon depending upon locale and disposal options available.
Disposal options such as bio-diesel, bio-fuel and polymer manufacture are technically interesting, but remain largely experimental, significantly less than cost effective, and the contamination issue remains.
A logical and somewhat historical parallel appears to be developing, of necessity, between the treatment of sewage and industrial discharges containing brown grease: bioremediation systems. I am not speaking of the application of bacteria, enzymes or chemical additives to conventional grease waste systems utilizing grease traps or grease interceptors. That practice is "removal" not "disposal"--a distinction necessary to this discussion and fundamental to long-term function of the collection system.
In situ and onsite bioremediation are widely utilized technologies for pollution abatement and clean up. In situ bioremediation is frequently utilized when a pollutant is detected in ground water, and the pollutant is treated in the ground. Onsite bioremediation refers to bioremediation at the site of pollutant generation prior to discharge. Essentially, a septic tank-drain field combination is a bioremediation system. The advantages of onsite bioremediation of water containing brown grease are:
- 1. It virtually eliminates the need for frequent cleaning and disposal of brown grease.
2. The pollutant is reduced to its basic elements (carbon, hydrogen and oxygen), usually in the forms of carbon dioxide and water.
3. A considerable quantity of food scraps is removed from facility effluent and can become a useful by-product.
The key word when considering bioremediation disposal is "system." Typically, bioremediation systems are engineered systems (Figure 1) of either fixed film, or moving bed types. Either type may have such features as dissolved air flotation, automated sludge removal, mechanical aerators and sequential batch processing.
"Fixed film" refers to a biofilm consisting of microbes attached to a structure designed to maximize pollutant/microbe contact. The structure can be stationary, as a trickling filter, or media, as illustrated in Figure 1. The structure can be moving, as in the case of rotating biological contactors, or buoyant media pieces. Fixed film systems are common in wastewater treatment plants.
The microbes can be indigenous to the waste stream, or as is predominantly the practice in industrial discharges carrying hydrocarbons such as brown grease, inoculated multiple species selected and bred specifically for their suitability to the oxidation of the various compounds present in the effluent.
Onsite bioremediation is the present evolutionary direction of disposal of noxious, polluting compounds such as brown grease. But, whatever the basic bioremediation design selection may be, the system is not a grease trap and it is not a grease interceptor. Generalizing terms and principals pertinent to those technologies is obstructive of the goal of pollution elimination. That goal can only be achieved through formation of workable policy based on sound engineering and uniform application of definition and terminology.