An outdoor grease interceptor takes mystery out of grease disposal

By Leslie Lichtenburg

Baltimore County Public Schools, the third largest school system in Maryland, will soon add another elementary school to its register. In the summer of 2005, Woodholme Elementary, a new facility for 720 students in grades kindergarten through five, will open as a prototype school in Northwest Baltimore County. When the school opens, hundreds of students will have a new venue for academic studies; additional buses will be added to the county fleet; numerous new faculty, administrators and custodial staff will be hired; and an abundance of nutritious school lunches and breakfasts will be served in the new cafeteria.

It is the latter consideration that led county officials to hire Mayer Brothers Inc., manufacturers of precast concrete products, to design and build a large outdoor precast concrete grease interceptor at the school site. Typically used in the restaurant industry and for other facilities that prepare large volumes of food, outdoor grease interceptors play an important role in minimizing the infusion of fats, oil and greases into public wastewater collection systems. The introduction of oil and grease in wastewater and the resultant combined sewer overflows is a growing problem in cities and towns across the country, due in part to aging sewage collection systems and limited oil and grease disposal options. The human health, environmental and economic impacts of these overflows has prompted state municipalities and the federal government to invest considerable resources in the study, regulation and management of sewer collection systems.

“The greatest threat of obstruction in Publicly Owned Treatment Works (POTWs) comes from polar fats, oils and greases (FOGs) of animal and vegetable origin,” according to an August 2004 Environmental Protection Agency’s guide from the Office of Wastewater Management.

Moreover, the EPA estimates that each year in the United States about 850 billion gallons of untreated wastewater is released as combined sewer overflows (CSOs), and between three billion and 10 billion gallons of untreated wastewater is released as sanitary sewer overflows (SSOs). The impact of these overflows is evident in numerous ways, including beach closures, contaminated drinking water supplies, and other environmental and public health concerns.

State and federal policy makers agree that regulations and enforcement plans for dealing with oil and grease are needed at the local level. Control of fats, oil and grease from restaurants, schools and other large industrial food processors by way of pretreatment programs is critical to keeping oil and grease out of the sewers. In a June 2002 report, the Tennessee Department of Environment and Conservation estimated that the minimization of oil and grease in the collection system could, by some estimates, reduce the risk of sewer line blockages as well as backup into service laterals by up to 50 percent.

The EPA crackdown and increased public scrutiny on sanitary sewer overflows has created a boon in the grease interceptor market. “Sanitary sewer overflows are a problem and grease buildup is a primary cause,” says Nancy Mayer, president of Mayer Brothers Inc. “Food service facilities and any others that introduce grease into a public sewer system must use a grease interceptor in order to reduce the amount of grease in wastewater to acceptable levels.”

For the Woodholme Elementary School project, Mayer Brothers adhered to a Baltimore County requirement of a 1,000-gallon tank or larger for any restaurant and large food processor. Mayer Brothers installed a precast concrete 1,500-gallon, two-compartment tank. Available as an option was a monitoring system that continuously assesses the tank’s condition. Using ultrasonic transducers and an embedded microprocessor to continuously evaluate both the influent and the temperature in the interceptor, it provides real-time information on incremental changes in the levels of solids and helps to avoid backups in the system.

The precast tank for the interceptor was engineered and constructed for traffic bearing conditions. Weighing approximately 20,000 pounds, the tank fit into a 6 foot by 15 foot excavation. The two-compartment configuration was chosen to provide greater efficiency and protection for the sewage system.

Despite efforts to control oil and grease, many jurisdictions still lack clear regulatory guidance regarding effective tools for handling grease-laden wastewater. Most agree, however, that retention time is the most important consideration for adequate grease removal, and that large-volume outdoor grease interceptors are the single most effective method for achieving this goal. A recently published National Precast Concrete Association report on the design considerations for large outdoor grease interceptors states, “…only properly sized outdoor grease interceptors provide acceptable effluent quality.”

For the Woodholme Elementary School project, the county chose to downsize to a 1,500-gallon tank from the 4,400-gallon tank originally specified for the job. With most school cafeterias having evolved from full cooking facilities to kitchens that are primarily responsible for food preparation and reheating, a smaller tank was deemed sufficient for the site. As a result, Baltimore County Public Schools was able to maintain job integrity by installing a grease interceptor that would more than adequately control and retain oils and grease originating from the cafeteria. The decision also saved a considerable amount of money.

“More and more municipalities are evaluating the design and sizing criteria of grease interceptors,” said Mayer, whose Elkridge, Maryland-based company manufactures a full line of precast concrete tanks for grease interceptors and other applications, and also provides training classes for plumbing inspectors and others in the industry.

While there are many and varied schools of thought regarding the design and sizing of grease interceptors, precast concrete is considered the material of choice, particularly for large outdoor tanks with 1,000-gallon capacities or larger.

“It is by far the most economical and durable option,” according to Mayer.

In a jurisdiction where more than 10.5 million lunches and breakfasts are served during the course of the school year, durability and reliability are key. Although complicated calculations involving waste flow rate and retention time may contribute to the complexity surrounding design and sizing criteria, numerous studies have shown that concrete provides the strength, quality, efficiency and environmental friendliness needed to effectively remove grease from the waste stream and bring the water to acceptable standards before it is discharged into public sewers.

In addition to providing greater storage capacity and longer retention times, large precast concrete grease interceptors offer the added benefits of structural integrity, design flexibility and long service life. Outdoor concrete interceptors provide a level of health safety that is lacking in indoor grease traps, since the outdoor interceptors physically remove the grease and collect it outside the kitchen area, thereby removing it from the vicinity of food preparation and storage. Large outdoor grease interceptors provide ease of maintenance and accountability as they are typically serviced by third-party maintenance contractors. As an added bonus, precast concrete interceptors are manufactured in a controlled environment, meaning they offer superior quality, are ready to be installed when needed and foul weather won’t delay the construction process while waiting for cast-in-place concrete to cure.