Frequently Asked Questions

The responses below apply to businesses and institutions that prepare or serve food.

• Why is the issue of overflowing sanitary sewers important?
  
  Overflowing sewers release disease-causing bacteria, viruses, and other human health pathogens to the natural waters of the state. Public health, safety and welfare can be jeopardized by sewage in waterways.
  
  
• Is there a law against overflowing sanitary sewers?
  
  Yes. The federal Clean Water Act and comparable state water quality regulations prohibit wastewater discharges into the waters of the state without a permit. An overflowing manhole is an example.
  
  
• Is there a difference between grease traps and grease interceptors?
  
  Not really. Both devices do the same thing - separate and retain free-floating oils and grease. The two terms can be used interchangeably. Grease traps are most often referred to as the small, in-floor or under-the-sink prefabricated steel units. Grease interceptors are the larger devices located outdoors that offer much longer hydraulic retention times. Both small and large devices are marketed as grease interceptors. These interceptors offer much longer hydraulic retention times and can store much more trapped and retained grease and oil.
  
  
• What does FOG stand for?
  
  Animal and vegetable fats, oils, and greases as extracted from a wastewater sample by select solvents in a laboratory.
  
  
• I plan to purchase an existing building for a food service or preparation establishment. It does not have a grease interceptor or the grease interceptor is inadequate. What will the Town of Cary require?
  
  In cases where there is a problem installing a properly sized grease interceptor, the Town of Cary may require the conditional hot water only pre-rinse procedure, which requires a specific washing procedure to help retain fats, oils and grease in smaller grease interceptors.
  
  
• What numeric limit should I adopt as a local limit for my FOG control program?
  
  Local limits should be protective of the sanitary sewer collection system, yet be readily attainable by well-designed and maintained grease interceptors. The N.C. Pretreatment Consortium recommends a range of values be considered as a performance based goal or target. The consortium does not strongly support the concept of a numeric limit, recognizing the inherent differences in analytical procedures and sample recovery. That range, expressed as hexane extractable materials (HEM) is 150 mg/L to 200 mg/L. The range takes into account ambient sewer pipe slope and sample collection and analysis variability.
  
  
• How do buildings with common sewer laterals for each retail flex space provide adequate grease interceptors for tenants within the building?
  
  For new construction, a good approach is installation of a dedicated sewer line for each space that connects to a dedicated common lateral that is plumbed to an individual grease or community grease interceptor. This method ensures that each facility within the flex space can install sufficient grease interception without having to perform major renovation of the space if the space is used for food service in the future.
  
  
• What formulas, other than the Town of Cary formula, are available for sizing grease interceptors?
  
  Sizing of grease interceptors is based on wastewater flow and can be calculated from the number and kind of sinks and fixtures discharging to the interceptor. In addition, a grease interceptor should be rated on its grease retention capacity, which is the amount of grease that the device can hold before its average efficiency drops below acceptable levels.
  
  The Environmental Protection Agency created a formula that relies on varying factors; e.g., type of highway, varying storage capacity factors, number of seats in the dining area, and gallons of wastewater generated per meal. The EPA recommended minimum size interceptor is 750 gallons. The Unified Plumbing Code also details a formula that relies on the number of meals served per maximum serving hour, waste flow rates from facilities with and without dishwashers and garbage grinders, and various retention times and grease storage factors. Both of these methods are somewhat subjective to individual interpretation.
  
  The Plumbing and Drainage Institute, a consortium representing prefabricated grease interceptors, developed a marketing-driven sizing formula based on a non-required one-minute or two-minute sink drain time. That method purports nondescript benefits of making sinks drain in one or two minutes, although no such requirement exists in state health regulations. Grease interceptor choice is then made on size of sinks and drain time.
  
  
• What is Cary's grease interceptor sizing formula?
  
  Cary's grease interceptor sizing formula relies on calculated peak hydraulic wastewater flow rates from all drainage units plumbed to the grease interceptor. Cary's procedure relies on a laboratory-derived hydraulic retention time of 24 minutes at peak hydraulic rate for effective oil-water separation. Subjective storage and loading factors are not used in Cary's formula.
  
  
• What is the difference in grease interceptor sizing formulas?
  
  EPA and Unified Plumbing Code sizing methods result in grease interceptors with much larger volumes than does the method used for small, prefabricated under-the-sink devices promoted by Plumbing and Drainage Institute. The Town of Cary hydraulic retention time formula results in interceptor sizes that closely match those of the EPA and Unified Plumbing Code.
  
• Can I clean the grease interceptor at my food service establishment?

  The N.C. Division of Waste Management requires a permit for anyone or any
  firm that pumps, transports, stores, treats, or disposes of grease septage.
  More information