Frequently Asked Questions
Grease traps or interceptors are passive mechanical devices required by municipalities to stop grease,
animal fats, vegetable oil (FOG's), or grit/sand from entering the
city's sanitary sewer system. They are also a required pre-treatment for septic
systems to prevent overloading.
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Each local sewer district establishes regulations concerning the operations of grease
traps within their jurisdiction. These regulations vary greatly with little
or no consistency even within states.
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Grease traps are sized to provide a minimum retention time (usually at least 10 min.) for the kitchen waste
based on maximum water flow rates.
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Illustration by Donald Smith, Town of Cary
- Grease interceptors allow wastewater flows to slow down
- With sufficient time fats, oils, greases, and solids separate from watewater
- Fats, oils, and greases are less dense than water and float
- Solids are denser than water and sink
- Grease interceptors are designed in a variety of sizes, shapes, and constructed
of various materials
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Waste water containing FOG's from the sinks, drains, and washing devices enter the
trap through the inlet drain line. The principle of operation is mechanical/hydraulic
separation by the difference in specific gravity of the water, oils, and grit.
The grit and sand settles to the bottom and the oils and grease rise to the top.
The water flows through the first compartment into the second compartment where
the same process occurs. Some of the liquor contains emulsified and dissolved
oils. The separation time depends on time, temperature, and turbulence of
the mixture. Also, high pH liquids and emulsifiers like soap and pumps can
affect the sizing and performance of the trap requiring longer retention time.
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