Page 154 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SOLID-LIQUID SEPARATION AND INTERCEPTORS

3.22 CHAPTER THREE

Grease Interceptors. Field testing has proven that solidified grease blockages are com-
mon in many public sewer systems. Very often, grease traps installed adjacent to fixtures
and equipment do not adequately separate grease from the wastewater due to inadequate or
no maintenance. It must be emphasized that grease traps adhering to applicable certification
standards and properly cleaned remove grease very effectively. Reports indicate they are
rarely emptied and maintenance is generally poor. As a result, animal and vegetable fats,
oils, and grease (AVFOG) flow through the grease trap into the public sewer system causing
stoppages. This situation has led many local jurisdictions to require the installation of a
larger grease interceptor outside the building on the site. This allows inspection by public
authorities to be more convenient, and the larger size provides adequate AVFOG separation
and retention. Compared to other extensive testing, rating and certification standards for
grease traps originated by the Plumbing and Drainage Institute (PDI) and the American
Society of Mechanical Engineers (ASTM), grease interceptors have not had the benefit of
wide attention to research concerned with establishing design, configuration, and effluent
discharge standards.
The following considerations are recommended for the selection of a grease interceptor:
1. The top of concrete units shall meet AASHTO A-20 guidelines where there is a possibility
that trucks could pass over the unit. If the unit is in a nontraffic area, the unit shall meet
minimum top loading standards.
2. An inlet diverter is necessary to increase the retention time and avoid short circuiting.
3. A rectangular interceptor is believed to be the optimum shape. An approximate ratio of
depth to width shall be 1 to 1.5. There should be at least 4 in above the water level for
venting.
4. An allowance of 6 to 12 in (152 to 300 mm) shall be provided on the bottom for sludge
accumulation.
5. A minimum depth of 42 in (1070 mm) is suggested.
6. A sample port should be provided to allow ease of sample taking.

Interceptor Sizing Guidelines. The most important consideration is conformance with
any code requirements or standards of the authorities having jurisdiction. When these codes
or standards are found, they must be followed.
The following guidelines have been adapted from various published standards and codes
and are intended to be used only when there are no other applicable requirements.
For establishments other than restaurants, the maximum flow rate method was selected
because it relates the flow rate for fixtures to the size of the interceptor.

1. Determine the number and size trap of fixtures and the size of a dishwasher (if any) in
the establishment discharging into the interceptor.
2. Add the dishwasher and only the single largest sink together. If there is no dishwasher,
add the two largest flows, in gallons per minute, together. This is the maximum probable
flow rate into the interceptor. This figure is based on the probability that no more than
two fixtures could discharge at the exact same time.
3. Multiply the flow rate by 30 to calculate the minimum pounds of grease required to be
retained. (This figure has a proven past history of success.) Pick a standard size intercep-
tor with a capacity equal to or larger than the calculated size.
4. A minimum size shall be chosen on a sliding scale based on the size of the establish-
ment, as follows:
a. For small shops, such as a pizza parlor or other similar establishments—200 gal (760 L)
capacity

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