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Recommendations
for Designing Grease Interceptors
And the added benefits of constructing them
with precast concrete
By Michael Fankel, CIPE
Large precast concrete grease interceptors
have a proven history of adequately separating grease from
wastewater before being discharged into sewers at regulation
quality. This is precisely what jurisdictions want a grease
interceptor to do. But as more of these large precast grease
interceptors are specified, questions arise about how to size
and configure them. The Water and Wastewater Section of National
Precast Concrete Association (NPCA) recently sponsored research
on large-volume grease interceptors to develop sizing and
geometry recommendations. This article contains the results
of that research and is considered to be recommendations only-not
standards-at this time.
During the research, it became evident that
not enough information is available to develop a standard
specification. Interceptor design testing and the effects
of various grease concentration discharge regulations is limited.
There are many design inconsistencies between various authorities
and jurisdictions as well. This draws the conclusion that
third-party comprehensive testing with controlled conditions
and protocol will be necessary to develop a comprehensive
standard design specification for large precast concrete grease
interceptors.
Currently, there are proven standard methods
for producing high-quality, watertight precast concrete grease
interceptors. There are also a number of current standards
available for sizing and configuring interceptors. Recent
empirical data shows that large grease interceptors can be
designed under these current standard methods and operate
efficiently.
Large precast grease interceptors adequately
separate grease from wastewater, greatly reducing the risk
of sewer clogging, because they provide more storage volume
and longer retention times. On the other hand, small grease
traps located near grease sources in food facilities have
been attributed to causing blockages in public sewer systems
because of the lack of maintenance.
Maintenance for large precast interceptors,
which are located outside the building, can be much more easily
regulated by service contracts, thereby providing assurance
that the grease-laden wastewater will be treated before entering
the sewer system or the ground. Servicing costs can be low
because large interceptors require less frequent cleaning.
Another benefit to the precast interceptor's
increased storage volume is that garbage grinders can be routed
directly into the interceptor, eliminating the need for a
separate solids interceptor. A grease interceptor constructed
of precast concrete provides the added advantages of structural
integrity, long service life, design variability, and easy
inspection by public authorities.
Addressing Concerns, Inconsistencies,
and Difficulties
Following are some reasons why a standard
specification cannot be developed without testing being done
first.
From the available research, the primary
FOG (fats, oils, greases) test medium was animal fat (lard).
This material is easier to capture and retain than typical
vegetable oils, which have a specific gravity closer to that
of water. The problem this presents is that, because most
establishments typically use vegetable oils in cooking, the
test data does not reflect actual situations.
Another FOG medium not tested is emulsified
oils, which result from the use of detergents in a dishwashing
machine. These emulsified oil particles are smaller in diameter
than other types of FOG and require a longer retention time
to separate from the wastewater. A report prepared by the
city of Toronto, Ontario, indicates that light oils, such
as olive and vegetable, are only a minor nuisance and not
a problem. Because other jurisdictions have different opinions,
there is no clear answer.
A controversial issue is the intermittent
baffle (used to modify wastewater flow) arrangement inside
an interceptor. The Washington, D.C., Suburban Sanitary Commission
recommends two sets of baffles to provide three compartments.
Austin, Texas, and other jurisdictions recommend a single
baffle. Some recommend no baffles at all. The Uniform Plumbing
Code (UPC) requires one baffle. However, each of the arrangements
apparently yields effluent FOG concentrations acceptable to
those authorities.
Geometry criteria for interceptors (length,
width, and height ratios) are significantly different between
various authorities. An Austin study concluded that the shape
was of minimal importance. Some believe that the length of
the interceptor is more crucial for separation than depth.
There has been no consideration of sizing
an interceptor to retain sufficient FOG for a reasonable cleaning
schedule.
General industry practice recommends cleaning
when grease occupies 50 percent of the volume between the
interceptor bottom to the outlet pipe invert. Another recommends
cleaning when 25 percent of that space contains grease, and
the Environmental Protection Agency (EPA) recommends cleaning
at 75 percent. Another accepted practice is to clean interceptors
between 60 and 120 days. There is no scientific basis for
any of these figures.
The minimum retention time is another point
of contention. Austin recommends a minimum of 12 minutes.
In other correspondence, the same author recommends a 15-minute
detention. Conclusions of the Toronto report recommend five
minutes. Each of these times apparently meets the requirements
of that jurisdiction.
A problem also exists with the lack of consistent
effluent discharge criteria between various jurisdictions
regarding the allowable level of FOG in the effluent discharge
from an interceptor. Following is a short listing:
- New York City: Interceptors must remove
an average of 90
percent of grease and other extractable matter in the
effluent. There is no requirement for FOG level in discharged
effluent.
- Toronto & the EPA: 150 milligrams
per liter.
- Dallas and Austin, Texas, and others:
200 milligrams per liter.
Since little has been done to study the
amount of FOG produced by various establishments per day or
meal served, it is difficult to establish the amount of grease
storage actually needed. The following areas have varying
capacity considerations: Massachusetts and Vancouver, British
Columbia, both require a grease retention capacity of two
pounds for each gallon-per- minute of flow; Austin requires
a retention capacity of maximum flow rate multiplied by the
required retention time of 12 minutes; Johnson County, Kansas,
requires a grease retention capacity in pounds of grease based
on the maximum flow rate (based on fixture size) multiplied
by the required retention time of 30 minutes; and Toronto
requires a minimum retention capacity of interceptors in pounds
of grease based on the flow rate in liters per second multiplied
by 300. This equates to a minimum retention time of five minutes.
Interceptor Design Criteria
The criteria mentioned throughout the
remainder of this article are based on information available
at this time and are only recommendations not meant to override
local governing standards.
Large precast concrete grease interceptors
have advantages that impact design. A key advantage is that
they can receive discharge from fixtures, garbage grinders,
and all other food preparation areas, therefore eliminating
the need for any solids interceptors for kitchen fixtures
and kitchen equipment. Since separation will occur by gravity
there is no need to include flow control devices and other
moving parts in the design.
Sizing
One factor that appears most critical
for performance is that separation efficiency is directly
related to the retention time, which in turn is affected by
storage capacity. A larger retention time provides more separation,
providing there is adequate clear space. If the clear space
becomes full of grease, the space needed for additional incoming
wastewater is eliminated, and separation cannot occur.
Grease interceptors should be designed large
enough to provide adequate retention time for all types of
FOG, because longer retention time equals larger interceptors
(emulsified oils require the longest retention times and vegetable
oils require longer retention times than animal fats); provide
enough grease storage capacity to meet cleaning requirements,
which will vary for different situations; promote longer time
intervals between cleaning to save money; manage various discharge
and influent flow rates with larger interceptors; accommodate
all loading levels using higher FOG concentrations for longer
retention times.
Recommended Configuration
Inlet baffles are necessary to increase
the retention time and avoid short-circuiting, and each inlet
and outlet baffle should extend at least four inches above
liquid level to within 12 inches of the inside interceptor
floor. There should be a four-inch minimum of freeboard above
the top of the outlet pipe as a vent space. If intermittent
baffles are used, the initial one should be the largest of
the compartments. Commonly accepted compartment ratios are
4/5 to 1/5 and 2/3 to 1/3. Two or more single compartment
interceptors can replace one large unit with intermittent
baffles. An access manhole should be provided over the inlet
and outlet, and have a watertight opening with a diameter
of 30 inches.
A liquid depth range should be 36 to 72
inches and an approximate ratio of depth to width should be
1 to 1.5. Six to 12 inches on the bottom should be allowed
for the accumulation of settled solids (the smaller figure
applies to interceptors smaller than 1,000 gallons), while
50 percent of the wetted height of the whole interceptor (each
compartment) should be allowed for the storage of grease.
The use of large precast grease interceptors
will continue growing because they perform well and many jurisdictions
prefer them. There are methods in use, like the UPC code,
that work for designing quality precast interceptors. Where
improvement lies is in consistency of all the different designing
methods. Therefore, NPCA is investigating third-party testing
that will determine optimum interceptor sizes and configurations
to use as a standard specification. Until testing is complete,
you should work with your local precasters, many of whom are
currently manufacturing high-quality interceptors that will
meet your needs.
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