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MC Magazine |
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Precast
Concrete Grease Interceptors: Simple Efficiency
By Dean A. Frank, P.E.
Dean Frank is NPCA’s director of
technical services.
With as many as 900,000 restaurants located
in the United States, the need for effective pretreatment
for the wastewater they generate is vitally important. Ineffective
grease removal from these waste streams results in the discharge
of large quantities of grease into public sanitary sewer systems.
Solidified grease can cause stoppages downstream in the piping
networks and create costly and time-consuming problems at
municipal wastewater treatment plants. Grease blockages can
also cause considerable unnecessary health hazards when raw
sewage backs up into residences or commercial establishments.
By trapping fats, oils and greases (FOG) and solids normally
found in food service establishments, a grease interceptor
is the last line of defense against potential problems downstream.
More and more sanitarians and other similar
authorities are realizing that precast concrete grease interceptors
are the best solution for FOG removal. This is supported by
an NPCA paper on grease removal practices required by various
authorities having jurisdiction in the United States (see
http://www.
precast.org/technical/issue_papers.htm).
Besides precast concrete grease interceptors,
alternative grease separation devices such as small under-the-sink
metal grease traps have been used at many food service establishments.
In order to allow for adequate separation of the FOG from
the waste stream, all grease separation devices must be designed
with the following criteria in mind:
Retention time.
Greases and oils have a lower specific gravity than water,
so when a grease-laden mixture is left undisturbed, they will
float to the surface. As such, there must be sufficient time
for this process to occur. Smaller devices should be carefully
evaluated for retention time. Metcalf & Eddy, an internationally
acclaimed company that delivers a full range of environmental
services, recommends a retention time of 30 minutes to adequately
separate a grease-laden mixture.
Flow. The
grease separation device must be sized and configured to allow
for sufficient retention time, taking into account the flow
rate of the influent. Furthermore, it must be configured such
that it minimizes turbulence to allow the suspended FOG to
separate. This is especially important in high-flow situations,
such as when draining a large sink.
Storage capacity.
The device must be large enough to allow for sufficient storage
of accumulated FOG between cleaning operations without affecting
the flow characteristics through the unit.
Cleaning frequency.
The grease separation device must be sized as to how often
it will actually be cleaned out. Under-the-sink grease traps
must be cleaned out routinely, maybe as much as once per day.
There is generally no assurance that restaurant personnel
will maintain the unit properly, so routine cleaning performed
by a third party will more likely result in maintenance and
supporting documentation.
In addition to all of the previously mentioned
design considerations, consider that soaps and other emulsifying
agents have a significant impact on the amount of time required
for the FOG to adequately separate. Here is a quick and easy
demonstration that shows how soap drastically increases the
time it takes for FOG to separate effectively:
- Take two identical jars with lids, some
vegetable oil and liquid dish soap.
- Add a few tablespoons of the oil to
each jar and fill them up with water.
- Add a couple of drops of the dish soap
to one of the jars, then close both lids tightly.
- Shake both jars for about one minute,
then set them down and start a timer.
- Note the amount of time is takes for
the oil to separate in each one.
Given that restaurants use soaps, it should
be apparent that soaps’ effects on separation time should
be considered in the design for grease separation devices.
There are many variables that affect the separation rate of
emulsified FOG. As such, consider that the separation time
will be increased by soaps in order to ensure an appropriate
minimum retention time.
Outdoor precast concrete grease interceptors
can easily accommodate these design requirements. Retention
times in excess of 30 minutes are easily achievable. Baffles
and large liquid volumes help reduce turbulence and provide
for a high storage capacity. Large outdoor interceptors can
provide maintenance accountability because of their reliance
on third-party maintenance contractors. Another consideration
is that outdoor interceptors provide a level of health and
safety that indoor traps cannot provide. By physically removing
the collection, maintenance and disposal of grease outside
the kitchen area, outdoor interceptors eliminate the health
concerns otherwise created by providing these functions in
the same workspace as food preparation. So technically speaking,
precast concrete grease interceptors are a simple, efficient
solution to a critical problem.
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