| PRACTICAL GUIDELINES FOR GREASE TRAP DESIGN By: Jeffrey C. Bahret, PE
WHAT'S THE DEAL WITH GREASE TRAP DESIGN?
Given a design scenario of a 200 seat, 24 hour per day restaurant; a manufacture's
recommendation would be typically an in-kitchen 30 gallon canister, whereas, following the
State or Federal design guidelines would require an outside buried tank of olympic
swimming pool proportions. Which is correct? Well, while an undersized grease trap may
adequately meet the local plumbing authority guidelines, it will not usually meet
pretreatment requirements. Whereas, the State or Federal design guidelines fall under the
category of " there is no such thing as too big of a grease trap"; it is not
cost-efficient engineering to burden a facility with unnecessary capital cost
expenditures. We will attempt to clear-up some of the "smoke & mirrors"
associated with grease trap design, and focus on the key items.
Why do restaurants and commercial kitchen
facilities need grease traps on their kitchen waste lines? It is because of one of the
following conditions:
A. ON-SITE DISPOSAL SYSTEM- The trouble
arises when the grease and oil reaches the soil absorption field, cooling and solidifying
to form a "waterproof" layer on the soil below the distribution pipe network,
causing the failure of the system.
B. PUBLIC-SEWER DISPOSAL SYSTEM- The coating
and clogging of a public sewer system with grease/oil can be a big maintenance problem for
the collection system, pump stations and treatment facility. To abate this situation,
public sewer systems through their "Use-Laws" will typically have a pretreatment
requirement for grease/oil of 100 parts per million, but, can be as low as 50 ppm.
In general, kitchen facilities must be served
by a separate sewer line in which a grease trap is placed upstream of the on-site
treatment system or public sewer service lateral. The only exception to this rule, are
kitchen sinks which contain garbage grinders. These disposal units cannot be placed on
line with the grease trap, and must go directly to the septic tank or public sewer line.
These systems are difficult to design due to
the fact that there are so many variables. We have isolated what we believe are the six
key items in conducting a proper, cost-effective grease trap design:
1. PEAK FLOW RATE- Using a flow rate of 35
gallons per day, per seat, is fine for determining average daily wastewater volumes for a
kitchen/cafeteria facility, but is pretty much useless when it comes to determining peak
flow rates. This rate is the factor which must be determined for adequate grease trap
design. Total the unload rate of the commercial dishwasher(s) (available from the
manufacturer) and add 5 gallons per minute for each sink tied to the grease trap pipeline
to determine the peak flow rate for design.
2. GREASE REMOVAL EFFICIENCY- The percentage
of total grease which is to be separated from the waste stream is very important in the
evaluation. A typical kitchen waste stream contains 300 mg/l of grease/oil. Depending upon
the specific pre-treatment requirements, the facility may need to remove 50% to 90% of the
total grease volume. This variable has a big impact on the trap design.
3. CLEANING CYCLE- How often can the grease
trap be cleaned? Can it practically be done every month, or because of location,
operation, weather, etc. only be cleaned every 6 months. This has a direct impact on the
available grease storage volume factored into the trap design. Once the design storage
volume is exceeded, the unit becomes ineffective, and removal efficiencies cannot be
achieved.
4. MAXIMUM FLOW VELOCITY- Because it is
necessary for the grease/oil to cool-down to solidify, the maximum longitudinal velocity
through the tank must not exceed 0.25 inches per second. This maximum through-put velocity
must be calculated for a given tank with its maximum grease storage volume factored into
the equation.
5. GREASE PARTICLE "DWELL TIME" AND
"RISE DIFFERENTIAL"- These items are key to the design since they describe the
mechanical characteristics of the individual grease particles. The minimum dwell time is
the period in minutes that a grease particle takes to move longitudinally across the
grease trap. To realistically achieve most effluent limits, this value must not be lower
than 5 minutes. A 300 micron droplet of oil at the average temperature of 70 degrees F,
will rise at a rate of 0.05 inches per second. Based on maximum velocities and minimum
dwell times, the rise differential in inches must at the least equal the distance between
the bottom of the tank and the base of the grease layer, when the storage volume is at its
maximum.
6. INLET AND OUTLET TANK BAFFLES- Typically,
grease traps have adequate outlet baffles or drop tees to prevent the accumulated grease
from exiting the unit. These components normally terminate from 6" to 12" above
the tank floor. However, most grease traps are lacking any inlet baffle or control tee. A
proper inlet baffle is important to keep the flow of grease laden water going in the right
direction for separation. Without an inlet control baffle, the grease will pile-up at the
inlet and short circuit the smooth, steady state flow through the tank necessary for
maintaining design efficiency.
In conducting a proper cost-effective design
of a grease trap, each one of these six key components has a direct relationship to the
others. This is why a design cannot be satisfied with any one formula, in a one size fits
all type of scheme. |
