Automatic
Grease Recovery System Sizing Methods
Grease Recovery Systems are passive
devices required by municipalities to stop fat,
oil and grease (FOG) from entering the city's sanitary
sewer system. These materials cause blockages in
the system, which cause backups and overflows.
Grease Recovery Systems are designed to separate
FOG from wastewater so that they can be removed
before they enter the sewer system. All restaurants,
caterers, school cafeterias and other commercial
cooking facilities must avoid discharging FOG into
the municipal sewer system. A system is utilized
with grease dishwashers, floor drains and mat washing
area drains before draining to the sanitary sewer
system. Recovery systems must typically be sized
for the peak wastewater flow from all contributary
sources. For grease recovery systems to function
properly they must also be regularly serviced and
maintained by qualified personnel.
The following are the three most popular sizing
methods currently acceptable to various jurisdictions.
Since there is little agreement among various authorities
on grease recovery sizing, and these methods are
somewhat arbitrary and subjective to individual
interpretation, a specifying engineer should consult
local authority before using any of the following
sizing methods, it is expected that a generally
accepted sizing protocol will be eventually established.
|
AGRS Sizing Based on Waste Pipe Diameter Size
For a waste pipe installed with a typical slope of
1/4" per foot, there is a maximum flow if water
flows on its own gravity. Based on this theory, a recovery
system can be selected based on the following chart.
Since the flow control is required to be installed
with the system, this method is simple and reasonable.
|
| 2" |
.240 |
20 GPM |
20 GPM |
10 GPM |
| 3" |
.240 |
60 GPM |
75 GPM |
35 GPM |
| 4" |
.240 |
125 GPM |
150 GPM |
75 GPM |
| 5" |
.240 |
230 GPM |
250 GPM |
125 GPM |
| 6" |
.240 |
375 GPM |
500 GPM |
250 GPM |
|
| 1 |
Determine volume of fixture by multiplying
length by width by depth |
A sink 48"
L x 24" W x 12" deep. Volume = 48 x
24 x 12 = 13,824 cu. in. |
| 2 |
Determine capacity in gallons, 1 gal. = 231
cu. in. |
Volume in gallons, 13,824/231 = 59.8 gallons |
| 3 |
Determine actual
drainage load. The fixture is normally filled
to 75% of capacity with water. The items being
washed displace about 25% of the fixture content;
thus, actual drainage load is 75% of fixture
capacity. |
Actual drainage
load: 0.75 x 59.8 p 44.9 gallons |
| 4 |
Determine flow rate
and drainage period. In general, good practice
dictates a one-minute drainage period: however,
when conditions permit, a two-minute drainage
period is acceptable. Drainage period is the
actual time period to completely drain the fixture,
Flow Rate = Actural Drainage Load / Drainage
Period |
Calculate flow rate
for a one-minute period.
44.9/1 = 44.9 GPM Flow Rate
Two-minute period
44.9/2 = 22.5 GPM Flow Rate |
| 5 |
Select AGRS, and
select unit which corresponds to the flow rate
calculations. NOTE: Select next largest in size
when flow rate falls between two sizes. |
For a one-minute
period - 44.9 GPM requires PDI size "50".
For a two-minute period - 22.5 GPM requires PDI
size "25". |
|
| 1
1/4" |
1 |
7.5 |
AGRS-10 |
| 1
1/2" |
2 |
15 |
AGRS-15 |
| 2" |
3 |
22.5 |
AGRS-25 |
| 2
1/2" |
4 |
30 |
AGRS-35 |
| 3" |
5 |
37.5 |
AGRS-50 |
| 4" |
6 |
45 |
AGRS-50 |
|
|
