# Convert grease trap flow (gpm) into dfu



## Mech (Jun 9, 2015)

2009 IPC

F-1 Bakery use (they mix seasonings together and try their new recipes / concoctions) with a dishwasher, three bowl sink, two bowl sink, and two hand sinks.  They have a range and a deep fryer to create grease.

The two bowl sink discharges into a floor sink with a loading of 2 dfu.   The three bowl sink discharges into another floor sink with a loading  of 2 dfu.  The dishwasher uses 5.4 gal per cycle, so I am saying that is  5.4 gpm which is equivalent to 10.8 dfu. IPC 709.3 Values for continuous  and semi-continuous flow, listed below, says 1 gpm = 2 dfu.

The flow used to size the grease traps for the two bowl sink, the three bowl sink, and the dishwasher is 61 gpm (using PDI G101.)  When the water leaves the grease traps and enters the building sewer, what dfu load do I use?  Do I use 14.8 dfu (the loading going into grease traps) or is it 122 dfu  (61 gpm * 1 gpm @ 2 dfu)?

Thanks is advance.

*709.3 Values for continuous and semicontinuous flow.* _Drainage fixture unit_  values for continuous and semicontinuous flow into a drainage system  shall be computed on the basis that 1 gpm (0.06 L/s) of flow is  equivalent to two fixture units.


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## jdfruit (Jun 9, 2015)

grease traps are based on holding capacity over time for assuring adequate grease separation in the waste stream. Use total gallons over the expected time to arrive at input for grease interceptor size. The required grease interceptor (more than 50gpm required interceptor) flow control will even out transient surge flows.

See ch 10 IPC


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## mtlogcabin (Jun 9, 2015)

1002.3.1

and dishwashers without prerinse sinks.

If there is a pre-rinse then the dishwasher discharge should not run through the interceptor

What type of interceptor is being proposed?

GREASE INTERCEPTOR.

Hydromechanical. Plumbing appurtenances that are installed in the sanitary drainage system to intercept free-floating fats, oils and grease from waste water discharge. Continuous separation is accomplished by air entrainment, buoyancy and interior baffling.

Gravity. Plumbing appurtenances of not less than 500 gallons (1893 L) capacity that are installed in the sanitary drainage system to intercept free-floating fats, oils and grease from waste water discharge. Separation is accomplished by gravity during a retention time of not less than 30 minutes.


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## Mech (Jun 9, 2015)

jdfruit - I totaled the two bay sink volume, multiplied by 75%, and  assumed a one minute discharge, per PDI G101.  Then I added the  dishwasher volume to arrive at 26.4 gpm and rounded up to a 30 gpm  grease trap.  The three bay sink was similar and came out to 34.5 gpm  and rounded up to a 35 gpm unit.

mtlogcabin - There are two grease traps, both hydromechanical.  One (30 gpm) serves the two bay sink and dishwasher, the other (35 gpm) serves the three bay sink in another room.  There is a garbage disposal in the two bay sink, so I figured there could be greasy waste going in there so that is why it is discharging through the grease trap.  Code requires a solids interceptor, so that will be installed also, and it will only receive discharge from the two bay sink.

Since I do not know how they will operate, I decided to pipe both the two bowl sink and the dishwasher to a grease trap.

The ultimate question is - what is the minimum pipe size per code?  If 14.5 dfu, they can opt to keep the existing 3" sanitary line.  If 122 dfu, they will be cutting up the floor and replacing pipe.


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## mark handler (Jun 9, 2015)

DEFINITION OF DFU

1 DFU is defined as 1 cubic foot per minute (cfm) or 7.5 gpm from the drain

outlet of a plumbing fixture. The following formula can be used to calculate

the outlet flow:

Q=13.17 d2h1/2

where:

Q = Discharge flow rate, gpm

d = Diameter of outlet orifice, inches

h = Mean vertical height of the water surface above the point of the outlet

orifice, feet


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## north star (Jun 9, 2015)

*~ ~ @ ~ ~*

Mech,

Is the location of these grease interceptors at the last point on the

sewer line before the enter the sanitary sewer system, or are they

located farther upstream in the building ?.......Also, are there other

dfu's being added to the overall capacity of the sanitary sewer line ?

There [ might ] be a whole lot of pipe upsizing if the dfu's are

exceeded for the entire length of the sanitary sewer line.

*~ ~ @ ~ ~*


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## mark handler (Jun 9, 2015)

Only FOG waste goes through the interceptor

So you do not put it "last"


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## Mech (Jun 9, 2015)

Northstar - the grease traps are only receiving waste from the two mentioned sinks and the dishwasher.  The handwash sinks will be piped into the sanitary line after the grease traps.  Yes, there may be pipe upsizing if the building main is insufficient in size.  Thank you for pointing this out / reminding me.  I do not have drawings for the building, so my plan will get the "contractor to verify pipe size and total dfu loading and upsize as required" note.  Unfortunately, the code reviewer will have to decide if the permit gets issued or if it gets a conditional permit and the client / building owner / contractor checks things out during construction.


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## JBI (Jun 9, 2015)

Not sure why you would assign an outflow value any higher than the inflow volume for a grease trap... Less comes out than goes in, not more.


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## fw. (Jun 10, 2015)

JBI said:
			
		

> Not sure why you would assign an outflow value any higher than the inflow volume for a grease trap... Less comes out than goes in, not more.


Less grease goes out, but the same volume of liquid that goes in comes out.


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## fw. (Jun 10, 2015)

JBI said:
			
		

> Not sure why you would assign an outflow value any higher than the inflow volume for a grease trap... Less comes out than goes in, not more.


Less grease goes out.  Same amount of liquid that goes in comes out.


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## JBI (Jun 10, 2015)

fw. said:
			
		

> Less grease goes out.  Same amount of liquid that goes in comes out.


Right. The grease and water go in together through the same waste line and only the water comes out.

Like I said less comes out than goes in...


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## Frank (Jun 11, 2015)

JBI said:
			
		

> Right. The grease and water go in together through the same waste line and only the water comes out. Like I said less comes out than goes in...


The same combined volume comes out as goes in--the level in the grease trap remains the same--if grease is retained then there is an equal volume of water that is lost from the grease trap --- the combined flow in equals the combined flow out.  When first installed there is almost all water in the grease trap, as it operates the amount of grease increases and the amount of water in it decreases.

See P709.2 for 2 inch traps you get 3 DFU each or 9 total 3 for both sinks and 3 for the Dishwasher assuming it discharges through a 2 inch trap.

3 inch drain is fine

The flow used for grease trap sizing is different than for drainpipe sizing


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