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COD on Grease Samples.
Posted: Wednesday, July 7, 2010 11:42 AM
Joined: 7/7/2010
Posts: 4



I'm curious to know if anyone out there has a prep method for COD for grease interceptor samples. We will be receiving samples that come from grease traps in restaurants. The idea is that they want to collect this material and then use it as a fuel source. I received a batch of these last year. The samples are multi-phased (solid grease on top, food particle layer, some water). They were awful to run and the uncertainty on them was huge because I did not have any type of established method to work from. I was trying to run them as though they were a typical COD, which they are clearly not.


My ideas for developing an in-house method to use would involve heating the sample to melt the grease and oil to a liquid phase and then homogenize the whole sample with an immersion blender. Then removing the aliquot from this more blended sample.


Has anyone else had to deal with this kind of sample matrix before. Do you have anything written up or any suggestions? I'm open to all ideas at this point since there is no established method. I just want to get something in place before the 50-100 samples I'll be getting start coming in.


Thanks for your help!


Victor Santa Cruz
Posted: Wednesday, July 7, 2010 2:08 PM
Joined: 11/11/2009
Posts: 38



A similar discussion was present in a different WW and W lab forum but it dealt with calculating COD of a sample that had glycerine present from the production of biofuel.  To shorten the discussion to this posting--the solution was to weigh out a representative aliquot.  If you are interested in viewing the discussion use the following link:

Posted: Wednesday, July 7, 2010 3:23 PM
Joined: 7/7/2010
Posts: 4

Thanks for your reply and the link. I read what was discussed and it's the same type of issues I had last year when I got a few of these samples in. There is just no good or acceptable way to dilute the samples appropriately and still main some integrity to the test. I'm going to discuss this with the group who is involved with this project and find out if another test (such as BTU) would be more appropriate. They want to know how much energy they could potentially get from this grease material. BTU seems to be the test to run in my mind, but I've been unable to convince anyone outside of the lab that this is true. Go figure, right? :\)

Posted: Wednesday, July 7, 2010 4:14 PM
Joined: 12/31/2009
Posts: 40

If they want the amount of energy present, they really need to go with BTU.  That, pretty much, is the definition of available energy.  Try having them read the wikipedia page for BTU.  Trying to do a COD on a non-aqueous sample, particularly one that is solid at room temperature, is going to be very difficult.  I wouldn't trust any of the results you get from it.

Victor Santa Cruz
Posted: Wednesday, July 7, 2010 4:46 PM
Joined: 11/11/2009
Posts: 38



Typically, I don't come back to post on this forum but this WEF Forum may not provide you with the answers that you need.  Is this project for the purposes of adding the grease to an anaerobic digester or literally burning the fat?  If it is for adding the grease to produce methane gas the use of COD as a means of monitoring energy is somewhat idiotic.  There is no oxygen in an anaerobic digester so the test is irrelevant.  What is is commonly used in this case is VS or VSS as a % of TS/TSS.  This is the organic portion that will be biodegraded and turned into methane gas which can then be used as a fuel source.


If the grease is going to be burned directly, then BTU would be the choice.


I always scratch my head as to how many times the wheel is reinvented.  This has been done numerous times by different people/agencies....


There are so many papers on anaerobic codigestion.  Let me know.

Posted: Wednesday, July 7, 2010 4:52 PM
Joined: 7/7/2010
Posts: 4

That was my thought exactly! I'm not working with scientific people, I'm beginning to believe. It's nice to know that someone else agrees with me though that this test does not make sense for this matrix. I'll continue trying to convince them to go with BTU testing.....That's the best I can do at this point!

Posted: Wednesday, July 7, 2010 5:01 PM
Joined: 7/7/2010
Posts: 4

I was just sent a copy of a paper produced by the Water Enviornement Research Foundation (WERF) that is called "Co-Digestion of Organic Waste Products with Wastewater Solids". I haven't had time to read this 80 page document (but I'll get to in the near future!), but apparently they did the study of adding grease to digesters in order to increase the methane production to produce gas as an energy source for the treatment plant. I think that this is where my organization is trying to head towards also (although it's difficult to get confirmation). The study does use COD as the preferred test method, which is still confusing to me, but at least now I have a direct source to contact and find out exactly how they did their testing so I can at least do the prep in the same fashion, especially if they are going to compare our data with this study. It'll be as close to an "Apples-to-Apples" match up as I can get given that they should've used a different test for their study, in my opinion.


Thanks to everyone though for your comments and suggestions! I really appreciate it! happy

James Royer
Posted: Friday, July 9, 2010 8:25 AM
Joined: 9/21/2009
Posts: 98

Working in the lab sometimes you have to perform analysis that does not always make sence but we must do it. The results must be so that other labs can reproduce the same results so that data can be shared and compared.


If I were going to do COD's on grease trap waste I would want a large sample in a wide mouth glass bottle to ensure as represenative sample as possible. I would warm the sample near 70 C to help with mixing to get a representative aliquot out of the container.


I would weigh the sample into a COD flask and perform the macro COD reflux digestion method to ensure as good results as possible. The Standard procedure would have you titrate the end digestate and calculate the COD back the the original sample. You could probably do a standard curve and read out the COD colormetrically if you wanted to avoid the titration.


I have not performed the reflux method for probably 30 years but it still works if you have the equipment. I would also do a %TS and %VS for my own comparison.

Keith Chapman
Posted: Friday, July 9, 2010 3:23 PM
Joined: 10/2/2009
Posts: 35

WW plant labs are going to be receiving more samples of this sort as biodiesel manufacturers look for places to dispose of their waste "sludge" and as WW plants look for ways to make more methane for cogeneration.  We tried to run COD's on oily samples using a macro reflux method, but the "goo" didn't entirely dissolve.  The WEF article you mentioned doesn't provide much help in actually measuring the COD of this stuff, unfortunately.


I looked around for a method and only found a hint from a biodiesel research article listing COD's in the millions for these sort of oils.  Seems like you mix the oily sample with water  and test the aqueous fraction for COD and also measure how much oil actually dissolved.  With this "oil solubility factor" you can back-calculate the COD of the oily sample.  (Too bad there isn't more glycerine in the sample since glycerine is water soluble.)


I wonder if you can dilute a small amount of the oily sample with, say, methanol and then dissolve that in water?  Haven't tried that yet.


If you really want some fun, try a metals digestion on the oily sample!?!?!  Not a pretty result. 


Wish I could be more help. 

Jeff Naumann
Posted: Tuesday, July 13, 2010 2:53 PM
Joined: 3/11/2010
Posts: 7


You might want to read the link that Victor inserted, above.  The poster on the "other - more active forum" tried all kinds of things, including dilution with MeOH.  Unfortunately, the gunk from biodiesel wouldn't dissolve.  Finally, I suggested just what James Royer mentioned.  Unfortunately, the original poster hasn't returned with any results yet.

Keith Chapman
Posted: Friday, July 23, 2010 12:54 PM
Joined: 2/5/2010
Posts: 5

We tried running a BOD on the oily waste and after using 1 ml of a 0.001 dilution of the stuff we found the O2 entirely depleted in the BOD bottle which gives an estimated BOD of > 2,100,000 mg/l......(this assumes a DO drop of 7 mg/l, ergo (7)(300)/(0.001).  We also had to sacrifice two DO membrane caps after measuring initial and final DO because of the oily film on top of the dilution water.


Our attempts to run a reflux COD with a titration finish gave rather random results and we noticed that there was always some oily residue floating on the COD acid. 


We've considered using Simple Green as a solvent, but haven't tried it yet.


I'll have to check out the other forum. 



Posted: Monday, June 27, 2011 1:18 AM
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Posted: Monday, June 27, 2011 5:13 AM
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Posted: Monday, June 27, 2011 1:57 PM
I rceokn you are quite dead on with that.