Hi

Which COD method do you use (colorimetric, titration) ?

I don't know if my answer will be helpfull but because i don't understand what you mean by "very high error" but:

Normaly, to analyse the COD, it is made with K2Cr2O7, a very high oxidising chemical product. So, if your sample is only made of organic compounds and well homogeneisated, it should work. But, if your sample contains plastic and other compounds, they will be also oxydised. A way to avoid this is to use KMnO4 to replace bichromate (i don't remember the name of the method). It is less oxydising. I used it to analyse solid  waste from rubbish going to methanisation.

On an other way you should be very carefull with the boiling time, it should be very precise. And pay attention to any other compounds who can made wrong your measure (chloride,...).

Hi Salmon,

Yes, we use the colorimetric method. And our problem is the homogeneity of our samples. So we are thinking to scale up the method. But that will require increasing the quantities of the toxic chemicals... 

Yep we will check the method you are proposing. Cheers

Hi Juan

The homogeneity is always a great problem.

Take first different samples in different places. Mix all these samples to have a single one. Dry it or froze it with liquid nitrogen, than crush it.  Do a few replicate with this single sample and calculate the average value. You will use less chemicals than for a big sample.

Here is an (old) article about different kind measurement of COD in soil.

http://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_5/b_fdi_04-05/05490.pdf

They don't talk about KMnO4 neither crushing but it can be usefull.

Anyway, KMnO4 is only good if you have non organic particles (plastic,...) which can be oxydised with bichromate. It seems it is not your problem.

COD, as every body know, is an expression for the extent of the presence of oxidizable organic components in water. You are talking about COD for solid food material. Your sample is all, oxidizable organic material. The result is not high at all. Simply you are testing a 100% oxidizable organic material! 

You need to dilute enough, even with difficult samples homogeneity, If you get properly dilute the analysis is reproducible and correct. I worked with distillation tower remains (vinasse) which had a large amount of solids, with a dilution of 100 - 200x, could adequately perform the analysis, because with the dilution tendency is to help homogenize. Place a magnetic stirrer in pipetting the sample.

I usually dry the samples and grind them as fine as possible, then do some mixing with the powder. You have to take a relatively large sample size from the powder and dilute it to fit for the range of the measurement. Yes, several replicates will be needed. 

Thank guys for the answers.

Cheers

COD is amount of degradable materials present per unit volume of liquid. It will be expressed in mg/litre or ppm. If solid food material is present in an effluent liquid, homogenize it using a homogenizer or a mixer grinder and then try to find the COD using open cup reflux method as described in the book standard methods for examination of water and wastewater.

My friend, COD is a measure of water organic load that need oxidation and also, a round figure when compared to BOD to have a picture if it will be easily or hard to be degraded in waste water treatment plant. In case of solid waste the picture is totally different.  Solid organic content is measured by ignition loss (subtracting moisture content ), this is a measure of carbonaceous content.  

We also run CODs on food waste samples in our lab.  We typically put the samples in a blender for a few minutes to homogenize and break up the larger pieces - if needed we'll dilute the samples before blending.  We then do a series of dilutions, and we'll often add another hour to the heating step to be sure the reactions have gone to completion.  Performing replicates is also a good idea.  Good luck.

Hi.

Performing total COD on heterogenous samples will always be tricky and subject to high variation in the results, even if you are very careful in your preparation. You can take a look at this paper, where 20 labs around the world compared their results...

Raposo F., V. Fernandez-Cegri, M.A. De la Rubia, R. Borja, J. Beltran, C. Cavinato, M. Clinckspoor, G. Demirer, E. Diamadopoulos, J.-C. Frigon, J. Koubova, M. Launay, R. Mendez, G. Menin, J. Noguerol, H. Uellehdahl and S. West. 2010. Quality improvement in determination of chemical oxygen demand in samples considered difficult to analyze, through participation in proficiency-testing schemes. Trends Anal. Chem., 29(9): 1082-1091.

Even if you are measuring COD on a fully organic sample, since it is measuring oxidation, you can have very different results. For example, 1 g of sucrose would give you 1.07 gCOD, but 1g butyrate 1.82 gCOD. So it is still worth doing. Also, I would not recommend drying the sample to make a powder. Yes your sample will be more homogenous but you could end up loosing quite a lot of volatiles, depending on your substrate.

Good luck.

I agree with Van Loosdrecht: n general the measurement of VSS is more appropriate and the relationship between COD and VSS is quite reliable

I would suggest TOC (Total Organic Carbon). This analysis gives you all of the organic content and is the corresponding analysis to COD which gives all the organic content in aquatic samples (biodegradable and non-biodegradable).

Although all the alternative measurement methods are plausible, sometimes COD is more convenient in situations such as modeling, where previous ADM1 models generally use COD for energy balance and many constants are expressed by COD. Any suggestions about this situation? Thanks!

TOC is appropriate for measuring organic carbon but this does not necessarily match with the oxygen demand

Dear Mark Loosdrecht. Yes we have also thought about measuring the combustion heat and correlate it to the O2 consumption. We have found just one article which try to correlate both parameters in literature. May I ask you if you know more literature in this regards. Cheers,

You may try this reference. Longbak we had rose such question.

https://www.researchgate.net/profile/Asheesh_Yadav/publication/7580736_A_modified_method_for_estimation_of_chemical_oxygen_demand_for_samples_having_high_suspended_solids/links/544158010cf2a6a049a57577.pdf?ev=pub_int_doc_dl&origin=publication_detail&inViewer=true

Article A modified method for estimation of chemical oxygen demand f...

You can try a kind of COD fractionation in your samples:

- By leaching and measurement of dissolved COD

- By filtering of sample solution and measurement of SS and VSS and conversion of VSS in COD (particulate COD) with 1,42 gCOD/gVSS (as Mr. Loosdrecht wrote)

One thing to consider with some of the methods that require drying the samples is that loss of volatiles can occur during the drying step, which will artificially lower your volatile content.  So, if you have a high concentration of soluble COD, say from acetic acid, a large percentage of this would be lost during the drying step at 105 C, causing errors in your analysis of TS and VS and other methods such as calorimetry.  Here's a link to a reference on this:

http://www.biotechnologyforbiofuels.com/content/4/1/44

We found some of our food waste has fermented considerably by the time we get it, resulting in relatively high concentrations of volatile compounds - which then creates an artifact in our TS/VS analysis.  It can also result in artificially high methane yields when expressed as volume of methane per mass of VS.

Good luck.

http://www.biotechnologyforbiofuels.com/content/4/1/44