Because BOD requires a minimum of 5 days incubation before analysis and suffers from poor reproducibility; the other oxidation tests were developed to reduce the time required for analysis in hopes of using these tests for control of wastewater treatment processes. None of these testa measure a chemical directly; they only attempt to measure the amount of oxygen required to oxidize the various chemicals in the wastewater.

Historically, BOD was the first test developed and is used to this day to evaluate discharge quality in spite of the short comings of the test. The ratios are used to predict BOD when the quicker tests are used. Unfortunately, none of these tests measure specific chemicals and so the correlation between the tests varies considerably with different wastes.

Incidently, while the 5 day incubation period was selected because this was the time required for water in the Thames River to travel from London to the sea. Biological oxidation of wastewater will continue after this period.

I hope this simplified explanation will inspire you to look for additional references,   grrun      

You ask, "What does the correlation of BOD, COD & TOC imply and affect?"

For waste water treatment, the ration of 1:1 is the ideal as mentioned.  That means that all the organic material present in the water will be broken down in an aerobic biological treatment system in 5 days or less.  For any other ratio, some of the organic pollutant in the water will be discharged somewhere.  This may or may not have an impact, depending on where it's going.

A great example is a tomato processing plant.  There, a lot of the organic material discharged is slowly degradable, like skin particles and the walls of the cells within the tomato.  Typically, this water winds up in an aerated pond and then into a settling pond.  The cellulous like material settles to the bottom and is slowly degraded under anaerobic conditions.  Under the right conditions, this seathing mess will decide to float to the surface and make a wonderful odor

Another example is the problem of discharging large quantities of slowly degradable organics into a river.  Somewhere downstream, it will degrade.  This can also cause problems.  Because of this, the TOC and COD numbers are being looked at more carefully and requirements are being based more on those measurements of degradability.

I don't think we've heard the last of this issue.

There are no firm numbers for these ratios. Most plants run these and come up with a ratio strictly as a process control. For example, at one of my plants, the BOD/COD ratio of 70%was used to estimated loadings from an anaerobic lagoon. However, BOD is a biological test and COD is a chemical degradation test. There will not be a linear relationship.

Hi,

I need to add a remark on Sobisch' previous post on this issue. For fully biodegradable matter (e.g. acetate) the BOD/COD ratio is not  zero, but around 0.65.
In the BOD-test, some of the substrate is converted to biomass, which is only partially respired (endogenous respiration), hence contributes to the oxygen depletion and thus to the BOD value. The biomass formed that is not respired does not contribute to the BOD value. The respiration (hence oxygen depletion) in the beginning of the test (so called "exogenous respiration", untill the substrate is depleted, of course also contributes to the BOD value.

One of the problems with BOD-determination (hence its numerical value) is that inoculum "A" reacts different from inoculum "B", because they can have different biokinetic values (Y, µmax, Ks,kd, KDO) for the different polluting compounds....

Besides that, the best inoculum is the bacterial consortium of the WWTP under consideration; they should be best adapted to the influent.

This, plus other factors contribute to the experimental error on BOD determination.

kind regards,


DS

Hi

If compunds are identified

C6H12O6 + 6O2 --> 6CO2 + 6 H2O

COD/TOC = 6M O2 / 6M C = 2.66 mg COD/mg organic carbon

Hi!

This link summerises well why i dislike BOD  
If I use it it is to look at the bod in the outlet to see if the plant is working well

http://www.activatedsludge.info/bod.asp

In stable conditions a chart with COD vs BOD of values form NORMAL operating conditions in your process should give useful information.

Hi Harri,

I have not read the link, but I am also a big "disliker" of BOD: too much time, effort and money for little information (and possible a lot of error, especially in industrial WW).

Martina, COD allways > BOD (except when BOD and COD are both =0, so non degradable matter, but that is a bit irrellevant)

I was asking about the possibility of BOD > COD and when/how this may happen since we have encountered a sample recently which gave what appears to be consistent BOD results close to 4000 mg/L, yet a COD of only 1500 mg/L. The COD was repeated 2 x and confirmed.

During a repeat BOD test a week later (now outside of holding time)  the sample behaved very differently using the same dilutions, & gave 3 widely differing but lower results. I'm inclined to think the initial results were more correct, but can not explain the BOD/COD discrepancy.  

Due to a very strong smell (sulphury?) for BOD it was given an initial dilution of x50, then 3 secondary dilutions of 10, 20 and 50 mL per 400 mL.

Overall initial plus secondary dilutions as follows:

0.05% dilution depleted form 8.32 to 5.82, calc'ed as 3884 mg/L

0.10% dilution depleted from 8.30 to 3.82, calc'ed as 4120   "

0.25% dilution depleted from 8.25 to 0.26, cal'c as 3203    "

The 3rd dilution depleted too much, so would be disregarded, but still gave an indication of the BOD being high.

Little else was analysed, so I don't have other data to draw on.

Using simple test strips I can see the pH is neutral, it contains no nitrates, but it does give a positive reaction for ammonia. ( 6ppm or higher)

I'm struggling to make sense of all this.

Can the presence of ammonia (or anything else?) elevate the BOD to this extent? What concentration will be needed to do so?

Any other thoughts/ideas on this issue will be very welcome.

Your BOD should never exceed your COD, at least not to any significant digits.  Values that you are reporting (4000 and 1500) should not happen.  My first suspicion is that you have some sort of chloride (or other halogen) interference in your sample.  Most labs, ours included, don't do any sort of chloride check prior to running a COD.  Standard Methods says that anything over 2000 mg Cl- /L shouldn't be run using traditional COD methods.  Another possibility is that pyridine 'and other related compounds' resist oxidation.  I don't know about the BOD of these types of compounds, but if they resist the COD reaction they probably will not oxidize under BOD conditions either.

Hi all

BOD can not  exceed than Cod but sometimes give these results 10 years befor I found same pb at that time we did our best what the result   ,,,
we found that the chemist do not take the sample as good method
and also the COD reagent was expier what I mean by this you have to check all of these
1- sample collection and presentative method
2- the correction of sample befor analysis
3- check all reagent used
4- make COD first then you can expect the suitable dilution factor for BOD
5 - do your best
waiting for your reply

best regards
ashraf  kotb

Hii ALL...

im handling wwtp from the pulp and paper .. im use ferric cloride as a cogulant aid. but cod and bod still high.. it will be nice if you all can give me the chemical treatment method to reduced the cod and bod .

Thanks consultant for your suggestion... Actually right now.. i try to combine primary method and advanced waste water treatment method.. with this combination method..i have reduce COD from 3300 to 300mg/l and BOD from 1000mg/ to 75 mg/l... but my target is 50mg/l for COD and 20 mg/l for BOD. Actually What the other cause cod and BOD high from wastewater pulp and paper.

hi asraf..

i used sand filteration system by mixing ac with sand