Because normally organic matter from residential and industrial effluents accounts for the largest share of pollution in rivers. Therefore, it is natural to use BOD as one of the main parameters for monitoring water quality.

Well,

In this case, it is necessary to measure COD as well as BOD.

Regards

If required to measure the quality of water near industrial estuaries, definitely it is done using COD, because using BOD near industrial estuaries will kill microorganisms (that considered essential for this kind of testing) due to the presence of toxic industrial substances

Most river water quality standards for different countries in the world set maximum permissible limits to both BOD and COD.

Dear Sandra,

Measuring COD takes a few hours, measuring BOD takes (typically) 5 days because bacterial degradation of organic matter (OM) is slower than is oxidation with harsh chemical oxidisers. So, COD is often measured where BOD is desired. As microbes typically cannot degrade all the OM in a sample, COD tends to be higher than BOD; maybe 1.5 times higher. If COD > 2 BOD, the sample might contains stuff that is not biodegradable, thereby signalling a potential problem of pollution by hazardous chemicals or, as Zaid states, pollution that also disinfects, killing bacteria.

BOD is typically done aerobically, because it is usually used for lakes, river, streams - surface waters in contact with the atmosphere and so oxygen containing. It typically measures degradation by oxic bacteria and archaea. It can be done anaerobically, but degradation under anaerobic conditions is very slow, so one would typically measure COD instead.

Thank you to all for your quick and in depth response to my query. I am grateful for your patience and time. thanks again. Sandra

Dear Sandra,

COD is equal to or higher than BOD. Never lower. Just what the ratio would be would depend on site-specific issues. Think of it in everyday terms. When measuring COD, powerful chemical oxidisers are used. These oxidisers can oxidise stuff bugs may have a hard time metabolizing (oxidising) when measuring BOD. Also, BOD is not a measure of the amount of organic matter bugs can oxidise, but the amount they can oxidise in a set time, typically 3 days or 5 days. Leave them longer and they will oxidise more, so BOD would get nearer COD as time passes (probably exponentially). But there is always a residuum that bugs don't like, which is why we have 'heavy oil' and 'tar sands' - crude oil that bugs have partially degraded though oxidation, but in which the easy-to-metabolise part (the caviar) has been oxidised (eaten) leaving the harder-to-metabolize stuff behind (the mouldy bread).

Thank you John M McArthur for the apt analogy and lucid explanation.

BOD gives the ability of flora & fauna to sustain in the waters.

There are simila discusions in RG.

https://www.researchgate.net/post/Does_anyone_know_about_the_ratio_between_BOD_and_COD_in_wastewater

https://www.researchgate.net/post/What_is_the_relationship_between_the_COD_and_BOD_values_in_Waste_water

I think the two parameters are very important to calculate the water quality index for the river, but its could be depending on the available data or cost of the test and the available time.

Pollution is the access to anything, within the limit, it is required like heavy metals and other nutrients. Industrial effluent holds a large number of toxic chemicals, which give boost or help in the growth of certain micro-life forms, which are harmful for the aquatic body as well as human health and hygiene.

To assess the level of contamination and understand the interactions of Biotic components with chemicals, Biochemical Oxygen Demand(BOD) is calculated and interactions of abiotic components and other chemicals, Chemical Oxygen Demand (COD). More the interactions more the oxygen requirement.

Dear Prof. John M McArthur

Just a question out of curiosity -- Not so sure about the tryptophan like fluorophore sensors, but can't a sensor based on microbial enzymes which may represent BOD (maybe: Catalase, Oxidase or CytC Oxidase), and pace up the analysis? The COD = X*BOD would be easily understood.

Dear Devanita,

Not sure what the question is.

I know nothing about microbilly-based sensors.

Ah sorry, what I mean is- Can measuring BOD and COD at the same time possibly alter the COD:BOD ratio?

I am trying to understand the lags in the traditional 3 or 5 days incubation protocol, which could be led by altered microbial metabolic oxidation rates due to laboratory conditions.

Hi Sandra Dsa

For the evaluation of the water quality of the river, it is necessary to analyze the two parameters, the BOD and the COD of the samples taken and calculate the BOD / BDCO ratio to assess the biodegradability of these waters in order to propose an adequate treatment for the depollution of the waters of this river.

Thank you Sir @ F. Bouteldjaoui

Dear Devanita,

The question remains unclear.

What do you mean by 'alter'? Alter the BOD/COD ratio from what?

BOD should be measured ASAP and no more than a day after sample collection, preferably less. The shorter the time from collection to lab and analysis, the more meaningful will be the result.

For COD, preservation with Hg or H2SO4 can extend the period of storage to maybe a week (site dependent) but the same principle applies. Do the analysis as quickly as possible.

Both BOD and COD change from the moment of collection, the rates of change being sample (and so site) specific.

John McArthur

Wherever the industrial effluents are disposing to the river the CPCB has made mandatory to test BOD and COD parameters. Also these to important for the deciding the river is polluted with the various chemical contaminants and making issue of biomagnification.