It depends on the own characteristics of the specific wastewater.
COD values are generally higher than BOD, however, there is no real ratio. It depends on the treatment process i.e if the same process is used all the time under the same conditions. Other factors can affect both factors such as composition of the waste being treated.
Dear Dr. Udaya Bhanu
Do you have some references "BOD/COD relationship for assessing toxicity" can you please share with me
The ratio BOD: COD is a characteristic of a particular wastewater and is a simple and direct measure related to its biodegradability. That is, there is no absolute value for any residual water, but will directly depend on its composition. It is usual to consider a wastewater that has a BOD ratio: COD with a value greater than 0.4 indicates that you can apply a biological treatment if necessary. However, as this relationship will depend on the data quality of BOD and COD, analytical quality assurance should be applied, so that the data are sufficiently reliable. In the case of the BOD, as this method is made applying an inoculum of microorganisms, among other things, you must ensure that toxic compounds are not present, in order to guarantee the correct grow of such microorganisms.
Thanks Dr. Maria Del Carmen Espinosa Lloréns for sharing your views with us
Is there any possibility for getting higher COD value than BOD value..?? whether it depends on the source of samples??
Thanks Munavar Iqbal and friends here...
I have a lot of unpublished work carried out in this area. However this para is intended for a better understanding of the concept.
BOD is based on activity of microbes. COD is based on chemical reaction (oxidation). If microbial seed we use for BOD is not active then it results in Higher BOD. It is as good as no microbe is added. Even if the same microbial seed is more active or more in volume, then also it results in lower BOD. It takes a lot of time and expertize to carry out BOD for 100% accuracy. Hence I believe in standardized BOD namely using a seed of uniform composition tested with glucose as standard.
To avoid all confusions Take it granted that COD of filtered samples free from inorganic compounds (mostly anions) alone is reliable.
Regarding using BOD5:CODCr ratio as an indicator of Toxicity, We need a lot of work to be carried out with different standard solutions, standardized microbes, toxicity tests etc.
Unfortunately I never had an oppurtunity to work on this demanding topic after my PhD days.
Prince of Songkla University
Polytechnical University of Valencia
Indian Institute of Technology Guwahati
Leibniz Universität Hannover
Sultan Qaboos University
University of Akron
Universiti Tun Hussein Onn Malaysia
Kharkiv National Academy of Municipal Economy
Eastern University Sri Lanka