Dear Dr Gupta

This is a tough question to answer until you don't know the nature of wastewater. Though there are some thumb rules which apply for municipal wastewater (0.4-0.6 of COD). But in case of industrial wastewater, it is highly specific. I had experienced the factor full range (Zero- One of COD) for industrial effluents.

 Regards

Pravin

As says Dr. Gupta, this is a difficult question. I recommend testing dilutions until a COD of 500 mg / L and this dilution to the BOD test.

Thanks Dr. Pravin and Jhonny. However, the question is still unanswered. It is general believe that the BOD is around 40-60% of COD but in most of the cases it is not. That is why I asked this question. 

Yup Dr Gupta,

BOD of around 40-60% of COD is  a TRUE CASE for Municipal Sewage.

Dear Sanjay

According to the reference of Standard methods for water & wastewater examination; it says:

"f. Dilution technique: Make several dilutions of sample that will result in a residual DO of at least 1 mg/L and a DO uptake of at least 2 mg/L after a 5-d incubation. Five dilutions are recommended unless experience with a particular sample shows that use of a smaller number of dilutions produces at least two bottles giving acceptable minimum DO depletion and residual limits. A more rapid analysis, such as COD, may be correlated approximately with BOD and serve as a guide in selecting dilutions."

I hope it is helpful...

regards

Thanks Dr. Ahmed

As discussed earlier with Dr. Praveen, We have a general understanding about correlation between BOD and COD however this doesn't work for all type of the samples. Again the question is how to to get best approximation about the specific dilution which give a residual DO of at least 1 mg/L and a DO uptake of at least 2 mg/L after a 5-d incubation specially for a sample having very high COD values.  

depends on the wastewater, for example municipal sewage COD is composed of about 50% BOD, but for some industrial wastewaters such as metal working fluids the ratio is BOD to COD is much lower (about 20%). hope that helps

Dear Sanjay 

You must try several dilution factors till you get the proper dilution factor that achieves the two criteria.

Regards

Dear Sanjay 

You must try several dilution factors till you get the proper dilution factor that achieves the two criteria.

Regards

For agroindustrial efluentes for instante You mau expect BOD5 variations betwen 10-80 % of COD values. Só you should try several dilution rates.

Antonio Pirra gives you suitable advice. Try a range of dilutions, assuming that BOD5 will be 10% of COD at the low end, and 80% of COD at the high end.

I agree with Dr. Ahmed Rahomi.

Dilution factor = BOD (mg/l)/ (Initial DO - Final DO)