it is not clear your explanations

the cod can not be negative even in very busy waters

can be an error of calculation, of measurement ..

by describing how you have done and what technique you have used, I can help you

also generally in the case of very heavy or very polluted water, dilution is carried out

If you use a COD kit method such as the one from Hach that quantify the residual chromic acid by light absorbance in the heat treated sample it is possible that to read an apparent negative COD values in some cases due to the persistent strong color of the sample.

The solution would be to use the classic COD analysis using Fe(II)-titration to quantify residual chromic acid. E.g. the laboratory method following the standard book for water analysis.

make sure the tube of Blank free from COD residue and Calculate TOC or TSS or BOD or Dissolved Oxygen (DO) to estimate theoretical COD value

How can?

Chek your equipment , your reactive before you answer the question like this .

It should be better.