about synthetic wastewater?

You make up a standard solution of the material based on the carbon content and reaction 10 give around 10000 COD.

sodium acetate + O2 = CO2 + H2O (needs to be balanced)

Then do dilutions off the standard until a COD test gives you the 100. Weighing the material will not work, that is not precise or accurate.

Such a wastewater will be lousy in experiments. Better recipes are found in the OECD 302 and 304 Methods. I believe the best is in the footnotes of 302C.