What is the target of your project? What is the project like?

sorry, but I have no idea what are COD and BOD

COD: chemical oxygen demand
BOD: biological oxygen demand

2 different way to measure the biological activity of effluents/water. Basically when they are low, most of the content of the effluent has been previously oxidized, and there is very little energy available anymore for bacteria to strive in that kind of effluents.

A good way to reduce them is to reduce organic molecules before releasing them. You can do that either biologically (bacteria will use up all organic material, then die and leave poorly usable waste) or chemically.
In any case if you want to study that you need to decide what kind of treatment you want to do (chemical/biological), what kind of equipment you can use (lagoons, fermenters,…) and what speed you can afford to treat your effluents. It is as much engineering than biology.

I guess you meant oxidize, didn’t you?

I did.
I just proved why you have to be careful with the answers you get from Internet. And hand my head in shame.