Do you know what type of ...
Published by Robert Duff, UtraPure Water/ Wastewater Engineer at SUNY Polytechnic Institute - CNSE
Do you know what type of compound is causing the COD? That might change the answer,.
But, not knowing the exact chemistry, I would put an activated carbon bed followed by an ultraviolet TOC destruct unit and a micro-filtration tank ahead of your RO membranes.
I have multiple Ultrapure water trains set up that way at my facility and get very good quality water from the ROs.
Good Luck