Varying feed (waste) nature and COD of final clear water

Q. The waste water from a paint factory- The waste splitting and settling is done with coagulants, flocculants, bit of chlorine and pH adjuster. The nature of the feed when treated varis sometimes and mostly extra additions of chemicals required. However, I manage to get clear water at the end. The final COD is the problem. I have tried passing compressed air, but no significant reduction. Waste contains some organic content like phenols and styrenated dispersions (additives like surfactants and emulsion binders used for water based paints). The final COD I got so far is 2450 and the requirement is 150. How can I get to these levels of COD? COD of the feed waste is in the range of 15,000- 20,000.