Color Removal from Pulp & Paper Industry Effluents.

I am trying to remove the color of the pulp and paper industry effluent. 

Our WWTP consists of the primary and secondary treatment (activated sludge) systems. 

We introduced a new washing system for the bleaching stage (acid effluent). Since then the final effluent color has significantly turned darker (around 1000 mg Pt/L). Additionally, the final effluent chemical oxygen demand - COD has also increased. 

What is the explanation for this significant color and COD increase? How can I remove the color of the effluent?