Dear Carlos, The increase ...

Dear Carlos,

The increase in COD and color after biological treatment is explained by the increase of recalcitrant (hard to biodegrade) organics including lignins (brownish color) resulting from the process changes (acid effluent from bleaching). Depending on the operating conditions in the activated sludge stage, the color may even increase due to intermediates.

That's why we upgrade the activated sludge process as to capture this fraction into regenerated biosludge for subsequent biodegradation by microbial enzymes within the biosludge. In this way we achieved over 80% color removal at 50% to 70% lower COD in a large pulp and paper mill back in 2010.

If interested, I would be happy to send you more information by email.

Regards, Bruno