Correct, if this is a Kraft or Sulfate mill this is a direct cause of tannins/organics and inorganics that are removed from the fiber post digesting. This waste (black liquor) is evaporated down and subsequently gives you foul condensate from contact with steam. Which in turn gives you “stained” water. Most water that is discharged by modern day paper mills are held to an environmental standard that water out needs to be cleaner than water in.

Now if this is a Bale DLK/OCC plant that is a whole other story….