Characterization of Treated Wastewater In Paper Mills
Biological processes can eliminate nearly 100 % of biologically degradable organic substances in the wastewater of paper mills. Considering the amount of organic substances determined as COD, the level of elimination is 80 to 95 %. A distinction must be made between a genuine biological degradation and a physico-chemical elimination, e. g. adsorption to anaerobic biosludge or activated sludge. COD and BOD concentrations and loads in biologically treated wastewater of paper mills producing different paper grades are given in .
Treated wastewater of paper mills is not normally toxic to water-borne organ¬isms, as evidenced by the results of fish and other biological tests. Due to the low nitrogen and phosphorus concentration in the treated effluents only a small risk of extensive growth of water plants in surface waters exists. The wastewater has very low heavy metal content.
The additives used in paper production generally contribute only slightly to the load of the treated wastewater. For example, some starches, soaps or surfactants are easily biologically degradable. Others, and especially higher molecular weight additives such as retention agents, wet strength agents or coating binders, adsorb strongly to the biosludge. Detailed research on the subject is described in  and .