Nutrients within integrated bleached kraft mills: Sources and behaviour inaerated stabilisation basins

As constraints on the discharge of nitrogen and phosphorus from the pulp an d paper industry become tighter, a thorough knowledge of nutrient sources, and their behaviour through treatment systems, is required. Once nutrient b ehaviour has been characterised, strategies to minimise discharge can be fo rmulated. A survey of nutrient sources around a modem ECF integrated bleach ed kraft mill showed that point sources of nitrogen and phosphorus could be identified. Approximately 40% of the nitrogen was contained in the foul co ndensate stream and 40% of the phosphorus contained in the acid stage of bl eaching. Three Aerated Stabilisation Basin treatment systems were studied, two of which were nutrient limited in terms of theoretical nutrient require ments. All three systems achieved approximately 90% ROD removal without nut rient supplementation. For the wastewater with a high BOD:N ratio (100:0.8) , nitrogen fixation was shown to be important. Benthic recycling was a more important mechanism when the BOD:N ratio was lower (100:2.1). For a severe ly phosphorus limited wastewater (BOD:P 100:0.04), treated in a system with a long retention time (85 d), benthic recycling was the predominant mechan ism for nutrient supply. The relative roles of nitrogen fixation and benthi c feed back to the nitrogen cycle in aerated stabilisation basins are discu ssed. (C) 1999 Published by Elsevier Science Ltd on behalf of the IAWQ. All rights reserved.