Cyclical Aeration System: A Synergistic Approach to Energy Conservation and Environmental Performance, 1995 Environmental Conference Proceedings

Om Bhatia

Rainy River Forest Products

Robert J. York

CH2M HILL

Robert M. Seamons

CH2M HILL

Rainy River Forest Products (Rainy River) operates a newsprint mill in Steilacoom, Washington. In August 1993, a recycle mill was started up to supplement the existing thermomechanical pulp mill. Modifications to the wastewater treatment system to handle the increased wasteload from the de-ink mill and the limits imposed by the revised NPDES permit included replacing the aeration system in the existing ASB and installing a new secondary clarifier after it. The new system optimized both aeration and mixing energy requirements by incorporating fine bubble membrane technology and automatically cycling air to individual headers in an on-off sequence. The upgraded wastewater treatment system consistently met revised NPDES limits and energy conservation goals. However, two issues remained: 1. Basin dissolved oxygen levels have routinely run below 0.5 mg/L. 2. The system would not meet proposed cluster rule requirements. There appeared to be restrained biological kinetics compared to ASB model predictions. Pilot-scale testing found that biological kinetics would be improved if dissolved oxygen and supplemental nutrients were added. Also, an apparent negative synergy between defoamer and an unknown constituent in the wastewater depressed oxygen trans-fer. The apparent alpha coefficient was only 0.2 to 0.3, well below traditionally used values. Further work is recommended to identify the cause of depressed oxygen transfer.