On Reconciliation of Traditional Water Quality Models and Activated Sludge Models

Models of the oxidation of organic material developed for river water quality management and for biological wastewater treatment differ widely in state variables and process descriptions due to their development history, environmental conditions and the objectives of the two approaches. The IAWPRC/IAWQ Activated Sludge Model No. 1 (ASM-I) resulted from a coordinated effort of a dedicated specialist group at the mid 1980s and thus free of the inconsistencies inherent in ambient water quality models such as QUAL2E developed in the course of the past three decades. The reconciliation of the ASM-1 and QUAL2E attempted in the present work may help in developing integrated pollution abatement strategies considering treatment and riverine processes in a unified way. It is shown that, after some modifications, a model similar to ASM-1 can be successfully applied to riverine conditions. It is also demonstrated that simple first-order kinetics models (such as the Streeter-Phelps and extended Streeter-Phelps ones) can be derived from the ASM-1 and QUAL2E as their asymptotic forms under the assumption that "fast" variables attain their long-term equilibrium levels. Models of ASM-1 type should, therefore, be applied when there are abrupt temporal or spatial changes in the system, otherwise simpler models adequately reflect the behavior of the oxidation system. Finally, an international effort to develop a standardized and improved river water quality model following the procedure of ASM-I looks more than desirable.