Analysis and design of suitable model structures for activated sludge tanks with circulating flow

Activated sludge models are widely used for planning and optimisation of wa stewater treatment plants and on line applications are under development to support the operation of complex treatment plants. A proper model is cruci al for all of these applications. The task of parameter calibration is focu sed in several papers and applications. An essential precondition for this task is an appropriately defined model structure, which is often given much less attention. Different model structures far a large scale treatment pla nt with circulation flow are discussed in this paper. A more systematic met hod to derive a suitable model structure is applied to this case. Results o f a numerical hydraulic model are used for this purpose. The importance of these efforts are proven by a high sensitivity of the simulation results wi th respect to the selection of the model structure and the hydraulic condit ions. Finally it is shown, that model calibration was possible only by adju sting to the hydraulic behaviour and without any changes of biological para meters. (C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserve d.