An integrated process model for activated sludge processes consists of four mechanisms, namely hydraulics, mass transfer, reactions, and separations. The structural forms of the models are classified as follows:
• 'Mechanistic models', which are based upon the actual physics, chemistry and microbiology that govern the system.
• 'Empirical models', which are based upon convenient mathematical functions that reasonably represent available data from the system, also called black-box or input-output models.
• 'Gray-box models', which include some of the mechanisms of the system and also empirical functions where the mechanisms are too complex or unknown.
Most process models are gray-box models. Commonly used dynamic models for activated sludge processes are classified in Table 9. In spite of the complexity and long computing time, the distributed parameter approach to the dynamics and control of activated sludge processes is to be preferred due to its generality, flexibility, and accuracy.
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