This paper provides the structural framework for the proposed modified version of Activated Sludge Model No. 3 (ASM3), where direct heterotrophic growth on readily biodegradable substrate is included as a new process and provision is made so that growth on internal storage compounds is started sequentially, after the depletion of the external primary substrate pool. The results have provided strong indication that there was a need for considering direct growth on primary substrate as a significant biological mechanism.

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