Nonsteady-State Modeling of Multispecies Activated-Sludge Processes

A nonsteady-state, multispecies model tracked reactor volume and 12 species critical for describing changes in soluble chemical oxygen demand (COD), biomass types, and nitrogen. Key to the model is its systematic partitioning of COD. The COD in all electron donors is partitioned among biomass synthesis, respiration to an electron acceptor, and soluble products released. Biomass decay is divided into respiration for maintenance energy, formation of inert biomass, and release of soluble products. Soluble products can be used as electron donors by heterotrophs, which are capable of using oxygen, nitrite, or nitrate as electron acceptors. Two modeling examples illustrate key features of the model. These demonstrate the importance of soluble products in controlling the soluble COD and indicate that different biomass types require different times to reach steady state and that a sequencing batch reactor reaches a global steady state in which concentrations of soluble species change dramatically within a cycle but biomass concentrations change relatively little.