Development of an extended ASM framework integrating rotifer–ciliate ecological mechanisms for predicting sludge reduction in septic tank systems

Abstract

Continuous accumulation of sludge in septic tank systems causes reduced treatment efficiency and increased maintenance costs. However, existing Activated Sludge Models (ASM) are limited to bacterial trophic levels, failing to quantitatively explain sludge reduction mechanisms driven by higher-level predators. This study proposes a novel extended model integrating the ecological dynamics of rotifers and ciliates, based on the Storage-Growth framework of ASM3. The model functionally divides the bacterial community into Bacteria Biomass (XB) and Filamentous Biomass (XFB), and incorporates ciliates (XP) and rotifers (XR) that selectively graze on them, comprising a total of 16 state variables and 13 processes. Specifically, the model mathematically structures predator grazing not merely as biomass conversion, but as a process mediated by internal storage products (XSTO) involving respiration and maintenance metabolism during famine conditions. Differential analysis of Total Suspended Solids (XSS) demonstrated that, in addition to conventional endogenous respiration, the additional carbon mineralization occuring at the predation stage is a key mechanism for sludge reduction. Furthermore, the model suggests the potential for controlling sludge bulking through rotifer predation on filamentous bacteria. By introducing ecological interactions into process modeling, this study provides an advanced quantitative framework capable of simultaneously predicting sludge reduction efficiency and operational stability in septic tanks.

Acknowledgements

Following are results of a study on the "Convergence and Open Sharing System" Project, supported by the Ministry of Education and National Research Foundation of Korea