Modeling and numerical simulation of secondary settlers: a method of lines strategy

Research advances and challenges in one-dimensional modeling of secondary settling tanks--a critical review

Sedimentation is one of the most important processes that determine the performance of the activated sludge process (ASP), and secondary settling tanks (SSTs) have been frequently investigated with the mathematical models for design and operation optimization. Nevertheless their performance is often far from satisfactory. The starting point of this paper is a review of the development of settling theory, focusing on batch settling and the development of flux theory, since they played an important role in the early stage of SST investigation. The second part is an explicit review of the established 1-D SST models, including the relevant physical law, various settling behaviors (hindered, transient, and compression settling), the constitutive functions, and their advantages and disadvantages. The third part is a discussion of numerical techniques required to solve the governing equation, which is usually a partial differential equation. Finally, the most important modeling challenges, such as settleability description, settling behavior understanding, are presented.

Dynamic one-dimensional modeling of secondary settling tanks and design impacts of sizing decisions

As one of the most significant components in the activated sludge process (ASP), secondary settling tanks (SSTs) can be investigated with mathematical models to optimize design and operation. This paper takes a new look at the one-dimensional (1-D) SST model by analyzing and considering the impacts of numerical problems, especially the process robustness. An improved SST model with Yee-Roe-Davis technique as the PDE solver is proposed and compared with the widely used Takács model to show its improvement in numerical solution quality. The improved and Takács models are coupled with a bioreactor model to reevaluate ASP design basis and several popular control strategies for economic plausibility, contaminant removal efficiency and system robustness. The time-to-failure due to rising sludge blanket during overloading, as a key robustness indicator, is analyzed to demonstrate the differences caused by numerical issues in SST models. The calculated results indicate that the Takács model significantly underestimates time to failure, thus leading to a conservative design.

ICA and me--a subjective review

ICA - instrumentation, control and automation - is a hidden technology. It is ubiquitous in all industrial processes, including water and wastewater treatment systems. Still, as long as everything works fine, it is not noted but when things go wrong it will be observed. ICA has now about forty years of history in water and wastewater systems and is well recognized. One early attitude was that ICA will be a necessary burden to be added to a plant in order to correct for a poor design. However, the key reason for ICA is the fact that all processes are subject to disturbances, externally via the wastewater influent, from the customers in a water supply system, or from operations in one unit process that will propagate as a disturbance to another unit within a plant. This paper is an attempt to describe the development of ICA in water and wastewater systems. Most of it is based on personal experiences with all their limitations. No single paper can fairly describe the development that is documented in thousands of research papers, practiced by so many operators and process engineers and implemented in so many treatment systems. Still, the hope is that the paper can give a flavour of the most important ingredients of this fascinating development.