Research Advances and Challenges in One-Dimensional Mathematical Modeling of Secondary Settling Tanks—A Critical Review

As one of the most significant components in activated sludge process (ASP), secondary settling tanks (SSTs) are often investigated with the mathematical models for design and operation optimization. The starting point of this paper is to present a review of the development of the settling theory and one-dimensional SST modeling techniques. The first part of this paper focuses on the batch settling theory and the development of flux theory, since these two theories play 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 those based on physical principles, various impact factors, those that used constitutive relations, with respect to both advantages and disadvantages. Finally, some modeling challenges, such as the description of settling velocity and compression behavior are discussed.