This paper aims to identify the optimal working effect of peripheral-inlet and outlet (PIO) circular secondary clarifiers (CSCs). For this purpose, the simplified multiphase mixture model was adopted for the 2D numerical simulation of hydraulic features of the solid-liquid two-phase flow in CSCs. Specifically, the closed-form time-averaged flow equations were established by the RNG k- ε turbulence model, the differential equations were discretized by the finite volume method, and the coupling velocity and pressure equations were solved by the pressure-implicit with splitting of operators (PISO) algorithm. Then, numerical simulations were performed to disclose how the retaining baffle-deflection baffle distance and retaining baffle depth influence the distribution of the velocity field and sludge volume concentration field in a PIO CSC. The simulation results show that the optimal performance of the CSC appeared at the baffle distance of 300mm and the retaining baffle depth of 600~1,000mm. All in all, a proper increase of distance and depth can enhance the sedimentation efficiency and outflow quality, but an excessive increase can only accomplish the very opposite. The research findings provide valuable references to the optimal design of actual secondary clarifiers.
circular secondary clarifier (CSC), peripheral inlet and outlet (PIO), numerical simulation, velocity field, sludge volume concentration field
 Roza T, Asterios P. (2013). CFD methodology for sedimentation tanks: The effect of secondary phase on fluid phase using DPM coupled calculations. Applied Mathematical Modelling 37(5): 3478-3494. https://doi.org/10.1016/j.apm.2012.08.011
 Roza T, Asterios P. (2014). The influence of lamellar settler in sedimentation tanks for potable water treatment--A computational fluid dynamic study. Powder Technology 268: 139-149. https://doi.org/10.1016/j.powtec.2014.08.030
 Ben L, Michael KS. (2014). Research advances and challenges in one-dimensional modeling of secondary settling Tanks-A critical review. Water Research 50: 160-170. https://doi.org/10.1016/j.watres.2013.11.037
 Elham R, Dorottya SW, Lars Y, Philip JB., Michael RR, Peter SM., Benedek GP. (2014). A new settling velocity model to describe secondary sedimentation. Water Research 66: 447-458. https://doi.org/10.1016/j.watres.2014.08.034
 Estelle G, Elham R, Murat K, Benedek GP. (2015). ICFD: Interpreted Computational Fluid Dynamics e Degeneration of CFD to one-dimensional advection-dispersion models using statistical experimental design - The secondary clarifier. Water Research 83: 396-411. https://doi.org/10.1016/j.watres.2015.06.012
 Nicolas D, Christian T, David D, Kris V. (2017). Batch settling curve registration via image data modeling. Water Research 124: 327-337. https://doi.org/10.1016/j.watres.2017.01.049
 Xu G, Yin F, Xu Y, Yu H. (2017). A force-based mechanistic model for describing activated sludge settling process. Water Research 127: 118-126. https://doi.org/10.1016/j.watres.2017.10.013
 Raimund B, Stefan D, Sebastian F, Ingmar N. (2012). On reliable and unreliable numerical methods for the simulation of secondary settling tanks in wastewater treatment. Computers & Chemical Engineering 41: 93-105. https://doi.org/10.1016/j.compchemeng.2012.02.016
 Elham R, Xavier FA, Gürkan S, Krist VG, Ulf J, Peter SM, Benedek GP. (2014). Influence of selecting secondary settling tank sub-models on the calibration of WWTP models-A global sensitivity analysis using BSM2. Chemical Engineering Journal 241: 28-34. https://doi.org/10.1016/j.cej.2013.12.015
 Patziger M. (2016). Computational fluid dynamics investigation of shallow circular secondary settling tanks: Inlet geometry and performance indicators. Chemical Engineering Research and Design 112: 122-131. https://doi.org/10.1016/j.cherd.2016.06.018
 Zahir B. Derradji C, Saci N. (2012). Dynamic modelling of the secondary settler of a wastewater treatment via activated sludge to low-load. Energy Procedia 18: 1-9. https://doi.org/10.1016/j.egypro.2012.05.012
 Qin B. (2012). Research on domestic wastewater treatment with radial flow inclined tube settling tank. Environmental Science and Management 37(4): 82-85.
 Liu YL, Zhang P, Wei W. (2013). Two-dimensional numerical simulation of properties of liquid-solid two-phase flow in a circular secondary clarifier. Engineering Journal of Wuhan University 46(4): 410-412.
 Liu YL, Zhang P, Wei WL. (2013). Numerical simulation of mechanical property of solid-liquid two-phase turbulent flow in a secondary sedimentation tank of radial flow. Journal of Water Resources＆Water Engineering 24(4): 25-27.
 Wei WL, Li PP, Hong YF, Liu YL. (2016). Influence of baffle length on flow and sludge concentration fields in a radial sedimentation tank. Journal of Northwest A&F University (Nat. Sci. Ed.) 44(7): 228-234. https://doi.org/10.13207/j.enki.jnwafu.2016.07.032
 Wei WL, Bai CW, Liu YL. (2016). 3D simulation for the influence of a feed baffle on the density current behaviors caused by temperature in a radial sedimentation tank. Journal of Xi’an University of Technology 32(1): 12-17. https://doi.org/10.19322/j.cnki.issn.1006-4710.2016.01.003