Improving secondary settling tanks performance by applying CFD models for design and operation

Abstract

This study uses computational fluid dynamics (CFD) models for an improved performance of secondary settling tanks, taking as a case study the wastewater treatment plant (WWTP) of Souk-Ahras City, Northeastern Algeria. Unlike previous studies, which focused primarily on simulations of real-time clarifiers by independent measures, this study combines differentoperating criteria and geometric standards on clarifiers to estimate the ultimate capacity of secondary sedimentation tanks (SST). Numerical simulations were performed toassess the SST hydraulic performance under different conditions in order to (i) eliminate vortex zones; (ii) optimize the surface overflow rate (SOR) and predict sludge blanket height (SBH), and finally, (iii) estimate SST ultimate capacity. Based on the current configuration of the SST of Souk-Ahras WWTP, its hydraulic treatment capacity was found to be 49,000 m3/d. The results obtained, which were validated by field observations, showed that optimizing the different parameters yields a significantly improved SST performance. Furthermore, the tests consideredwere shown to be satisfactorily correlated with SOR. Validation using the receiver operating characteristic (ROC) curve analysis yielded anarea under curve (AUC) value of 90%, implying thereby a reasonably good performance. Indeed, optimization resulted in increased solidremoval efficiency and increased SOR to 1.4 m.h−1 (compared to 0.8 m.h−1 in literature) without overflow beyond the SBH. HIGHLIGHTS Secondary settling tanks (SST) may be achieved with a surface overflow rate (SOR) of 1.4 m·h−1, which exceeds the guide value for conventional configurations (0.8 m·h−1).; Computational fluid dynamics (CFD) was shown to be an interesting method for optimization of the capacity of circular SSTs.; The characterization of sludge settling is necessary to determine sedimentation velocities and evaluate purification efficiency.;

Keywords

• computational fluid dynamics
• sludge blanket
• turbulence model
• wastewater treatment plant