Modeling the effect of wind in rectangular settling tanks for water supply

Abstract

A 2-D CFD model is applied to investigate the effect of co-current and counter-current wind velocities of up to 7.5 m/s in the settling tanks of EYDAP Water Treatment Plant of Aharnes, in Greece, and the following conclusions are drawn: (1) without wind, the flow field is characterized by a large recirculation region with significant short-circuiting, while for windy conditions a two-layer flow is observed, in which the surface layer follows the wind direction. The suspended solids’ concentration fields strongly depend on the corresponding flow fields. (2) Calculated local removal efficiencies, without wind, near water surface show a satisfactory agreement with measurements. When the sludge removal mechanism is operating, the calculated efficiency without wind is equal to 83.1%; this value is lower than the experimental value (86.0 ± 1.0%) and the predicted value by a 3-D model (85.7%). When co-current wind velocity increases, short-circuiting increases and the efficiency decreases; however, not noticeably (<0.3%). When counter-current wind velocity increases, short-circuiting is reduced and the efficiency increases by up to approximately 1.0%. (3) When the sludge removal mechanism is out of operation, the calculated efficiency without wind decreases to 68.1%, a value that is again lower than the experimental value (70.8 ± 1.0%) and the predicted value by a 3-D model (70.9%); the wind effect is similar to the case of operating sludge removal mechanism, with a somehow larger difference of up to 1.3%.

Keywords:

• Wind
• Settling tank
• Water treatment plant
• CFD modeling
• Suspended solids
• Settling efficiency

Acknowledgments

The present work was conducted within the framework of a research project entitled “Improving the settling efficiency in the old unit of WTP of Aharnes using modern flocculants and appropriate mathematical models, for parametrization in real operation conditions.” The financial support by EYDAP Athens Water Supply and Sewerage Company S.A. for this project is gratefully acknowledged. Furthermore, the authors would like to thank the members of the Project Monitoring Committee: Mr A. Magoulas, Mrs M. Xanthaki, Mrs A. Synodinou, Dr F. Miskaki, Dr N. Defteraios, Dr T. Kaloudis, Dr E. Smeti and other scientific and technical personnel of EYDAP S.A., for the useful discussions and for providing the required information and data. A part of this work was carried out within the MSc Thesis of the second author in the TUM under the supervision of the third author, while he was Visiting Professor at the TUM; thanks are also due to the DAAD, the TUM, and the NTUA.

Notes

Presented at CEST2015—14th International Conference on Environmental Science and Technology, Rhodes, Greece, 3–5 September 2015