Modelling of water mixing and renewal time in storage tanks

The current paper aims at presenting the main developments and achievement attained in research project IMiST – Improving mixing in Storage Tanks for safer water supply, funded by Fundação para a Ciencia e Tecnologia, carried out between 2018 and 2022. This project aimed at a better understanding of flow dynamics inside water supply systems’ storage tanks to find practicable solutions to improve the design and the rehabilitation and to support operation of the existing tanks. The project comprised the development of an extensive experimental programme of small-scale tests, full-scale testing in a real storage tank and advanced numerical modelling. Small-scale tests were carried out in the Laboratory of Hydraulics and Water Resources of Instituto Superior Técnico, Portugal, for different tank’s configurations and operating conditions. Two different cross-section tanks (circular and rectangular) were tested with and without interior structures (baffles) and with the inlet and outlet pipes at different locations. Three sets of experimental tests were carried out using different instrumentation to collect complementary data, namely traditional tracer tests, dye tracer tests and velocity field measurements by Particle Image Velocimetry (PIV). A methodology for assessing mixing conditions in full-scale storage tanks was developed and tested in a real tank with a rectangular cross-section and 3500 m3 capacity with inner baffles. Advanced numerical modelling using Computational Fluid Dynamics was carried out for the submerged and plunging get in a circular tank for betted understanding the velocity fields and the flow patterns. This complementary experimental and numerical modelling have allowed drawing conclusions concerning improving mixing measures, by readjusting the tank configuration (e.g., location, number and diameter of inlet/outlet pipes, jet inflow, fluctuating stored water volume and geometry) and changing operating conditions (e.g., extreme tank levels, pump scheduling). New knowledge on water storage tank hydrodynamics has been created and guidelines for upgrading tank design, rehabilitation and operation were prepared with design criteria for new tanks and measures for the improvement/rehabilitation of existing tanks in supply systems, that will be briefly summarised herein.