Improving mixing and renewal in drinking water storage tanks: lessons learnt and practical measures

ABSTRACT This paper investigates the effect of different types of structural and operational measures on water mixing and renewal time in circular and rectangular cross-section water storage tanks, aiming at a better understanding of the flow dynamics to find practicable solutions to improve their design, rehabilitation and operation. An experimental programme, including traditional tracer and dye tracer tests, was carried out in small-scale tanks for different configurations and operating conditions. Two tanks were tested with and without interior structures, with the inlet/outlet pipes at different locations and for constant and variable water level. The main findings are that: i) the most effective measure is operating with fill-and-draw cycles, however, for tanks operated nearly full structural measures are recommendable; ii) reducing the inlet pipe diameter and installing nozzles near the tank bottom improve the mixing conditions; iii) the use of baffles is recommendable when the inlet and outlet pipes are very close.