Reduction of Mixing in Jet-Fed Water Storage Tanks

Abstract

Contrary to usual mains-water practice, mixing in water storage tanks used in rainwater harvesting systems is undesirable because pathogen die-off can occur in the unmixed water prior to its extraction for use. The principal cause of mixing in these tanks is the momentum of the inflow during a rainfall event. We investigate the effect of inflow-jet configuration on the proportion of stored water in a tank which mixes with the slightly cooler inflow of rooftop water. Scale experiments are conducted which show that the nondimensional height of the mixing front above the jet inlet is proportional to the inflow-jet densimetric Froude number for both single and multijet arrangements of various geometries. For each arrangement a coefficient of mixing is found. The results are then used to assess the level of mixing in full-scale rooftop rainwater harvesting storage tanks and determine whether residence time in such tanks is a viable strategy for pathogen reduction. For such applications, a radial manifold of jets outwardly directed from the tank center is found to be the most promising.