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Private water tanks inside buildings are common in developing countries, due to intermittent water supply. They are also used in multi-storey buildings which are too tall to be supplied throughout by the normal pressure in the public water mains. Their hydraulic and water mixing behavior has not been studied. This paper outlines results of a study of residence time and disinfectant mixing in roof tanks, based on physical and numerical models. A real scale transparent wall laboratory model of a roof tank was constructed and equipped with high resolution water meters, continuous chlorine monitor, and tracer test instruments. Tracer and disinfectant decay tests were carried out. A computational fluid mechanics model of the tank was implemented, and an equation for the loss of disinfectant in water tanks with stagnation space and bypassing was obtained. Methodology and results can be extrapolated to large municipal water tanks as well.