A study was conducted to assess the effects of storage tank design and operation on mixing regimes and effluent water quality. The influent and effluent flows of three tanks with diameter-to-height ratios ranging from 3.5:1 to 0.4:1 were monitored for chlorine residual. Chlorine levels were also measured within the water column of each tank. Although chlorine profiles revealed some stratification in tanks with large height-to-diameter ratios, completely mixed models were more accurate than plug-flow models in representing the mixing behavior of all three tanks. Modeling further indicated that the quality of the effluent from completely mixed tanks deteriorated with decreasing volumetric change. Standpipes were the least desirable tank design with respect to effluent water quality. The authors conclude that if all reservoirs behave more or less as completely mixed reactors, then storage reservoirs should be designed to ensure maximum volumetric change. Includes 11 references, tables, figures.