This paper presents the results of a research aiming at evaluating the effects of storage on wastewater characteristics. Wastewater discharged from the Caltagirone (Eastern Sicily) plant after secondary treatment was stored in an earth reservoir with a capacity of about 25,000 m3 and a depth of 5 m. Wastewater inflow in the reservoir was continuous throughout the experimental activities, while discharge was discontinuous, depending on irrigation demand. Removal efficiency resulted highly influenced by the operation regime of the reservoir and by influent wastewater characteristics. BOD5 and COD removal efficiency was above 50%. Removal efficiency of faecal coliforms, faecal streptococci and Escherichia coli was between 2 and 5 log units. Single and multiple regressions were tested to determine the reservoir design characteristics and operation parameters that most significantly affected water quality changes.