The pollution reduction opportunities of a drainage basin, located in Barcelona (Spain), were analysed as a case study in order to assess strategies for optimised management of sewerage networks in the Mediterranean region. The analysis results show that the introduction of a large storm tank against combined sewer overflow significantly reduces the negative impacts associated with discharges during rainy weather. The total annual mass into the receiving waters is reduced by 45%, measured in terms of suspended solid and chemical oxygen demand. The performance of large storm tanks can be optimised taking advantage of the natural sedimentation of the retained rainwater. At optimum stratification conditions, the annual regulation capability can be increased by 15% compared to the traditional operating conditions. Apparently, the environmental impacts associated with the materials used in the construction of the tank will also be reduced per unit volume of regulated water with the increasing annual regulation capacity.