Modelling rainwater harvesting and greywater reuse for tank size optimizations Sara Simona Cipolla 1 and Marco Maglionico 2 1 Interdepartmental Centre for Industrial Research- Building and Construction, University of Bologna, Viale del Risorgimento 2, 40136 - Bologna, Italy 2 Department of Civil, Chemical, Environmental and Materials Engineering, University of Bologna, Viale del Risorgimento 2, 40136 - Bologna, Italy. sara.cipolla@unibo.it, marco.maglionico@unibo.it Abstract In the light of water shortages, frequently affecting many regions worldwide, domestic rainwater harvesting and greywater reuse systems represent an alternative source to provide non-potable water in buildings, reducing the water demand from mains water supply systems. This study fits this framework providing a methodology, based on a hydraulic/hydrological model developed by means of the EPA’s Storm Water Management Model, which allow optimizing the system design by giving the opportunity to the user to consider different catchments surfaces (impervious, gravel and green roofs), plant's configurations, user’s habits, water end-uses, and climate conditions. The model has used to model a residential building, located in the city of Bologna (Italy), and equipped with a hybrid greywater/rainwater system. Continuous simulations were performed with 13 years daily rainfall data, and the long-term performance of different system combinations were evaluated. The case study showed a non-potable water saving efficiency of 75.86%, which accounts by 26.71% mains water withdrawal. The final goal of this paper is those of presenting the hydrological/hydraulic model that has been used as engine of a calculator tool for sizing and planning hybrid rainwater/greywater systems. 1 Introduction Global population is facing with contrasting phenomena, on one hand climate change, which determines an increase in the frequency of drought and floods in many countries. On the other urban population growth, which already affects every continent and causes a growing demand for potable water and the increase in urban waterproof areas (Bitterman et al., 2016; Ward et al., 2012). Proper and sustainable management of these phenomena is both challenging in terms of water supply systems management and of urban drainage (Campisano et al., 2017; Casadio et al., 2013; Cipolla et al., 2016a). Engineering EPiC Series in Engineering Volume 3, 2018, Pages 460–469 HIC 2018. 13th International Conference on Hydroinformatics G. La Loggia, G. Freni, V. Puleo and M. De Marchis (eds.), HIC 2018 (EPiC Series in Engineering, vol. 3), pp. 460–469