A rainwater harvesting system (RHS) was designed for a waste treatment facility located near the town of Mirandela (northern Portugal), to be used in the washing of vehicles and other equipment, the cleaning of outside concrete or asphalt floors, and the watering of green areas. Water tank volumes representing 100% efficiency (V-r) were calculated by the Ripple method with different results depending on two consumption scenarios adopted for irrigation. The RHS design was based on a precipitation record spanning a rather long period (3 decades). The calculated storage capacities fulfilled the water demand even when prolonged droughts occurred during that timeframe. However, because the drought events have been rather scarce the V-r values were considered oversized and replaced by optimal volumes. Notwithstanding the new volumes were solely half of the original V-r values, the projected RHS efficiency remained very high (around 90%) while the probability of system failure (efficiency < 100%) stayed very low (in the order of 5%). In both scenarios, the economic savings related to the optimization of V-r were noteworthy, while the investment's return periods decreased substantially from the original to the optimized solutions. A high efficiency with a low storage capacity is typical of low demanding applications of rainwater harvesting, where water availability (V-w) largely exceeds water demand (C-w), that is to say where demand fractions (C-w/V-w) are very low. Based on the results of a literature review covering an ample geographic distribution and describing a very large number of demand fraction scenarios, a C-w/V-w = 0.8 was defined as the threshold to generally distinguish the low from the high demanding RHS applications. (C) 2015 Elsevier B.V. All rights reserved.