Modelling of a roof runoff harvesting system: the use of rainwater for toilet flushing

• Published in: Water science & technology. Water supply Vol. 11; no. 2; pp. 151 - 158
• Main Authors: VIALLE, C, SABLAYROLLES, C, LOVERA, M, HUAU, M.-C, MONTREJAUD-VIGNOLES, M
• Format: Journal Article
• Language: English
• Published: London International Water Association 01.01.2011; IWA Publishing
• Subjects: Applied sciences; Building technical equipments; Buildings; Buildings. Public works; Case studies; Chemical and Process Engineering; Computation methods. Tables. Charts; Computer models; Computer simulation; Computing time; Correlations; Drinking water; Engineering Sciences; Exact sciences and technology; Flushing; Flushing (water); Food engineering; Harvesting; Hydrologic data; Life Sciences; Mathematical models; Modelling; Numerical simulations; Plumbing. Sanitary facilities; Rain; Rain water; Rainfall; Rainfall data; Rainfall-runoff modeling; Rainwater recovery systems; Roofs; Runoff; Storage capacity; Storage conditions; Structural analysis. Stresses; Water balance; Water conservation; Water demand; Water harvesting; Water supply. Pipings. Water treatment; Flushing equipment; Water harvesting; Instruments; rainwater collection; Fresh water; Modeling; modelling; Case study; Experimental result; Roof; water saving efficiency; System performance; Simulation; Toilet water; Efficiency; Runoff; System description; Rain water; system efficiency; simulation
• ISSN: 1606-9749; 0273-1223; 1607-0798; 1996-9732
• DOI: 10.2166/ws.2011.031

The water balance of a four-people family rainwater harvesting system was calculated in a case study. The experimental water saving efficiency (WSE) was calculated as 87%. A simple computer model was implemented to simulate the behaviour of the rainwater harvesting system. In general, the rainwater collector volumes predicted by the daily model had shown a good correlation with the experimental values. The difference between the experimental and the predicted values for the stored volume can be explained by the lack of maintenance of the system that can affect its performance. On the basis of a long-term simulation of 20-year rainfall data, the following parameters were calculated: rainfall, water demand, mains water, rainwater used, over-flow and WSE. The collection of rainwater from roofs, its storage and subsequent use for toilet flushing can save 42 m3 of potable water per year for the studied system. The model was also used to find the optimal size of the tank for the single-family household: a storage capacity of approximately 5 m3 was found to be appropriate. The storage capacity and tank size were distinguished. The importance to take into account the dead volume of the tank for the sizing was indeed highlighted.