Sizing the First Flush and its Effects on the Storage-Reliability-Yield Behavior of Rainwater Harvesting in Rwanda
Kelly Doyle, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge
Rooftop rainwater harvesting is a technology used to supply water for domestic purposes in developing countries. Rooftop rainwater harvesting involves collection of rainwater from a rooftop via a guttering system and storage in a cistern. During dry weather, dust and other pollutants accumulate on the roof surface and are washed off at the beginning of the next rain. The initial contaminated volume of water flowing from a roof after a rainstorm is known as the first flush and research has shown that diverting it from the main supply can improve the quality of stored water.
This thesis considers a variety of design options for rainwater harvesting, set in the context of rural Bisate Village in northwestern Rwanda. Fieldwork sought to quantify the first flush by measuring the water quality of runoff samples. Subsequent work assessed the reliability of the rainwater harvesting systems in the village by studying their storage-reliability-yield (SRY) behavior. A SRY simulation model was run to mimic daily use of a harvesting system and calculate the reliability while considering the first flush fieldwork results. The recommended first flush diversion was simulated and evaluated against several management options and overall recommendations for first flush diversion were assessed in the context of the existing systems.