First flush diversion is a simple treatment method for rainwater harvesting systems in both developed and developing countries. Diverting the first flush from the main supply can improve the quality of stored water but at the cost of some loss of water supplied. This paper considers the impact of first flush removal both on the quality of the stored water and on the reliability of three rainwater harvesting systems for the 8,500 residents of Bisate Village, Rwanda.To quantify the efficacy of first flush removal, discrete samples of rainwater were collected using an easily constructed PVC bottle array. After a rain event, the samples were tested for a variety of water-quality parameters, including turbidity. The resulting plots of turbidity reduction over runoff depth follow exponential decay, as expected. Based on the water-quality data an easy-to-use recommendation for first flush operation was proposed: divert the first millimeter of runoff from a roof after three consecutive dry days. Previous researchers (Martinson and Thomas, 2005) have found that diverting the first millimeter of rain halves the contaminant load from the roof. Our similar results suggest this is a robust and broadly applicable conclusion. The recommendation was then used to evaluate the impact of first flush removal on the reliability of the system.Reliability of a rainwater harvesting system is measured by its ability to deliver the water needed by the system users. While storage-reliability-yield (SRY) behavior has been characterized for the United States by Hanson (2007), adequate precipitation data are often scarce in developing countries. To compensate for sparse rainfall records, a synthetic pseudo-daily rainfall record was generated. Using a combination of the synthetic data, the proposed management option, and the known storage and yield for three systems in Bisate Village, we calculated the impact of first flush diversion on reliability of three in-place systems. The simulations showed that for these systems the reliability would be reduced by less than 2% with diversion of the recommended first flush, while the turbidity would be decreased by 50%. Rainwater harvesting with the diversion of the first millimeter provides good quality, low-cost water in a properly sized cistern for people with few resources.
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