Effect of first flush diversion on the reliability of urban rainwater harvesting systems in Dhaka

First flush diversion, which involves discarding the initial, dirtiest flush of runoff from a rainfall event, is one of the most commonly adopted and effective means of improving stored water quality in rainwater harvesting (RWH) systems. Discarding a certain fraction of the runoff can diminish the amount of water available for use and may affect the reliability of the RWH systems. We developed storage-reliability-yield (SRY) relationships within practical ranges of daily demand and storage volumes and analysed the reduction of reliability of RWH system for Dhaka city resulting from different first flush diversion schemes recommended in national and international guidelines. It is found that a maximum 5% reduction in reliability can occur if various strategies of run-off diversion are applied. While analysing the operational performance of four RWH systems in Dhaka city, it was found that the baseline reliabilities of three out of the four RWH systems were very poor although first flush diversion will not significantly reduce their performance any further. The study highlights that the SRY relationship can be effectively applied to determine the reliability of existing RWH systems and to assess the effect of first flush diversion schemes on its performance.