The effects of hydraulic performance and operation interventions on water quality: The case of Phakalane water distribution system in Botswana

Abstract

This study demonstrates the application of water distribution system (WDS) simulations to investigate the effect of hydraulic performance, network configurations and alternative operational interventions (AOI) on water quality in a WDS. The study explores the viability of introducing AOI to the current (existing) and plausible (possible) network configurations to improve water age and chlorine residuals in the Phakalane WDS. The criteria used to determine the performances of the WDS are (1) the percentage of nodes and (2) the percentage of volume of water demand with water age or chlorine residuals, which are within the acceptable or unacceptable limits. The current network configuration and operational interventions of Phakalane WDS were analysed. The results revealed that flow velocities in more than 95% of the pipes are less than the minimum acceptable limit (0.3 m/s), which is suspected to contribute to increase in water age and decay of chlorine residuals due to accumulation of deposits in the pipes. Furthermore, all the nodes and 87% of the volume of water demand had water age of more than 2 days. Also, only 43% of the nodes and 28% of the volume of water demand had chlorine residuals within the standard limits (0.3–0.6 mg/L). In order to evaluate possible interventions that would improve water age and chlorine residuals in the Phakalane WDS, several AOI and network configurations were analysed. The results revealed that the AOI introduced to the plausible network configuration would generally improve water age and chlorine residuals better than the AOI that are introduced to the current network configuration. Also, there is a link between the areas in the WDS with decreased water age and increased chlorine residuals. Finally, sensitivity analysis revealed that the levels of water demand, chlorine injection rate and the chlorine decay rate would need monitoring in order to introduce interventions that improve the performances of the WDS.