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Water distribution systems (WDSs) construction, operation and disposal processes contribute to undesirable greenhouse gas (GHG) emissions. GHG concentration in the atmosphere is strongly associated with global warming and climate change. In order to meet the consequent challenge of limiting GHG emissions, the problem of WDS (re)design is formulated here as a multi-objective optimisation problem. The three objectives are as follows: (1) minimisation of total (re)design cost, (2) maximisation of the WDS resilience and, (3) minimisation of GHGs emissions. In addition to the frequently considered conventional (re)design intervention options (new pipes, pipe duplication or replacement, addition of pumps, tanks, etc.), various water demand management interventions (e.g. water efficient appliances and domestic Rainwater Harvesting Systems) are considered here too. A number of different rainwater tank sizes and water saving appliances provided to different parts of the households have been evaluated. This methodology was applied on the New York Tunnels and the Anytown network problems. The output from the Non-dominated Sorting Genetic Algorithm (NSGA2) optimisation process is a Pareto front containing optimal solutions traded-off in terms of the three objectives analysed. The results obtained demonstrate that using demand management technologies in the (re)design of WDSs can lead to a similar cost, resilience and GHG emissions but with cost savings. In a pumped WDS where cost savings and GHG emission reduction are relatively significant, demand management technologies led to more cost effective, resilient and climate change mitigating solutions as compared to the conventional (re)design.

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Assessment of probable causes of chlorine decay in water distribution systems of Gaborone city, Botswana Gaborone city water distribution system (GCWDS) is rapidly expanding and has been faced with the major problems of high water losses due to leakage, water shortages due to drought and inadequate chlorine residuals at remote areas of the network. This study investigated the probable causes of chlorine decay, due to pipe wall conditions and distribution system water quality in the GCWDS. An experimental approach, which applied a pipe-loop network model to estimate biofilm growth and chlorine reaction rate constants, was used to analyse pipe wall chlorine decay. Also, effects of key water quality parameters on chlorine decay were analysed. The water quality parameters considered were: natural organic matter (measured by total organic carbon, TOC; dissolved organic carbon, DOC; and ultraviolet absorbance at wavelength 254, UVA-254, as surrogates), inorganic compounds (iron and manganese) and heterotrophic plate count (HPC). Samples were collected from selected locations in the GCWDS for analysis of water quality parameters. The results of biofilm growth and chlorine reaction rate constants revealed that chlorine decay was higher in pipe walls than in the bulk of water in the GCWDS. The analysis of key water quality parameters revealed the presence of TOC, DOC and significant levels of organics (measured by UVA-254), which suggests that organic compounds contributed to chlorine decay in the GCWDS. However, low amounts of iron and manganese (< 0.3 mg/L) indicated that inorganic compounds may have had insignificant contributions to chlorine decay. The knowledge gained on chlorine decay would be useful for improving water treatment and network operating conditions so that appropriate chlorine residuals are maintained to protect the network from the risks of poor water quality that may occur due to the aforementioned problems.

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