Chlorine degradation in continuous and intermittent drinking water supply networks

Chlorine is a widely used disinfectant in developing countries due to its efficiency and affordability. Millions of people in these countries receive water intermittently through piped networks. However, very little is known about chlorine decay in intermittent networks. In this work, field chlorine decay measurements were carried out in adjoining continuous and intermittent networks. Hydraulic modelling was done to determine the water velocity in networks using EPANET. Overall chlorine decay rates were estimated using field measurements and showed a good first-order fit. Decay rates in continuous and intermittent networks were compared, considering the influence of pipe velocity, material and diameter. The average overall decay rate for ductile and cast-iron pipes in intermittent network (1.7 hr−1) was 1.5 times higher than that in continuous network (1.1 hr−1). These results indicate that intermittency has a long-term effect on the overall chlorine decay in supply networks and its implications are discussed in this study.</p

CuO Nanoparticle Dissolution and Toxicity to Wheat (<i>Triticum aestivum)</i> in Rhizosphere Soil

It has been suggested, but not previously measured, that dissolution kinetics of soluble nanoparticles such as CuO nanoparticles (NPs) in soil affect their phytotoxicity. An added complexity is that such dissolution is also affected by the presence of plant roots. Here, we measured the rate of dissolution of CuO NPs in bulk soil, and in soil in which wheat plants (<i>Triticum aestivum)</i> were grown under two soil NP dosing conditions: (a) freshly added CuO NPs (500 mg Cu/kg soil) and (b) CuO NPs aged for 28 d before planting. At the end of the plant growth period (14 d), available Cu was measured in three different soil compartments: bulk (not associated with roots), loosely attached to roots, and rhizosphere (soil firmly attached to roots). The labile Cu fraction increased from 17 mg/kg to 223 mg/kg in fresh treatments and from 283 mg/kg to 305 mg/kg in aged treatments over the growth period due to dissolution. Aging CuO NPs increased the toxicity to <i>Triticum aestivum</i> (reduction in root maximal length). The presence of roots in the soil had opposite and somewhat compensatory effects on NP dissolution, as measured in rhizosphere soil. pH increased 0.4 pH units for fresh NP treatments and 0.6 pH units for aged NPs. This lowered CuO NP dissolution in rhizosphere soil. Exudates from <i>T. aestivum</i> roots also increased soluble Cu in pore water. CaCl₂ extractable Cu concentrations increaed in rhizosphere soil compared to bulk soil, from 1.8 mg/kg to 6.2 mg/kg in fresh treatment and from 3.4 mg/kg to 5.4 mg/kg in aged treatments. Our study correlated CuO NP dissolution and the resulting Cu ion exposure profile to phytotoxicity, and showed that plant-induced changes in rhizosphere conditions should be considered when measuring the dissolution of CuO NPs near roots