Chemical Technology January 2015

Water Treatment

will be low as well. Secondly, to keep the metal-dissolving properties of the water sufficiently low, the pH needs to be maintained in an optimal range. In hard water, it will not be possible to have a sufficiently high pH, because the water will then become supersaturated with respect to calcium carbonate and cause scaling. Allowing for a sufficient pH increase, together with the need to have water in calcium– carbonic acid equilibrium, necessitates the softening of water. On the other hand, it should be recognised that the efficacy of chlorine disinfection decreases as pH increases, and it is especially weak beyond about pH 7-8. Water treatment practices in the Netherlands Central softening has been applied in the Netherlands since the late 1970s. Almost all drinking-water in the Netherlands

less leaching of copper, less salt usage by private soften- ers, lower use of chemical inhibitors); and • better consumer comfort due to better looking clothes, glassware, etc. It is essential to arrive at an optimal composition of the drinking-water to be distributed, and the central aspects of this optimal composition of the public drinking-water should include: • calcium–carbonic acid equilibrium; • a non-corrosive pH; and • sufficient alkalinity. First of all, the water will need to be in calcium–carbonic acid equilibrium. This means that the Saturation Index (SI) should be close to 0. The preferred range for SI values is -0,2 < SI < +0,3. In this region of SI, the corrosion of cement pipe will be minimal, while at the same time the scaling effects

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Chemical Technology • January 2015