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Pellet softening of drinking water can provide aesthetic, socioeconomic and environmental benefits in areas with hard water. Calcium carbonate pellets are the main by-product from pellet softening and their characteristics determine their reuse potentials. We characterized pellets from a pilot-scale pellet reactor treating 16 water types at 8 Danish drinking water treatment plants to investigate the variations in pellet characteristics and how they depend on the influent water composition. The pellets consisted of up to 100% calcium as calcium carbonate, but contained often also impurities such as strontium, magnesium, iron and sodium each contributing with up to 1.3% of the pellet mass. Other elements, including heavy metals, accounted for <0.04% of the pellet mass. The quartz sand seeding material contributed with up to 15% of the pellet mass and can be a barrier for pellet reuse. Therefore, replacing this with calcium carbonate (limestone) seeding material increases the pellet purity. Modelling the chemical speciation indicated that elements not forming carbonates (e.g. potassium and magnesium), are only incorporated into pellets to a limited extent. The concentrations of strontium, magnesium, manganese, iron and nickel in the pellets had a strong positive correlation with the influent water concentration. Consequently, the pellet purity increases if the concentration of these elements is reduced in the water before softening by other treatment technologies. Potassium, arsenic and zinc showed no or only a weak correlation. The pellets precipitated as calcite, and had a reactivity of ≤25.7% and a specific surface area of ≤0.32 m2/g, which limits the potential reuse in agriculture. The pellet mineralogy was independent of the investigated range of influent water quality and seeding materials. Including pellet quality when designing the softening process can improve pellet reuse, ultimately leading to a more environmentally sustainable drinking water supply.
Original languageEnglish
JournalScience of the Total Environment
Volume652
Pages (from-to)538-548
ISSN0048-9697
DOIs
StatePublished - 2019
CitationsWeb of Science® Times Cited: No match on DOI

    Research areas

  • Calcite, Calcium carbonate, Chemical water treatment, Residuals management, By-product reuse, Drinking water treatment
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