Softening of drinking water by the pellet reactor - Effects of influent water composition on calcium carbonate pellet characteristics

Abstract 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 2 /g, which limits the potential reuse as soil amendment in agriculture. The pellet mineralogy was independent of the investigated range of influent water quality and seeding materials. Including pellet characteristics when designing the softening process can improve pellet reuse, ultimately leading to a more environmentally sustainable drinking water supply.